Braking a surgical robotic system

Abstract

A control system 300 for a component of a surgical robotic system, the component comprising a driven joint and a linkage, the driven joint being attached to an configured to alter a position of the li

Description

Background It is known to use robots for assisting and performing surgery. Figure 1 shows a surgical robot arm 102 which comprises a base 101 and is attached to an implement 103. The base supports the rest of the surgical robot arm, and is itself attached rigidly to, for example, the operating theatre floor, the operating theatre ceiling or a trolley. The arm further comprises a plurality of arm segments 105 connected by a plurality of driven joints 104 extending between the base 101 and a terminal arm segment 105t. The surgical robot arm 102 is articulated by the multiple driven joints 104 between the arm segments which are used to position the implement 103 at a desired location and orientation relative to a patient. The implement 103 is attached to the distal end of the terminal arm segment 105t of the robot arm. During an operation, the implement can penetrate the body of a patient at a port so as to access a surgical site. In the example seen in figure 1, the implement is a surgical instrument having a pair of jaws. Other surgical instruments may include a knife, a cauterizer, an electrosurgical instrument and a pair of scissors. In other examples, the implement may be an endoscope. Figure 1 illustrates that the surgical robot arm 102 forms part of a system 100 also including a surgeon command interface 106 (which may be referred to as a "surgeon console") and a control unit 107. The control unit comprises a processor 108 and a memory 109. The memory 109 of the control unit 107 stores in a non-transient way software that is executable by the processor 108 to cause the arm 102 to move as desired. The control unit is coupled to motors (not shown) in the surgical robot arm 102. The motors are coupled to the joints 104 in the robot arm such that driving the motors causes the angles of the joints to change, causing the arm to move. The control unit 107 can therefore effect motion of the robot arm 102 in accordance with outputs generated by execution of the software. The control unit 107 may, or may not, be remote from the arm 102. The control unit 107 may, or may not, be remote from the surgeon command interface 106. The processor 108 of the control unit 107 can execute software to drive the arm in dependence on inputs from a surgeon command interface 106. The surgeon command interface 106 may comprise one or more surgeon input devices whereby a user (a surgeon) can request motion of the implement 103 in a desired way. The control unit 107 is configured to receive an input signal from the surgeon input device 106. The input devices could, for example, be manually operable mechanical input devices such as hand controllers or joysticks, or contactless input devices such as gesture sensors. In the following description, the surgeon input device is at least one hand controller, as illustrated in figure 1. Figure 1 illustrates that the hand controller 106 comprises a base 110, a grip portion 113 and a plurality of linkages 111 connected by a plurality of driven joints 112 extending between the base 110 and the grip portion 113. The surgeon can hold the grip portion 113 and move the hand controller so as to request the desired motion of the implement 103. The software stored in memory 109 of the control unit 107 is configured to respond to input signals received from the surgeon input device and the processor 108 is configured to execute the software to send an output signal to the motors of the surgical robot arm. The control unit 107 is therefore configured to send an output signal to cause the joints 104 of the arm 102 to move in accordance with the input signal received from the surgeon input device 106 to achieve the requested motion of the implement 103. As well as driving motion of the arm 102 in response to inputs from the hand controller 106, it may also be desirable to prevent motion of the robot arm. For example, during surgery it may be desirable to hold a joint of the robot arm in place so as to prevent any accidental movement of the arm if nudged by another object or due to gravity acting on joints of the arm. This will prevent any accidental movement of the surgical implement 103. The ability to hold joints of the robot arm in place can also help to avoid collisions between parts of the robot arm or implement, for example when multiple robot arms are being used for the same surgical procedure. This can help to reduce damage caused to the arm and implement. The present application relates to improved motion prevention of components of a surgical robotic system. Summary There is provided a control system for a component of a surgical robotic system, the component comprising a driven joint and a linkage, the driven joint being attached to and configured to alter a position of the linkage, the system comprising a motor for driving the driven joint of the component of the surgical robotic system, a drive circuit configured to drive the motor; a brake circuit configured to induce braking of the motor; and a trigger circuit configured to, in response to a falling motion under gravity of the linkage, store in the trigger circuit a charge generated by the falling motion; and activate the brake circuit using the stored charge to arrest the falling motion of the linkage by inducing braking of the motor. The brake circuit