Sensor control for linear electromagnetic surgical impactor

Abstract

A surgical impactor comprising a linear motor, having a first set of one or more magnetic sensors 310 at a first end of a stator of the linear motor; and a second set of one or more magnetic sensors 3

Description

TECHNICAL FIELD The application relates to methods and systems for controlling surgical impactors as well as surgical impactors with improved sensing. In particular, the application relates to improved sensing of striker position within a linear electromagnetic for a surgical impactor. BACKGROUND When performing surgery, particularly hip replacement surgery, it is sometimes necessary to provide an impact force. This can be through the use of a surgical mallet and hip hammer. For instance, in total hip replacement surgery, a surgeon may utilise a broaching tool to remove less dense (cancellous) bone from a femoral canal without damaging the hard cortical bone surrounding it. This broaching may require multiple strikes, both in the forward and reverse direction. Thereafter, the surgeon may seat an implant in the prebroached femur and seat a head on the implant. Similarly, when seating an acetabular cup, the acetabular may be reamed using a reamer before an acetabular cup is inserted. The acetabular cup may be held in place by a press-fit. Seating the acetabular cup may require a number of impactions (e.g. 8-10 impactions) to ensure a secure fit. Surgical impactors can be driven in a variety of ways, for instance, pneumatic drive. Pneumatic drives can be bulky and can provide limited control over impact power. Pneumatic drives can also require connection to a gas supply, which can limit movement and make the impactors more difficult to use. SUMMARY In general, the application relates to surgical impactors that are powered through linear electromagnetic motors. In particular embodiments, sensors are positioned at both sides of a stator of the electromagnetic motor. The sensors sense the position within the impactor of a striker that is being driven by a stator. The position is used to control the coils to drive the striker. By positioning the sensors on both sides of the stator, the striker can be made smaller, reducing the size and weight of the impactor. By positioning the sensors outside of the stator, the stator can be made smaller, and the sensors experience reduced magnetic interference from the stator. Reducing the size and weight of the impactor is important for surgical uses where access the surgical site being impacted (e.g. a joint or bone) can be limited, and therefore reduced size can help the surgeon to better orient and position the impactor. According to an embodiment, a first set (e.g. a front set) of one or more sensors is placed on a first side (e.g. a front side) of the stator, and a second set (e.g. a rear set) of one or more sensors is placed on a second side (e.g. a rear side) of the stator. At any one time, the striker will be primarily located over the first or second set. To ensure consistency of readings at any one time, measurements from only one of the sets is used to control the motor. Particular embodiments provide a means of effectively switching which of the first and second sets of one or more sensors are being used to control the coils at any one time. For instance, the set of one or more sensors that provide the strongest magnetic readings at the current time may be used to control the motor at that time. Where a set of one or more sensors includes multiple sensors, then a combined strength value can be determined, for instance, by determining a sum of normalized or squared values of the measurements. According to an aspect there is provided a system configured to control a surgical impactor comprising a linear motor, the system comprising circuitry configured to: receive a first set of one or more magnetic field measurements from a first set of one or more magnetic sensors at a first end of a stator of the linear motor; receive a second set of one or more magnetic field measurements from a second set of one or more magnetic sensors at a second end of the stator, the second end being opposite to the first end; select one of the first set of one or more magnetic field measurements and the second set of one or more magnetic field measurements based on a relative strength of the first set of one or more magnetic field measurements and the second set of one or more magnetic field measurements; and issue control signals to control the linear motor based on the selected one of the first set of one or more magnetic measurements and the second set of one or more magnetic measurements. According to an embodiment selecting one of the first set of one or more magnetic field measurements and the second set of one or more magnetic field measurements comprises: determining a first strength value, indicating a strength of the first set of one or more magnetic field measurements; determining a second strength value, indicating a strength of the second set of one or more magnetic field measurements; and selecting the one of the first set of one or more magnetic field measurements and the second set of one or more magnetic field measurements based on the first strength value and the second strength value. According to an embodiment selecting the stronger of the first set of one or more magnetic field measurements and the second set of one or more magnetic field measurements comprises selecting the set of one or more magnetic field measurements that has the highest strength value. When switching from a currently selected set to another set, an offset may be applied when comparing the magnetic field measurements to apply hysteresis. For instance, the selected set of one or more magnetic field measurements may be raised by an offset, and / or the other set of one or more magnetic field measurements may be decreased by an offset. According to an embodiment the one of the first set of one or more magnetic field measurements and the second set of one or more magnetic field measurements is selected in response to determining that the corresponding strength value for the one of the first set and second set is above a first threshold. According to an embodiment the circuitry is further configured to, subsequent to selecting the one of the first set of one or more magnetic field measurements and the second set of one or more magnetic field measurements, select the other of the first set of one or more magnetic field measurements and the second set of one or more magnetic field measurements in response to determining that the strength value for the one of the first set of one or more magnetic field measurements and the second set of one or more magnetic field measurements has dropped below a second threshold that is lower than the first threshold. According to an embodiment: the first set of one or more magnetic field measurements comprises a first plurality of magnetic field measurements; the second set of one or more magnetic field measurements comprises a second plurality of magnetic field measurements; and the circuitry is further configured to, for each magnetic field measurement, calculating a processed magnetic field measurement by performing one or both of rectifying the magnetic field measurement and raising the magnetic field measurement to a power that is greater than one. Determining the first strength value comprises calculating an accumulation of the processed magnetic field measurements determined from the first plurality of magnetic field measurements. Determining the second strength value comprises calculating an accumulation of the processed magnetic field measurements determined from the first plurality of magnetic field measurements. According to an embodiment: determining the first strength value comprises calculating a sum the processed magnetic field measurements determined from the first plurality of magnetic field measurements; and determining the second strength value comprises calculating a sum of the processed magnetic field measurements determined from the first plurality of magnetic field measurements. According to an embodiment: the first set of one or more magnetic field measurements comprises a first plurality of normalized magnetic field measurements; the second set of one or more magnetic field measurements comprises a second plurality of normalized magnetic field measurements: selecting the one of the first set and the second set comprises: for each set: determining a corresponding vector for the set by allocating each normalized magnetic field measurement in the set to a corresponding dimension in the vector; and calculating a length of the vector; and selecting the one of the first set and the second set based on the corresponding lengths of the vectors. The dimensions of each vector may be orthogonal to each other. According to an embodiment selecting the one of the first set and the second set based on the corresponding lengths of the vectors comprises selecting the set that has the longest corresponding vector. When comparing vector length, an offset may be applied to apply hysteresis. According to an embodiment each magnetic field measurement indicates a relative strength of a magnetic field generated by one or more magnets connected to a striker that is configured to be driven to move linearly through the stator. According to an embodiment: the circuitry is configured to determine a position of the striker relative to the stator based on the selected set of one or more magnetic field measurements; and the one or more control signals are issued based on the determined position. According to an embodiment the one or more control signals are configured to control each of one or more sets of electromagnets within the stator to generate a corresponding magnetic field to drive the striker to move linearly within the stator. According to a further aspect there is provided a surgical impactor comprising an electromagnetic linear motor comprising: a striker comprising one or more magnets and configured to move linearly within the