may be configured to induce passive braking of the motor. The motor may comprise a rotor, a stator and three windings mounted on one of the rotor and the stator for acting on the other of the rotor and the stator. The drive circuit may be configured to drive the motor by dynamically energising the three windings. Activating the brake circuit may cause the brake circuit to connect the three windings so as to form a closed current path and induce braking of the motor. The trigger circuit may be configured to activate the brake circuit in response to the charge stored in the trigger circuit reaching a threshold value. The trigger circuit may be configured to activate the brake circuit at a predetermined time after the start of the falling motion under gravity of the driven joint. The trigger circuit may comprise a capacitor and the stored charge is stored in the capacitor. The trigger circuit may be configured to activate the brake circuit in response to a voltage across the capacitor as a result of the charge stored in the capacitor, reaching a threshold voltage value. The control system may be configured to arrest the falling motion of the linkage before the component engages in a collision. The time taken to charge the capacitor to the threshold value may be less than the time between the start of the falling motion of the linkage and the time at which the component would engage in a collision. The drive circuit may comprise a drive circuit power source; a plurality of voltage-controlled switching elements configured to selectively couple the power source to the three windings of the motor; and a diode connected across each of the plurality of voltage-controlled switching elements, wherein the drive circuit is configured to act as a three-phase rectifier. The system may be configured such that the falling motion under gravity of the linkage drives rotation of the rotor relative to the stator such that the motor generates a back emf. The drive circuit may be configured to rectify the back emf generated by the motor and store the rectified voltage as the charge in the trigger circuit. The control system may be configured to arrest the falling motion of the linkage when the drive-circuit power source of the drive circuit is in a powered-off state. The charge stored in the trigger circuit at the start of the falling motion due to gravity of the driven joint may be zero. The brake circuit may comprise a plurality of voltage-controlled switching elements connected to the three windings of the motor and activating the brake circuit may cause the three switching elements to close to create a closed current path between the three windings. The voltage-controlled switching elements may be MOSFETs. The closed current path may include a common rail and / or a power source. The surgical robotic system may comprise a surgical robot arm and the component may be a part of the surgical robot arm. The surgical robotic system may comprise a set of hand controllers and the component may be a hand controller. There may be provided a surgical robotic system comprising a surgical robot arm, a set of hand controllers for controlling motion of the surgical robot arm and the control system. Brief Description of the Figures Figure 1 illustrates a surgical robotic system. Figure 2 illustrates a surgeon command interface. Figure 3 illustrates a control system according to the present invention. Figure 4 illustrates a trigger circuit. Figure 5 illustrates a timeline of events which occur when the control system prevents motion of a component of the surgical robotic system. Detailed Description As discussed above, it has previously been found to be useful for the control system of a surgical robotic system to be able to prevent motion of joints of a surgical robot arm. It is also desirable to prevent unwanted motion of surgeon input devices of the surgical robotic system. For example, it is desirable to prevent unwanted motion of a hand controller to reduce damage caused to the hand controller due to collisions, for example, with other parts of the surgeon console or the ground. It is therefore advantageous for a control system to prevent unwanted motion of components of a surgical robotic system. Existing control systems with the ability to prevent motion of a certain joint in a surgical robot arm may include a drive circuit for driving the motor at the joint in response to signals input at the surgeon input device during a "driving mode" and a brake circuit which can brake the motor at the joint during a "braking mode". Another situation in which it may be desirable to brake the motor may be as part of a safety mechanism in the event of a fault condition, such as a power supply failure. In the case of a surgical robotic arm, it may be desirable to brake the motor at one or more joints when there is a power failure during a surgical procedure so as to prevent injury if the arm is close to a patient, or even holding a tool that is within a patient, the control system may be configured to store a charge that is built up during the "driving mode" and, in the event of a power failure, to use the stored charge to cause the brake circuit to implement the "braking mode". However, in a scenario in which not enough charge has been built up to power the brake circuit, for example when the system has not operated in the "driving mode" for long enough to build up sufficient charge, when the system has not recently operated in the "driving mode" and / or when the robot arm has been in storage, it is not possible