impactor; a stator comprising a series of coils configured to drive the striker linearly within the impactor; a first set of one or more magnetic field sensors at a first end of the stator, the first set of one or more magnetic field sensors being configured to produce a first set of one or more magnetic field measurements based on a position of the striker within the impactor; and a second set of one or more magnetic sensors at a second end of the stator, the second end being opposite to the first end, the second set of one or more magnetic field sensors being configured to produce a second set of one or more magnetic field measurements based on the position of the striker within the impactor. According to an embodiment: the striker comprises a series of magnets spaced along a length of the striker, wherein the magnets are configured to generate a combined magnetic field that alternates sinusoidally over a wavelength, A, based on relative position along the striker; and one or both of the first and second sets of one or more magnetic field sensors comprise two sensors, wherein a pitch between the sensors in the one or both of the first and second sets of one or more magnetic field sensors is substantially equal to (n + 1 / 4)2, wherein n is an integer greater than or equal to zero. According to an embodiment the striker comprises a series of magnets spaced along a length of the striker, wherein the magnets are configured to generate a combined magnetic field that alternates sinusoidally over a wavelength, A, based on relative position along the striker; and one or both of the first and second sets of one or more magnetic field sensors comprise three sensors, wherein, in the one or both of the first and second sets of one or more magnetic field sensors, a pitch between a first sensor and a second sensor is substantially equal to (n, + 1 / 3)2 and a pitch between the first sensor and a third sensor is substantially equal to (n2 + 2 / 3)2, wherein n± and n2 are integers greater than or equal to zero. According to an embodiment the surgical impactor further comprises: a first anvil located on a first side of the striker; a second anvil located on a second side of the striker, opposite to the first side; and a connection connecting the first anvil to the second anvil. The striker is configured to be driven in a first direction to impact the first anvil to convey an impact in the first direction, and to be driven in a second direction, opposite to the first direction, to impact the second anvil to convey an impact in the second direction. According to an embodiment the first set of one or more magnetic field sensors is located between the first end of the stator and the first anvil and the second set of one or more magnetic field sensors is located between the second end of the stator and the second anvil. According to an embodiment the first and second sets of one or more magnetic field sensors are located outside of the stator. According to an embodiment the length of the striker in the direction of movement of the striker is greater than the length of the stator in the direction of movement of the striker. According to an embodiment there is provided a surgical impactor as described above further comprising a system as described above configured to control the surgical impactor. BRIEF DESCRIPTION OF THE DRAWINGS Arrangements of the present invention will be understood and appreciated more fully from the following detailed description, made by way of example only and taken in conjunction with drawings in which: FIG. 1A shows a surgical impactor fitted with a broaching tool according to an embodiment; FIG. 1B shows a surgical impactor fitted with an acetabular cup implant according to an embodiment; FIG. 2 shows a cross-section of a surgical impactor according to an embodiment; FIG. 3 shows a zoomed in cross-section of the motor according to an embodiment; FIG. 4 shows a zoomed in cross-section of a first set of sensors according to an embodiment; FIG. 5 shows a zoomed in cross-section of a second set of sensors according to an embodiment; FIG. 6 shows a perspective view of an underside of the stator assembly according to an embodiment; FIG. 7 is a circuit diagram showing a connection arrangement between coils according to an embodiment; FIG. 8 shows a plot of the summed magnetic field strength values from the first set of magnetic field sensors and the summed magnetic field values from the second set of magnetic field sensors according to an embodiment; FIG. 9 shows a plot of the summed absolute magnetic field strength values from a front set of magnetic field sensors and the summed absolute magnetic field values from a rear set of magnetic field sensors according to an embodiment; FIG. 10 shows a plot of the summed squared magnetic field strength values from a front set of magnetic field sensors and the summed squared magnetic field values from a rear set of magnetic field sensors according to an embodiment; FIG. 11 shows a system for determining a strongest of a first and second set of magnetic field measurements according to an embodiment; FIG. 12 shows a further system for determining a strongest of a first and second set of magnetic field measurements according to an embodiment; and FIG. 13 shows a block diagram of a control system for controlling a surgical impactor according to an embodiment. DETAILED DESCRIPTION Embodiments provide a surgical impactor with an improved sensor design that allows the size of the striker and stator to be reduced. Further embodiments include a method and system for controlling which of a plurality of sets of one or more magnetic sensors are used to control the motor. FIG. 1A shows a surgical impactor 10 fitted with a broaching tool 150 according to an embodiment. FIG. 1B shows a surgical impactor 10 fitted with an acetabular cup 160 according to an embodiment. The surgical impactor 10 comprises a handle 110, a body 120 and an attachment coupling 130. The body 120 and handle 110 may collectively form a housing for the surgical impactor 10. The body 120 houses an anvil 170 and a motor comprising a striker 180 and a stator 200 (see FIG. 2). The stator 200 is configured to drive the striker 180 to impact the anvil 170. The anvil 170 is connected to the attachment coupling 130 such that impact force is transmitted from the anvil 170 to the attachment coupling 130. The attachment coupling 130 is configured to receive and secure an attachment. For instance, as shown in FIG. 1A, a broaching tool 150 may be attached to an adaptor 155 which may be received and secured within the attachment coupling 130. Similarly, as shown in FIG. 1B, an acetabular cup 160 may be attached to an adaptor 165 which may be received and secured within the attachment coupling 130. When the anvil 170 is impacted, the impact force is transmitted to the attachment via the attachment coupling 130. The attachment is therefore driven to provide an impact force (e.g. to bone). The broaching tool 150 comprises an elongate body having teeth. The teeth may be in the form of ridges running around the circumference of the elongate body. The broaching tool 150 may be mounted onto an adaptor 155 which may be secured within the attachment coupling 130. The broaching tool 150 may be used for broaching (e.g. broaching a femoral canal). When driven to reciprocate forwards and backwards, the teeth cut into the cavity being broached (e.g. into the bone) and then pull loose material out of the cavity being broached. Alternately broaches may be configured to compact the soft cancellous bone, in which case these tend to be driven predominantly in the forward direction. The acetabular cup 160 comprises an external curved (e.g. hemi-spherical) surface. The external curved surface may be convex. The external curved surface may be roughened. A socket on an opposite side to the curved external surface may be configured to receive a femoral implant consisting of a stem and a head. The acetabular cup 160 may be mounted to an adaptor 165 which may be secured in the attachment coupling 130. The acetabular cup 160 may be seated within an acetabulum through repeated impacts, driven by the impactor 10. After seating, the adaptor 165 is removed from the acetabular cup 160 to allow the femoral implant to be fitted within the acetabular cup 160. As noted above, the impactor 10 may also be fitted with various adaptors. The adaptors may act as a means of mounting different attachments (e.g. broaches, acetabular cups, etc.) to the impactor 10. Different adaptors may be used depending on the surgical approach and type of attachment being connected to it. They can also be used on their own (connected to the attachment coupling 130) for impacting other components (e.g. the femoral implant, femoral head, acetabular liner, etc.) and for revision surgery. The surgical impactor 10 may be provided with one or more of a trigger 140, a mode selector 145, and a power selector 147. The trigger 140, mode selector 145 and power selector 147 are all input devices, allowing the user to adjust operating parameters, instruct the device to operate and change operating mode. The operation of the surgical impactor may be controlled by a processor (not shown). The trigger 140 is configured to, in response to being depressed, provides an input to a processor to instruct the surgical impactor to drive the striker 180 to impact the anvil 170. The trigger 140 may be configured to output a varying signal depending on the extent that the trigger is depressed. The processor may be configured to adjust the impact force and / or impact frequency based on the extent that the trigger 140 is depressed (e.g. increase impact force and / or impact frequency with greater depression and decrease impact force and / or impact frequency with lesser depression). The mode selector 145 may be configured to input a mode signal to the processor. The mode signal may define an operating mode of the impactor 10. The mode may be selected from forward mode, backward mode, and reciprocating mode. In one embodiment, the mode selector 145 is a sliding switch, with different positions of the sliding switch defining different modes. For instance, a forward position may define forward mode, a middle position may define reciprocating mode, and a backward position may define reverse mode. In forward mode, the striker 180 may be driven to impart a primary force in a first direction (defined herein as a forward direction). The forward direction may be a direction in which the attachment coupling 130 (and by extension, an adaptor or attachment secured in the attachment coupling 130) moves away from the body 120 (e.g. along a longitudinal axis of the body 120). In reverse mode the striker 180 may be driven to impact a primary force in a second direction opposite to the first direction (defined herein as a reverse direction). The reverse direction may be a direction in which the attachment coupling 130 (and by extension, an adaptor or attachment secured in the attachment coupling 130) moves towards the body 120 (e.g. along the longitudinal axis of the body 120). In reciprocating mode, the striker 180 is driven to alternately drive the striker 180 in the forward direction and in the reverse direction to alternately impact the anvil 170 to impart a first primary force in the forward direction and to impact the anvil 170 to impart a second primary force in the reverse direction. Whilst the forward and reverse modes of operation drive the striker 180 to impart primary forces in the forward and reverse directions respectively, they also may drive the striker 180 to move opposite to these directions (e.g. to prepare to be driven). When preparing to be driven, the striker 180 may impart a secondary force to the anvil 170 in an opposite direction to the primary force in that mode of operation. This secondary force may be smaller (e.g. may have a smaller magnitude) than the primary force. The power selector 147 may be configured to input a power signal to the processor to control an impact force or energy. In the present description, the terms “power”, “force” and “energy” may be used interchangeably when discussing the force of impact. It will be appreciated that these terms are related to each other, such that a larger impact force will impart a larger amount of kinetic energy and that, more powerful impacts (e.g. through driving the striker 180 more powerfully) will impart more force. The power selector 147 may be in the form of a rotating dial, although alternative forms of input device may be used. The power selector 147 may have varying positions related to specific power modes. For instance, one or more power modes may relate to respective levels of impact force. A safety mode may stop the impactor 10 from being driven (e.g. by disabling the driving of the impactor 10 and / or locking the trigger 147). An automatic mode may allow the operating parameters (e.g. impact force) to be adjusted automatically by the processor. The automatic mode may allow the operating parameters to be adjusted by an external device. FIG. 2 shows a cross-section of a surgical impactor 10 according to an embodiment. The surgical impactor 10 comprises an anvil 170 and a linear motor comprising a striker 180 and a stator 200. The anvil 170 (otherwise known as an anvil system) includes a front anvil 172, a rear anvil 174 and a connector 176. The front anvil 170 is connected to (e.g. integrated with) the attachment coupling 130, such that movement of the anvil 170 causes the attachment coupling 130 (otherwise known as a tool holder) to also move. Any adaptor or attachment held within the attachment coupling 130 is therefore driven. For instance, a broaching tool or acetabular cup may be driven via a corresponding adaptor secured within the attachment coupling 130. The front anvil 172 and the rear anvil 174 are positioned on either side of the striker 180. The connector 176 connects the front anvil 172 to the rear anvil 174 such that the anvil 170 forms a single unit. In the present embodiment, the connector 176 is in the form of a shaft that runs along a longitudinal axis of the impactor 10. The striker 180 is mounted within the body 120 such that it may slide forwards and backwards within a cavity within the body 120, between the front anvil 172 and the rear anvil 174. The striker 180 may include bearings (e.g. linear bearings) which are configured to assist the striker 180 in moving longitudinally. The bearings may act on an internal wall within the impactor 10. The internal wall may define a longitudinal cavity within which the striker 180 is housed. The longitudinal cavity may be cylindrical. In one embodiment, the striker 180 is configured to move along the connector 176. For instance, a shaft may pass through the striker 180 through which the connector 176 passes. For instance, the shaft may be centrally located (e.g. along a longitudinal axis of the striker 180). The striker 180 may be cylindrical. Having said this, alternative arrangements are possible, which have a variety of different shapes of striker 180. Similarly, alternative arrangements are possible which may have a different shaped connectors 176 and / or different numbers of connectors 174. The striker 180 is configured to be driven by the stator 200 to impact the anvil 170 to impart a force to the attachment coupling 130. The stator 200 is an electromagnetic driving mechanism. The striker 180 includes magnets 182 (referred to herein as “striker magnets”) configured to be driven by the stator 200. The striker magnets 182 may be annular (ring shaped). The magnets 182 may be magnetised along a direction parallel to the longitudinal axis of the impactor 10 (e.g. parallel to the direction of movement of the striker 180). The polarity of the magnets 182 may alternate down the length of the striker 180. By alternating the polarity, the strength of the magnetic field generated by the magnets 182 is increased. The magnets 182 may have a predefined magnet spacing (a pitch of the magnets 182). In the present context, the term “pitch” may refer to a distance between two objects, measured from a centre of each object (in this case, the distance between the centres of each of two adjacent magnets). The stator 200 may comprise a number of electromagnets. The stator 200 may form part of a rectilinear motor configured to drive the striker 180 forwards and rearwards along the longitudinal axis. A driving force may be provided by the stator 200 through adjusting the current passing through each electromagnet to provide a moving magnetic field. The force provided by the stator 200 may be controlled by adjusting the magnitude and rate of change of the currents. The arrangement of the stator 200 and striker 180 shall be described in more detail below. Processing circuitry (not shown) is configured to control the stator 200. The processing circuitry is configured to control the stator 200 to drive the striker 180 in response to an input from the user through the trigger 140. When the striker 180 is driven in a forward direction, it impacts the front anvil 172 and imparts a force in the forward direction. When the striker 180 is driven in a reverse direction, it impacts the rear anvil 174 and imparts a force in the reverse direction. The forward and reverse directions are opposite to each other and are both parallel to the longitudinal axis. The terms “forward”, “reverse”, “backward” and “rearward” are intended to be relative terms (i.e. relative to the structure of the impactor 10) and are not intended to relate to any orientation of the impactor 10 in use. A centring system may be provided to provide biasing forces to centre the anvil 170 after it has been displaced. The centring system may comprise a number of resilient members (e.g. springs, dampers, etc.). The centring system may be configured to bias the anvil system 170 towards a resting position. For instance, after the anvil system 170 has been driven forward within the body 120, the centring system may be configured to bias the anvil system 170 rearwards, towards the resting position of the anvil system 170. Similarly, after the anvil system 170 has been driven backwards within the body 120, the centring system may be configured to bias the anvil system 170 forwards, towards the resting position of the anvil system. The stator 200 can be controlled to drive the striker 180 to provide a single impact, or a sequence of impacts. Once the user engages the trigger 140, the striker 180 is driven to impact the anvil 170. If the user continues to engage (hold down) the trigger 140, then the processing circuitry controls the stator 200 to reciprocate the striker 180 to provide a continuous sequence of impacts. At the point that the user releases the trigger 140, then the stator 200 disengages and the striker 180 is returned to a resting position. The processing circuitry may be configured to control the stator 200 according to one of three selectable modes of operation: forward mode, reverse mode, and reciprocating mode. The processing circuitry may be configured to switch between these modes of operation based on an input from the user through the mode selector 145. In forward mode, the impactor 10 is configured to provide a primary force in the forward direction. In reverse mode, the impactor 10 is configured to provide a primary force in the rearward direction. In reciprocating mode, the impactor 10 is configured to alternately provide forward and backward primary forces. The strength of the primary force(s) imparted by the impactor 10 can be set through input from the user via the power selector 147. When driven (e.g. in response to an input via the trigger 140), the stator 200 may initiate the striker 180 by moving the striker 180 to an initiation position. In forward mode, the initiation position may be rearward of the resting position of the striker 180. In reverse mode, the initiation position may be forwards of the resting position of the striker 180. In reciprocating mode, the initiation position may depend on whether the first primary impact is to be a forward or rearward impact. In certain embodiments, the first