to power the brake circuit and therefore the "braking mode" cannot be implemented. The present invention involves harnessing the kinetic energy resulting from a part of the arm falling under gravity and using the harnessed energy to power a braking mode to prevent any further motion of the arm. The same approach can be used effectively to prevent unwanted motion of other components of the surgical robotic system, for example a hand controller. The example hand controller illustrated in figure 1 comprises a base 110 which can be placed on flat surface, but according to the example of figure 2, the hand controller may form part of a larger surgeon command interface 200, as seen in figure 2. Figure 2 illustrates an example of a surgeon command interface 200 including a hand controller 201 having a grip portion 202 and a series of linkages 203 separated by a series of driven joints 207. Since the joints 207 can be driven, the position of the grip portion 202 of the hand controller can be maintained even if the surgeon stops holding the grip portion. According to the example of figure 2, a pair of hand controllers 201 (only one shown) may be attached to a console unit 204 such that the plurality of jointed linkages 203 extend approximately horizontally from the console unit. Figure 2 shows that the hand controller is placed in a cradle 206 when not in use. According to this example, the grip portion 202 of the hand controller is stored in a cradle when not in use. In other words, for each hand controller, the user removes the grip portion from its cradle before use and returns the grip portion to the cradle when the user has completed instructing the robot arm to perform a desired movement. It has been found that in scenarios in which the surgeon console is not powered, for example due to a power supply failure, or when the console is not connected to a mains power supply e.g. when in storage, the joints 207 cannot be driven and are acted upon by gravity. A hand controller may therefore be easily damaged when not held by its cradle for example by falling and colliding with another part of the surgeon console or the floor. It is therefore desirable for the control system to prevent motion of the joints of the linkages of the hand controller to fix the position of the grip portion of the hand controller when it is not held in a cradle to avoid damage being inflicted. Figure 3 illustrates a control system 300. The control system 300 includes a motor 301, a drive circuit 302, a brake circuit 303 and a trigger circuit 304. The motor 301 comprises a rotor, a stator and a plurality of windings mounted on one of the rotor and the stator for acting on the other of the rotor and stator. The motor 301 in figure 3 includes three windings 305a, 305b, 305c mounted on the rotor of the motor. According to other examples, the motor could have more than three windings. In motors comprising a rotor and a stator, the interaction of the magnetic fields of the rotor and stator cause rotation of the rotor relative to the stator, generating a torque, which in this case is harnessed to drive joints in a surgical robotic component. The motor seen in figure 3 is electronically commutated and therefore could be any suitable motor that requires electronic commutation to function. The motor 301 could be a brushless DC motor, a switched reluctance motor or a stepper motor. Figure 3 shows three connections 307a, 307b, 307c between the drive circuit 302 and the windings 305a, 305b, 305c. In other words, each winding has one of its ends coupled to a respective motor input 307a, 307b, 307c. In other examples, each end of the winding may be coupled to a motor input. The drive circuit 302 comprises a drive circuit power source 306 positioned between an upper supply rail 309 and a lower supply rail 310. The drive circuit 302 further comprises a series of voltage-controlled switching elements 308a-f arranged in a three-phase bridge arrangement. Each of the voltage-controlled switching elements 308a-f includes a respective diode connected across it. Switching elements 308a-c are high-side switching elements for coupling respective motor inputs 307a-c to upper supply rail 309. Switching elements 308d-f are low-side switching elements for coupling respective motor inputs 307a-c to lower supply rail 310. Switching elements 308a-f could be, for example, FETs (e.g. N-MOSFETS), bi-polar transistors, TRIACS, SCRS, IGBTs, SiC based devices etc. The drive circuit 309 may therefore comprise a three-phase MOSFET bridge. Each MOSFET may include a respective body diode. The control system 300 is configured to operate in a driving mode and a braking mode. In the driving mode, the control system 300 uses the drive circuit power source 306 to dynamically energise the windings 305a, 305b, 305c via the motor inputs 307a, 307b, 307c to cause the motor's rotor to rotate relative to its stator (i.e. to electrically commutate the motor). In order to rotate the rotor relative to the stator, a subset of the windings 305a, 305b, 305c may be energised. A winding may be energised, or activated, by passing a current through the winding. Applying current through the winding causes a magnetic field to be generated around the winding. As the rotor rotates to align its field with that of the stator, a next subset of windings is energised to continue to generate a torque. A subset of windings may include one or more windings. Applying current through the windings with a phase offset between each winding in this manner may be referred to as