primary impact in reciprocating mode is a forwards impact. Positioning the striker 180 at the initiation position provides the striker 180 with an increased distance over which it can be accelerated towards the anvil 170. The initiation movement may result in the striker 180 contacting the anvil 170. In addition, this initiation movement may result in the striker 180 moving the anvil 170. For instance, when initiating ahead of a forward drive (e.g. in forward mode) the striker 180 may contact the rear anvil 174. When initiating ahead of a rearward drive (e.g. in reverse mode) the striker 180 may contact the front anvil 172. Whilst this initiation movement may provide a force (e.g. a secondary force) on the anvil system (that is opposite to the primary force), this secondary force is less than the primary force that is imparted when the striker 180 is driven. Accordingly, in forward mode, a secondary force may be imparted rearwards before the striker 180 is driven forwards to provide a primary force forwards. Similarly, in reverse mode, a secondary force may be imparted forwards before the striker 180 is driven rearwards to provide a primary force rearwards. After initiation, the striker 180 is driven to impact the anvil 170 to impart the primary force (e.g. via a tool secured in the attachment coupling 130). In forward mode the striker 180 is driven forwards to impact the front anvil 172 and impart a primary force in the forward direction. In reverse mode the striker 180 is driven rearwards to impact the rear anvil 174 and impart a primary force in the rearward direction. After being driven, the stator 200 may return the striker to a resting position. If the striker 180 is being driven again (e.g. if the user continues to engage the trigger 140) during forward mode or reverse mode, then the stator 200 may move the striker 180 back to the initiation position before driving the striker 400 to impact the anvil 170 again. In reciprocating mode, the striker 180 may be moved to the initiation position ahead of the first impact in a similar manner to forward or reverse mode (depending on the direction of the first impact), but after this point the driving process may differ. Instead of using the stator 200 to move the striker 180 to the initiation position, the striker 180 may instead be driven in the opposite direction to produce an impact with a primary force in the opposite direction. For instance, after a forward impact, the striker 180 may be driven to impact the rear anvil 174 to provide a rearward impact. Similarly, after a rearward impact, the striker 180 may be driven to impact the front anvil 172 to provide a forward impact. In this manner, the anvil system may be reciprocated to provide alternating forward and rearward impact forces. As the range of motion of the striker 180 is greater than the distance between the front anvil 172 and rear anvil 174, the striker 180 may be reciprocated to alternately impact the front anvil 172 and rear anvil 174. This process can continue until the user releases the trigger 140. Reciprocating mode can be useful when performing broaching. In a broaching process, a broaching tool (a broach) is driven into a channel and then is pulled out of the channel. As the tool is pulled out of the channel, debris that has been built up within the channel is pulled out of the channel. This frees up space in the channel for the broach to be further inserted into the channel. By removing debris from the channel, radial forces against the walls of the channel are reduced, thereby reducing the chance of breakage of the material being broached. Broaching may be used to increase the size of a channel or cavity within a bone. For instance, in femoral broaching (e.g. in total hip arthroplasty) a channel is formed in the medullary canal to receiving a femoral stem portion of a hip implant. By reciprocating the broaching tool, debris is cleared from the channel, thereby reducing the risk of radial splitting of the femur during the broaching process. Forward mode can be used to hammer an object. For instance, an implant (e.g. an acetabular cup) may be hammered into a channel or fitting through the use of an adaptor. Reverse mode can be used to pull an object. For instance, a pulling attachment may be connected to, secured to, engaged with, or grip an object, and the reverse mode can be used to pull the object (e.g. pull the object out of another object). For instance, reverse mode can be used to pull (or extract) an implant from a channel or fitting in which the implant is fitted. In addition, reverse mode can be used to help pull a tool or attachment out of an object (e.g. when the tool or attachment becomes lodged or stuck within a cavity). The above description refers to a “resting position” of the anvil 170. It will be appreciated that as the front anvil 172 and the rear anvil 174 are separated from each other, they may have different respective resting positions. The use of the term “resting position” this generally refers to a position particular component (e.g. the anvil 170) when not receiving any external forces. It should be noted that position of the anvil 170 may move depending on external forces being applied to the anvil 170. For instance, if the impactor 10 is being pushed against an object, a rearward force may be applied to the anvil 170. This may move the anvil 170 rearwards within the body 120. Similarly, if the impactor 10 is being pulled away from an object (e.g. through the use of a pulling tool), a forward force may be applied to the anvil 170. This may move the anvil 170 forwards within the body 120. Moving the position of the anvil 170 through the application of external forces (e.g. pushing / pulling the impactor 10) can alter the forces applied by the impactor 10, by adjusting the relative positions of the anvil 170, striker 180 and centring system. The impactor 10 may be powered via a battery (e.g. a removable battery) (not shown). The battery may be aseptic or sterile. The battery may be connected to the base of the impactor 10 (e.g. a bottom of the handle 110) and communication between the battery and controller may be enabled through a hard wire connection. The battery may contain a wireless transmitter / receiver, which can enable the impactor 10 to communicate with external devices. Alternatively, the impactor 10 may comprise a wireless transmitter / receiver. The external device could be a battery charger or a user interface such as a tablet, touchscreen, navigation system, robotic system or smart device. These devices could be used to provide real time information about the impactor 10 and anatomical patient data. FIG. 3 shows a zoomed in cross-section of the motor according to an embodiment. FIG. 4 shows a zoomed in cross-section of a first set of sensors according to an embodiment. FIG. 5 shows a zoomed in cross-section of a second set of sensors according to an embodiment. FIG. 6 shows a perspective view of an underside of the stator assembly according to an embodiment. Referring to FIG. 3, the stator 200 comprises a magnetic core 210, wires 230, sensors 300 and coils 250. The magnetic core 210 forms a magnetic circuit. The magnetic core 210 may be a magnetically soft material (e.g. a soft ferromagnetic material, such as magnetically soft iron or magnetically soft steel). The magnetic core 210 may therefore be magnetisable, but may not maintain magnetisation over a prolonged period, once the external magnetic field has been removed. The magnetic core 210 comprises a central region within which the coils 250 are located. The magnetic core 210 may also comprise a first magnetic extension portion 220 and a second magnetic extension portion 225 that extend from opposite ends of the central portion, and act to conduct magnetic flux along the path of motion of the striker 180, beyond the extent of the coils 250. This improves the range of motion of the striker 180 over which the striker 180 is effectively driven. As shown in FIG. 3-6, the magnetic core 210 has a cavity running down its length. The cavity defines the path of motion for the striker 180. In the present embodiment, the cavity is cylindrical (has a circular cross-section); however, alternative shapes are possible (e.g. square or rectangular cross-section). The central region of the magnetic core 210 comprises coil cavities. A coil 250 is located within each coil cavity. Accordingly, whilst the term “magnetic core” is used, the magnetic core 210 may surround the coils 250 (e.g. may extend along side surfaces and an external surface of each coil 250). The magnetic core 210 therefore need not be centrally located within the coils 250. The central portion of the magnetic core 210 may comprise a set of rings that are secured around the coils 250. Each ring may comprise an external wall that extends around the channel, and may comprise or be connected to at least one side wall that protrudes towards the centre of the channel. A coil cavity may be formed between each adjacent pair of side walls. A bearing sleeve 205 is located within the magnetic core 210, and may act as a bearing along which the striker 180 may slide. A single bearing sleeve 205 may extend along the whole length of the cavity, or multiple bearing sleeves 205 may be located along the cavity. For instance, a bearing sleeve 205 may be located within each of the first extension region 220 and the second extension region 225. An internal surface of each bearing sleeve 205 may define a bearing surface along which the striker 180 may slide. Where the bearing sleeve 250 do not extend into the centre of the coils 250, the internal surface of potting compound used to insulate the coils 250 may align with the internal surface of each bearing sleeve 205 to define the cavity through which the striker 180 travels. Each bearing sleeve 205 may be formed of a non-conductive material, such as a non-metallic material (e.g. a ceramic, such as zirconia) to prevent eddy currents being produced within the bearing sleeve 205 which could oppose the driving force of the motor. The striker 180 comprises a series of magnets 182 located down its length. Each magnet 180 may encircle a core 188 of the striker 180. The polarity of the magnets may alternate down the length of the striker 180. Each magnet 180 may be ring shaped (e.g. toroidal). Spacers 184 may be located between adjacent magnets 182. A cover may be provided over the magnets 182 to protect the magnets during use. One or more bearings 186 (e.g. linear bearings) may be located on an external surface of the striker 180 to slide along the bearing sleeve(s) 205. The one or more bearings 186 may be made of a non-conductive material, such as a non-metallic material (e.g. a polymer or ceramic), and may act as an electrical isolator. Alternatively, the one or more bearings 186 may be metallic (e.g. formed of steel). Referring to FIG. 6, a break 214 is provided along the length of the magnetic core 210 (e.g. down the length of each of the first extension region 220, the second extension region 225, and the central portion). The break 214 may be in the form of a channel that runs from a first end (e.g. a front) of the magnetic core 210 to a second end (e.g. a rear) of the magnetic core 210 (e.g. may run longitudinally along a direction parallel to the path of motion of the striker 180). The channel may also run from an outside surface of the magnetic core 210 to an inside surface of the magnetic core 210 (e.g. may run radially along a direction perpendicular to the path of motion of the striker 180). In this manner, the break 210 may form an air gap within the magnetic circuit, which reduces or prevents eddy currents being generated within the magnetic core 210. The air gap may break the magnetic material to prevent the magnetic material forming a complete loop around the longitudinal axis of the device (the path of motion of the striker 180). The term “air gap” refers to a break in the magnetic material (e.g. a region in which magnetic material is not located) within a magnetic circuit (e.g. along a closed loop path through which magnetic flux passes during use); however, the air gap need not be filled with air (e.g. the air gap may contain other components, such as wires and / or sensors, as discussed below). Without the inclusion of the break 214 the motion of the magnets 182 in the striker 180 as the striker 180 is driven can generate eddy currents within the magnetic core 210. Without the break 214, these eddy currents can run circumferentially around the magnetic core 210, producing a force that resists the motion of the striker 180. Due to the fast response time of the impactor, this resistive force can affect the output of the device. By forming a break in the magnetic circuit around a circumference of the magnetic core 210, there is no continuous path within the magnetic core 210 that fully encircles the cavity within which the striker 180 moves. This therefore helps prevent eddy currents generating forces that resist the motion of the striker 180, thereby enabling the striker 180 to be driven more efficiently, to provide a higher impact force. To help reduce the overall size of the motor, wiring 230 to the coils 250 may be provided within the break 214. In addition, or alternatively, one or more sensors 300 may be located within the break 214 for sensing the position of the striker 180 within the cavity. In the present embodiment, both the wiring 230 and the one or more sensors 300 are located within the break 214, although it will be appreciated that only one of these need be located within the break 214 in order to achieve a reduction in the size of the motor. Referring to FIGs. 3 and 6, the wiring 230 runs along a length of the motor (e.g. along a direction parallel to the direction of motion of the striker 180). The wiring 230 includes a plurality of wires 230. One set of wires 230 is provided for each phase of the motor. In the present case, the motor is a three phase motor, meaning that three sets of wires 230 are provided. Each set of wires 230 connects to a different subset of the coils 250, to allow each subset of the coils 250 to be driven in a differently. This enables the different phases of the motor to be driven to generate magnetic fields of different polarities. This enables a linear force to be generated to drive the striker 180 along the path of motion. For instance, in the present embodiment, nine coils 250 are utilized, in three sets of three. One set of wires 230 is provided for each of the three coils within the set. Each set of wires 230 connects a different subset of coils 250 in series to form a chain of coils 250. Every third coil 250 is connected. Each coil 250 may be driven in either a first direction (e.g. clockwise) or a second direction (e.g. anticlockwise). At each time point within the driving cycle, one set of wires 230 may be driven to provide a clockwise current to its respective subset of coils 250, one set of wires may be driven to provide an anticlockwise current to its respective subset of coils 250, and one set of wires 230 may not be driven (e.g. may be grounded or may be floating). As the striker 180 moves along the stator, the polarities of the coils 250 may be switched (commuted) to continue to drive the striker 180. The driving and switching of the coils may be controlled by electronic circuitry within the impactor 10. Each set of wires 230 is insulated from the coils 250 that they are not connected to (e.g. through sheath(s) of insulation). Within each set of coils 250, the winding direction of the coils 250 may be alternated, and the input and output end of the coils 250 may be alternated. For instance, a first coil 250 may have an input at an outer end of the coil 250, and an output at the inner end of the coil 250 and may be wound in a first direction (e.g. clockwise). The next coil 250 in the chain (e.g. a fourth coil 250) may have an input at the inner end of the next coil 250 that is connected to the output (the inner end) of the first coil 250. An output of this next coil 250 may be located at an outer end 250 and this next coil 250 may be wound in a second direction (e.g. anticlockwise). In this manner, the length of the wires 230 between coils 250 may be minimized, whilst ensuring that all coils 250 in the chain are driven to provide a current in the same direction (e.g. clockwise or anticlockwise). FIG. 7 is a circuit diagram showing a connection arrangement between coils 250 according to an embodiment. Nine coils 250 are connected, in three sets of three. Each set includes a plurality of coils connected in series. The winding direction alternates down the set. In addition, the location of the input and output terminals of the coils alternates down the set. The first, fourth and seventh coils are connected in series in a first set. The second, fifth and eight coils are connected in series in a second set. The third, sixth and ninth coils are connected in series in a third set. Each of the first, second and third sets are wound and connected in a similar manner. Accordingly, only the winding and connections in the first set are described below, although it will be appreciated that this description applies equally to the second and third sets. The first set comprises the first, fourth and seventh coils. The fourth coil has an opposite winding direction to the first and seventh coils. The first coil has an input at an inner end of the first coil and an output at the outer end of the first coil. The outer end of the first coil is connected to the outer end of the fourth coil. The fourth coil has an input at the outer end of the fourth coil and an output at the inner end of the fourth coil. The inner end of the fourth coil is connected to the inner end of the seventh coil. The seventh coil has an input at an inner end of the seventh coil and an output at the outer end of the seventh coil. The output of the seventh coil is the output of the first set. The outputs of each set of coils is connected to the outputs of the other sets of coils. Accordingly, depending on how the coils are driven, a positive current can pass through one set of coils and a negative current can pass back through another set of coils. The final set of coils may be switched off to prevent current passing through the third set. By selectively switching the coils, the magnetic field can be varied within the channel to drive the striker 180. Referring to FIGs. 3-6, magnetic sensors 300 are provided for measuring the position of the striker 180 within the cavity. A first set of one or more sensors 310 is provided at a first end (e.g. a front end) of the stator 200 and a second set of one or more sensors 320 is provided at a second end (e.g. a rear end) of the stator 200, opposite to the first end. The sensors 300 may be magnetic sensors (e.g. Hall effect sensors) configured to measure a magnetic field provided by the magnets 182 in the striker 184. By providing sets of one or more sensors 300 on each side of the coils 250 the length of the striker 180 may be reduced relative to its range of motion whilst still allowing continuous sensing of the striker 180 position. By locating the sensors 300 outside of the coils 250, the size of the coils 250 can be reduced, and the sensors 300 are affected less by the magnetic field produced by the coils 250. One or more of the sensors 300 may be provided within the break 214 to reduce the overall size of the impactor 10. The magnets 182 may produce a magnetic field that alternates in polarity down the length of the striker 180. Where the polarity of magnets 182 alternates, the wavelength of the changing magnetic field generated by the magnets 182 (e.g. the peak to peak distance) may be equivalent to the distance between two adjacent magnets 182 of equivalent polarity (twice the magnet pitch). The location of the striker 180 along the cavity may be mapped onto an equivalent rotational position or phase (e.g. an angular position if the motor was adapted to be a rotational motor). This can allow rotational control methods to be applied to control the linear motor. In this instance, as the magnetic field repeats at a given wavelength (or as