dynamically energising the windings. Dynamically energising the windings means current may be passed through the windings according to a sequence, i.e. the windings are sequentially activated. Thus, by dynamically energising the windings of the rotor, the rotor can be continuously rotated to drive the motor. It will be appreciated by those skilled in the art that when the rotor and stator rotate relative to one another, such rotation of the motor causes a current to be induced in the motor's windings and a small back emf is generated in the motor. The voltage-controlled switch elements 308a-308f including body diodes seen in figure 3 act as a three-phase rectifier to convert the back emf to a DC supply voltage which may act across drive circuit power source 306 when the motor rotates during the driving mode. Energising the windings in this way may be controlled electronically. Those trained in the art will appreciate that there are many well-known methods of controlling the energisation of the rotor winding. Thus, during the driving mode, the drive circuit 302 is configured to drive the motor to change the angle at joints of a surgical component. As previously discussed, the control system could be used to drive the joints 207 of the hand controller and / or the joints 104 of the surgical robot arm. The brake circuit 303 comprises voltage-controlled switching elements 312a, 312b, 312c. The voltage-controlled switching elements 312a, 312b, 312c are separate from the voltage-controlled switching elements 308a-f of the drive circuit 302. Each of the voltage-controlled switching elements 312a, 312b, 312c is connected between one of the motor inputs 307a, 307b, 307c and a common rail, indicated by 313. According to other examples, each switching element may be connected between one of the motor inputs and a brake circuit power source (not shown). In the example seen in figure 3, the switching elements of the brake circuit are MOSFETs but in other examples may take the form of other types of switching elements. As previously mentioned, the control system 300 can operate in a driving mode and a braking mode. During the driving mode in which the windings of the motor are dynamically energised by sequentially opening and closing voltage-controlled switching elements 308a-f of the drive circuit 302 as previously described, the brake circuit 303 is not activated. However, in the braking mode, the brake circuit 303 is activated and each switching element 312a-c is configured to couple a respective motor input 307a-c to the common rail 313. Specifically, during the braking mode, each switching element 312a-c is closed such that a closed current path (or closed circuit) is created between the three windings 305a-c and the common rail 313. The common rail 313 may be grounded. Closing each of the switching elements 312a-c results in the drive circuit 302 being decoupled from the motor 301. By forming a closed circuit which includes all three windings, braking of the rotor relative to the stator may be induced. The closed circuit may include any number of other components so long as the three windings form a closed current path. In the closed current path, the three windings 305a-c are connected together. The closed circuit means that the motor is no longer being electrically commutated by the drive circuit. In the closed current path configuration, the amplitude of the magnetic field created in the windings is zero. Rotation of the rotor relative to the stator in the presence of zero magnetic field generates a back electromotive force (emf) in the motor. Therefore, once the brake circuit forms the closed current path containing the windings, any subsequent rotation of the rotor relative to the stator causes a back emf to be generated in the motor. This causes a current to flow in the windings in a direction so as to generate a magnetic field that retards the relative motion of the rotor in accordance with Lenz's law. In other words, a braking torque acts on the motor, thereby arresting motion of the rotor and braking the motor. This may be known as passive motor braking. Once motion of the rotor has been arrested, so long as the switching elements 312a-c remain closed, the closed current path may further function as a holding brake to resist any further rotation of the rotor caused by an external force. For example, an external force may act on the motor due to the mass of the linkages surrounding the joint which is driven by the motor. This external force would induce a driving torque on the motor. However, the braking torque as a result of the passive motor braking described above will oppose the driving torque ensuring that the motor remains braked. According to a different example of a control system, during the braking mode, some or all of the switching elements 308a-f of the drive circuit 302 are closed to form a closed current path containing the three windings 305a-c. The control system 300 seen in figure 3 further includes a trigger circuit 304. The trigger circuit 304 includes an energy harvesting circuit 314 and a control circuit 311. The trigger circuit 304 is coupled to the drive circuit 302 as indicated by dotted line 315. The energy harvesting circuit 314 includes a capacitor 316. The capacitor 316 may be located elsewhere in the trigger circuit. The capacitor 316 may be a capacitor bank. The capacitor may be isolated from the rest of the control system. The capacitor may be connected to ground. The control circuit 311 is configured to output a control signal to the brake