the pattern of the magnets 182 repeats over this wavelength), then this wavelength may be equated to a 360° change in phase. The sensors within a set may be spaced apart such that they have a 90° phase difference. Where the magnet polarity is alternated, a full 360° “rotation” may be equivalent to twice the magnet pitch. In this case, half the magnet pitch would be equivalent to a 90° phase difference. This can result in magnetic readings that oscillate sinusoidally, with one reading being 90° out of phase from the other. For instance, one sensor in the set may provide a sine signal, and one sensor in the set may provide a cosine signal. In one embodiment, each set of sensors 300 comprises two or more sensors 300. In one particular embodiment, each set of seniors 300 comprises two sensors 300. In this case, each set sensors 300 may have a spacing between adjacent sensors (e.g. a sensor pitch) that is substantially equal to (n + 1 / 4)2, where n is an integer greater than or equal to zero, and 2 is the wavelength of the magnetic field generated by the magnets 182. There the polarity of the magnets 182 alternates, 2 = 2s where s is the magnet 182 spacing (e.g. magnet pitch). This provides a sensor reading from each sensor that is 90° out of phase from the adjacent sensor(s) (assuming that the magnet polarity is alternated). This allows the striker 180 position to be determined based on an angular phase calculated based on the magnet 182 spacing. Based on the above, the motor may be controlled as if it were a rotational motor that was unwound into a straight line. For instance, by mapping the magnetic fields onto a phase (e.g. angle) measurement, the motor can be controlled using field oriented control (FOC). Output voltage drive may be calculated using demand and feedback current. Current readings together with calculated angle may be used to produce torque and flux currents and voltages. Proportional and integral controllers may be used to minimise flux current and control torque current to the desired current. Voltage output together with calculated phase can be used to generate a three phase output that is connected to the motor. The phase 6 of the motor can be determined from first and second magnetic field measurements m1,m2 from first and second sensors 300 in a given set. Raw measurements may be calibrated (e.g. normalized) to produce the first and second measurements m1,m2. The sensors 300 are configured to generate analogue signals (e.g. signals indicating magnetic field strength). The first and second sensors may be either in the first set 310 or the second set 320. The first sensor (e.g. front sensor) is located further forward (further towards the attachment coupling 130) than the second sensor (e.g. rear sensor). As the magnets 182 pass the sensors 300, the first sensor may generate a sine signal, and the second sensor may generate a cosine signal (alternatively, the second sensor may generate a sine signal and the first sensor may generate a cosine signal). The phase can be calculated using the following trigonometric relation: This 6 value gives the phase relative to the position of the set of sensors. It can be converted to a phase position relative to the coils in the stator by applying a known offset based on the position of the set of sensors relative to the coils. Accordingly, as two sets of sensors can determine two different phase values, these values can then be mapped onto a common frame of reference by adding the corresponding offset. The motor can be controlled based on the phase as if it were a brushless rotational motor (e.g. using field oriented control). In addition, the number of peaks can be counted to determine how far down the impactor 10 the striker 180 has travelled. Accordingly, the position can be determined based on the phase and the number of peaks. Alternatively to the above, three digital magnetic (e.g. Hall effect) sensors could be used instead of two analogue Hall effect sensors. In this case, the three sensors would be spaced 120° degrees (or some factor of 120°) apart from each other to create digital sectors for motor position. For instance, a pitch between a first sensor and a second sensor may be substantially equal to (n, + 1 / 3)2 and a pitch between the first sensor and a third sensor may be substantially equal to (n2 + 2 / 3)2, wherein n± and n2 are integers greater than or equal to zero, and 2 is the wavelength of the magnetic field generated by the magnets 182. As the impactor 10 comprises two sets of sensors 310, 320, a method is required to select which of the two sets of sensors 310, 320 to use for controlling the impactor 10 at any one time. As the striker 180 moves along the impactor 10, it will transition from being above one of the sets of sensors 310, 320, to being above the other of the sets of sensors 310, 320. Once the striker 180 is no longer directly above a given set of sensors 310, 320, it may not be possible to determine the location of the striker 180 based on measurements from those sensors 300. There is therefore needed a method for determining when to transition between using the two sets 310, 320. In order to determine a relative strength of the magnetic field values from a given set, the individual measurements from the sensors in that set may be added. Having said this, the sinusoidal nature of the readings makes it difficult to compare these directly summed values. For instance, FIG. 8 shows a plot of the summed magnetic field strength values from a front set of magnetic field sensors and the summed magnetic field values from a rear set of magnetic field sensors according to an embodiment. In this case, the values for each set are normalized (based on maximum and minimum field values that may be predefined or determined during a calibration step) before being added together. As shown in FIG. 8, as the striker 180 moves from the rear to the front of the impactor 10, the combined sensor values from the rear set are initially stronger, and then they attenuate, with the front sensor readings increasing in strength. Given the oscillating positive and negative nature of the readings, it is difficult to compare the summed values to determine the exact point to transition from using the rear measurements to using the front measurements for control. To resolve the above issue, embodiments described herein process the magnetic field measurements. For instance, the magnetic field measurements may be normalized (e.g. rectified). This removes the negative components of the individual measurements, allowing the overall strength of the signals to be determined effectively, thereby enabling effective comparison of the combined signals. FIG. 9 shows a plot of the summed absolute magnetic field strength values from a front set of magnetic field sensors and the summed absolute magnetic field values from a rear set of magnetic field sensors according to an embodiment. When using absolute values, the combined (or accumulated) strength value S from a set of N magnetic field values can be determined through: N i As shown in FIG. 9, by summing absolute values, a clear transition can be identified between the front and rear sensors. Accordingly, the system can select the sensor measurements from the set of sensors that provide that highest combined strength value. Alternatively, the system can select the sensor measurements from the set of sensors that provide a combined strength value above a given threshold. When comparing two strength values, an offset may be added to the strength value of the currently selected sensor set and / or an offset may be subtracted from the strength value of the unselected sensor set to add hysteresis to the system. In addition or alternatively, the magnetic field measurements may be raised to a power greater than one (e.g. may be squared). The power may be an integer that is greater than one. Raising the measurements by a power greater than one can help accentuate changes in the measurements, thereby allowing the transition point for switching between the sensors to be identified more clearly. In certain embodiments, the power is an even integer greater than one. By implementing an even power, negative components are removed. In one embodiment, a more accurate transition can be determined by summing the squared magnetic field measurements: N S = ^mi i This can be considered equivalent to determining, for each set, a corresponding vector for the set by allocating each magnetic field measurement in the set to a corresponding dimension in the vector and calculating a length of the vector. The magnetic field measurements for a set may be mapped onto a corresponding vector. For each vector, the dimensions of the vector may be orthogonal to each other. The set of magnetic sensors can then be selected based on the corresponding lengths of the vectors (e.g. selecting the set of sensors with the largest vector). When comparing two lengths, an offset may be added to the length of the vector for the currently selected sensor set and / or an offset may be subtracted from the length of the vector for the unselected sensor set to add hysteresis to the system. Where the measurements are normalized, the selection can be a selection that picks the set of sensors that have a vector length that is closest to one. The normalization of the measurements can be performed based on predefined calibration factors (e.g. predefined maximum and / or minimum field measurements for each sensor). FIG. 10 shows a plot of the summed squared magnetic field strength values from a front set of magnetic field sensors and the summed squared magnetic field values from a rear set of magnetic field sensors according to an embodiment. As can be seen, by squaring the measurements before summing, the point at which the strength values cross each other becomes more defined, therefore allowing for a more accurate determination of the point to switch between the selected sensor set. When selecting the strongest set of measurements, hysteresis may be added to the selection. For instance, an offset may be added such that the other combined strength value must be at greater than the strength value of the currently selected sensor set by more than the offset before the selection is switched. Alternatively to the above, thresholds may be set for the selection of a given sensor set. A given sensor set may be selected in response to its strength value being greater than a first threshold. The selected sensor set may be deselected in response to the strength value of the selected sensor set falling below a second threshold, which is less than the first threshold. FIG. 11 shows a system for determining a strongest of a first and second set of magnetic field measurements according to an embodiment. FIG. 12 shows a further system for determining a strongest of a first and second set of magnetic field measurements according to an embodiment. Both the system of FIG. 11 and the system of FIG. 12 rectify the input measurements signals, add the rectified signals for each set together, and then compare the summed values to determine the highest summed value. The adding of rectified values for each set may be performed by corresponding summing amplifiers. In FIG. 11, rectification is performed through a comparator, and inverting amplifier and a multiplexer. The comparator is used to select whether an inverted or non-inverted version of the input measurement signal is selected by the multiplexer. A voltage reference may be used to cause the comparator to select whichever of the inverted and non-inverted signal is above the voltage reference (e.g. above zero) at any given time. In FIG. 12, rectification is performed by inputting the measurement signal and a DC offset into a DC offset circuit and rectifier. The DC offset circuit adds a DC offset to the input signal. The rectifier rectifies the DC offset signal. Accordingly, the selection of the sensor set can be determined using analogue circuitry. Alternatively, the selection of the sensor set can be determined digitally (e.g. using one or more processors). FIG. 13 shows a block diagram of a control system 500 for controlling a surgical impactor 10 according to an embodiment. This control system 500 may be integrated within the surgical impactor 10, or may be external to the surgical impactor 10 but communicatively connected to the surgical impactor 10 (e.g. via a wired or wireless connection). The control system 500 includes a bus 510, a processor 520, a memory 530, a persistent storage device 540, an Input / Output (I / O) interface 550. Optionally, the control system 500 may include a network interface (not shown). The bus 510 interconnects the components of the control system 500. The bus 510 may be any circuitry suitable for interconnecting the components of the control system 500. For example, the bus 510 may be an internal bus located on a computer motherboard of the control system 500 or may be a global bus of a system on a chip (SoC). The processor 520 is a processing device configured to perform computer-executable instructions loaded from the memory 530. Prior to and / or during the performance of computer-executable instructions, the processor may load computer-executable instructions over the bus from the memory 530 into one or more caches and / or one or more registers of the processor. The processor 520 may be a central processing unit with a suitable computer architecture, e.g. an x86-64 or ARM architecture. The processor 520 may include or alternatively be specialized hardware adapted for application-specific operations. The memory 530 is configured to store instructions and data for utilization by the processor 520. The memory 530 may be a non-transitory volatile memory device, such as a random access memory (RAM) device. In response to one or more operations by the processor 520, instructions and / or data may be loaded into the memory 530 from the persistent storage device 540 over the bus 510, in preparation for one or more operations by the processor 520 utilising these instructions and / or data. The persistent storage device 540 is a non-transitory non-volatile storage device, such as a flash memory, a solid state disk (SSD), or a hard disk drive (HDD). A non-volatile storage device maintains data stored on the storage device after power has been lost. The persistent storage device 540 may have a significantly greater access latency and lower bandwidth than the memory 530, e.g. it may take significantly longer to read and write data to / from the persistent storage device 540 than to / from the memory 530. However, the persistent storage 540 may have a significantly greater storage capacity than the memory 530. The I / O interface 550 facilitates connections between the control system 500 and other components or devices. The I / O interface 550 may receive signals from a given other component or device (e.g. sensor(s) and / or input device(s)), convert them into a format intelligible by the processor 520 and relay them onto the bus 510 for processing by the processor 520. The I / O interface 550 may also receive signals from the processor 520 and / or data from the memory 530, convert them into a format intelligible by a given external component or device (e.g. a display, audio output (e.g. speaker), light output (e.g. light emitting diode), motor, etc.) and relay them to the given other component or device. The control system 500 may receive sensor data from one or more sensors 560 in the surgical impactor 10. In addition, the control system 500 may receive one or more inputs from one or more input devices 570 of the surgical impactor 10. Based on the sensor data and input(s), the control system 500 is configured to output one or more control signals for controlling the driving mechanism (in this case, a motor 580) of the surgical impactor 10. The inputs from the sensor(s) 560 and / or input device(s) 570 may be received via the I / O interface 550 or may be received directly (e.g. via a direct connection to the processor 520 and / or bus 510). The output to the motor 580 may be sent via the I / O interface 550 or may be sent directly (e.g. via a direct connection from the processor 520 and / or bus 510). A network interface may be provided to facilitate connections between the control system 500 and one or more other computing devices over a network. For example, the network interface may be an Ethernet network interface, a Wi-Fi network interface, or a cellular network interface. The processor 520 may be configured to control the surgical impactor 10 to perform the processes discussed herein. For instance, the processor 520 may be configured to determine a strongest set of one or more magnetic field measurements to use for controlling the motor 580. Whilst particular examples have been provided in the context of hip replacement surgery, embodiments described herein may be utilised in other forms of orthopaedic surgery, such as shoulder replacement, where a glenoid sphere is fitted within a shoulder socket and a humeral stem is fitted within a humeral canal. In addition, whilst various embodiments discuss surgical impactors for use in orthopaedic surgery, the embodiments described herein may be used in other forms of surgery. Given the above, the design and manufacture of implementations of the subject matter and the operations described in this specification can be realized using electronic circuitry (e.g. digital and / or analogue electronic circuitry), or in computer software, firmware, or hardware, including the structures disclosed in this specification and their structural equivalents, or in combinations of one or more of them. For instance, electronic circuitry (such as processors, microprocessors, electronic components, integrated circuits, etc.) may be used to implement one or more of the methods described herein. Portions of the processing described herein can be performed using analogue circuitry and other portions may utilised digital circuitry. For instance, analogue circuitry may be utilised for the selection of a set of sensors and digital circuitry can be used for motor control. Motor control may be managed by a motor controller. Digital processes may be performed by one or more processors, such as one or more field programmable gate arrays (FPGAs) and / or one or more application specific integrated circuits (ASICs). Implementations of the subject matter described in this specification can be realized using one or more computer programs, i.e., one or more modules of computer program instructions, encoded on computer storage medium for execution by, or to control the operation of, data processing apparatus. Alternatively or in addition, the program instructions can be encoded on an artificially-generated propagated signal, e.g., a machine-generated electrical, optical, or electromagnetic signal that is generated to encode information for transmission to suitable receiver apparatus for execution by a data processing apparatus. A computer storage medium can be, or be included in, a computer-readable storage device, a computer-readable storage substrate, a random or serial access memory array or device, or a combination of one or more of them. Moreover, while a computer storage medium is not a propagated signal, a computer storage medium can be a source or destination of computer program instructions encoded in an artificially-generated propagated signal. The computer storage medium can also be, or be included in, one or more separate physical components or media (e.g., multiple CDs, disks, or other 5 storage devices). Accordingly, embodiments may include transitory or non-transitory computer readable media storing computer-executable code. While certain arrangements have been described, the arrangements have been presented by way of example only, and are not intended to limit the scope of protection. 10 The inventive concepts described herein may be implemented in a variety of other forms. In addition, various omissions, substitutions and changes to the specific implementations described herein may be made without departing from the scope of protection defined in the following claims.

Claims

1. A system configured to control a surgical impactor comprising a linear motor, the system comprising circuitry configured to:receive a first set of one or more magnetic field measurements from a first set of one or more magnetic sensors at a first end of a stator of the linear motor;receive a second set of one or more magnetic field measurements from a second set of one or more magnetic sensors at a second end of the stator, the second end being opposite to the first end;select one of the first set of one or more magnetic field measurements and the second set of one or more magnetic field measurements based on a relative strength of the first set of one or more magnetic field measurements and the second set of one or more magnetic field measurements; andissue control signals to control the linear motor based on the selected one of the first set of one or more magnetic measurements and the second set of one or more magnetic measurements.

2. The system of claim 1 wherein selecting one of the first set of one or more magnetic field measurements and the second set of one or more magnetic field measurements comprises:determining a first strength value, indicating a strength of the first set of one or more magnetic field measurements;determining a second strength value, indicating a strength of the second set of one or more magnetic field measurements; andselecting the one of the first set of one or more magnetic field measurements and the second set of one or more magnetic field measurements based on the first strength value and the second strength value.

3. The system of claim 2 wherein selecting the stronger of the first set of one or more magnetic field measurements and the second set of one or more magnetic field measurements comprises selecting the set of one or more magnetic field measurements that has the highest strength value.

4. The system of claim 2 wherein the one of the first set of one or more magnetic field measurements and the second set of one or more magnetic field measurements isselected in response to determining that the corresponding strength value for the one of the first set and second set is above a first threshold.

5. The system of claim 4 wherein the circuitry is further configured to, subsequent to selecting the one of the first set of one or more magnetic field measurements and the second set of one or more magnetic field measurements, select the other of the first set of one or more magnetic field measurements and the second set of one or more magnetic field measurements in response to determining that the strength value for the one of the first set of one or more magnetic field measurements and the second set of one or more magnetic field measurements has dropped below a second threshold that is lower than the first threshold.

6. The system of any of any of claims 2-5, wherein:the first set of one or more magnetic field measurements comprises a first plurality of magnetic field measurements;the second set of one or more magnetic field measurements comprises a second plurality of magnetic field measurements;the circuitry is further configured to, for each magnetic field measurement, calculating a processed magnetic field measurement by performing one or both of rectifying the magnetic field measurement and raising the magnetic field measurement to a power that is greater than one;determining the first strength value comprises calculating an accumulation of the processed magnetic field measurements determined from the first plurality of magnetic field measurements; anddetermining the second strength value comprises calculating an accumulation of the processed magnetic field measurements determined from the first plurality of magnetic field measurements.

7. The system of claim 5, wherein:determining the first strength value comprises calculating a sum the processed magnetic field measurements determined from the first plurality of magnetic field measurements; anddetermining the second strength value comprises calculating a sum of the processed magnetic field measurements determined from the first plurality of magnetic field measurements.

8. The system of any of any of claims 2-5, wherein:the first set of one or more magnetic field measurements comprises a first plurality of normalized magnetic field measurements;the second set of one or more magnetic field measurements comprises a second plurality of normalized magnetic field measurements:selecting the one of the first set and the second set comprises:for each set:determining a corresponding vector for the set by allocating each normalized magnetic field measurement in the set to a corresponding dimension in the vector; andcalculating a length of the vector; andselecting the one of the first set and the second set based on the corresponding lengths of the vectors.

9. The system of claim 8 wherein selecting the one of the first set and the second set based on the corresponding lengths of the vectors comprises selecting the set that has the longest corresponding vector.

10. The system of any preceding claim wherein each magnetic field measurement indicates a relative strength of a magnetic field generated by one or more magnets connected to a striker that is configured to be driven to move linearly through the stator.

11. The system of claim 10, wherein:the circuitry is configured to determine a position of the striker relative to the stator based on the selected set of one or more magnetic field measurements; andthe one or more control signals are issued based on the determined position.

12. The system of claim 10 or claim 11 wherein the one or more control signals are configured to control each of one or more sets of electromagnets within the stator to generate a corresponding magnetic field to drive the striker to move linearly within the stator.

13. A surgical impactor comprising:an electromagnetic linear motor comprising:a striker comprising one or more magnets and configured to move linearly within the impactor;a stator comprising a series of coils configured to drive the striker linearly within the impactor;a first set of one or more magnetic field sensors at a first end of the stator, the first set of one or more magnetic field sensors being configured to produce a first set of one or more magnetic field measurements based on a position of the striker within the impactor; anda second set of one or more magnetic sensors at a second end of the stator, the second end being opposite to the first end, the second set of one or more magnetic field sensors being configured to produce a second set of one or more magnetic field measurements based on the position of the striker within the impactor.

14. The surgical impactor of claim 13 wherein:the striker comprises a series of magnets spaced along a length of the striker, wherein the magnets are configured to generate a combined magnetic field that alternates sinusoidally over a wavelength, A, based on relative position along the striker; andone or both of the first and second sets of one or more magnetic field sensors comprise two sensors, wherein a pitch between the sensors in the one or both of the first and second sets of one or more magnetic field sensors is substantially equal to (n + 1 / 4)2, wherein n is an integer greater than or equal to zero.

15. The surgical impactor of claim 13 wherein:the striker comprises a series of magnets spaced along a length of the striker, wherein the magnets are configured to generate a combined magnetic field that alternates sinusoidally over a wavelength, 2, based on relative position along the striker; andone or both of the first and second sets of one or more magnetic field sensors comprise three sensors, wherein, in the one or both of the first and second sets of one or more magnetic field sensors, a pitch between a first sensor and a second sensor is substantially equal to (n, + 1 / 3)2 and a pitch between the first sensor and a third sensor is substantially equal to (n2 + 2 / 3)2, wherein n± and n2 are integers greater than or equal to zero.

16. The surgical impactor of any of claims 13-15 further comprising:a first anvil located on a first side of the striker;a second anvil located on a second side of the striker, opposite to the first side; anda connection connecting the first anvil to the second anvil,wherein the striker is configured to be driven in a first direction to impact the first anvil to convey an impact in the first direction, and to be driven in a second direction, opposite to the first direction, to impact the second anvil to convey an impact in the second direction.

17. The surgical impactor of claim 16 wherein:the first set of one or more magnetic field sensors is located between the first end of the stator and the first anvil; andthe second set of one or more magnetic field sensors is located between the second end of the stator and the second anvil.

18. The surgical impactor according to any of claims 13-17 wherein the first and second sets of one or more magnetic field sensors are located outside of the stator.

19. The surgical impactor according to any of claims 13-18 wherein the length of the striker in the direction of movement of the striker is greater than the length of the stator in the direction of movement of the striker.

20. The surgical impactor of any of claims 13-19 further comprising a system configured to control the surgical impactor according to any of claims 1-12.

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