circuit 303 to activate the brake circuit. Specifically, the control circuit is configured to activate the switching elements 312a-c when the control system is in braking mode to couple each motor input 307a-c to the common rail 313. The control circuit 311 may output the control signal to cause the control system 300 to switch from the driving mode to the braking mode. When the control circuit 311 of the trigger circuit 304 outputs a control signal to activate the brake circuit, each of the switching elements 312a-c is closed thereby connecting up each motor winding 305a-c to form a closed current path and induce braking of the rotor relative to the stator. As explained above, during normal operation of the control system 300, the drive circuit is powered by the drive circuit power source 306 to drive the motor. The brake circuit 303 may also be powered by the drive circuit power source 306 or may have its own power source (not shown). The drive circuit power source 306 may also be used to charge the capacitor 316 during the driving mode so that the capacitor's charge can be used as a "backup" power supply in case there is some interruption to the drive circuit power source's ability to provide power to the control system. Thus, in situations in which it is desirable to activate the brake circuit 303 to brake the motor 301, the drive circuit power source 306 and / or capacitor 316 may be used to provide the control circuit with enough power to send a control signal to the brake circuit and provide the brake circuit with enough power to close switching elements 312a-c and keep them closed. However, it has been observed that scenarios exist in which the drive circuit power source 306 is unavailable (for example due to a fault), the brake circuit does not have a dedicated power source and the capacitor 316 has not been sufficiently charged to power the brake circuit during the driving mode. Such a scenario can arise when the control system has only operated in the driving mode for a short period of time such that the capacitor 316 of the trigger circuit has not been charged sufficiently to power the brake circuit. Another scenario is that the surgical robotic system has been in storage and therefore has not operated in the driving mode for some time, meaning that any charge that had built up in capacitor 316 during a previous driving mode has been drained to ground such that there is zero charge stored in the capacitor. As previously explained, it is desirable to be able to brake the motor 301 (by activating the brake circuit 303) in such situations. As will be explained in more detail below, the trigger circuit 304 of the present invention is configured to harvest energy from movement of the motor 301 and interact with the drive circuit 302 to store a charge in the capacitor 316 of the trigger circuit 304. The trigger circuit is configured to use the charge stored in the capacitor 316 to activate the brake circuit 303 and cause the control system to operate in the braking mode. This process is described in more detail with respect to figure 5. Figure 4 illustrates the trigger circuit 304 which includes the energy harvesting circuit 314 and the control circuit 311. The energy harvesting circuit includes a resistor 401, a diode 402, a diode 403 and the capacitor 316. The diode 403 is connected to ground. The diode 403 may be a Zener diode or a Schottky diode. The diode 402 may be a Zener diode or a Schottky diode. The capacitor 316 is connected to ground. The capacitor 316 may be a capacitor bank. The control circuit 311 includes two resistors 404 and 405. The control circuit may include a further resistor connected to ground. The control circuit 311 further includes a MOSFET 407 and a capacitor 406 connected to ground. Figure 5 illustrates a timeline of events 500 relating to an example in which the control system of the present invention is used to prevent motion of a component of the surgical robotic system. As previously described, the component could be a hand controller used for controlling the surgical robot arm or a part of the robot arm itself. In either case, the motor referred to is the motor used to drive at least one of the joints of the component. The timeline 500 includes a series of steps which the control system is configured to perform in order to brake the motor to prevent the component from getting damaged by colliding with another part of the surgical robotic system or the floor. As previously explained, the timeline of events 500 occurs when the drive circuit power source 306 is unavailable, for example when it is switched off or has failed. In other words, when power to the control system 300 has been lost. The surgical robotic system may be configured to detect that power to the control system is lost. For example, a component of the system may be configured to detect that the voltage in the control system (e.g. from the drive circuit power source 306) has dropped below a threshold value. This step may be performed by an FPGA present in the surgical robotic system. Therefore, the control circuit 311 may receive a signal e.g. from the FPGA indicating that power has been lost. Figure 4 illustrates that the control circuit 311 may receive a signal 410. The signal 410 may indicate that the power supply to the control system 300 has been lost. The timeline of events 500 may occur after the signal 410 has been received at the control circuit 311 of the trigger circuit 304.At 501, the component begins to experience a falling motion under gravity i.e. free fall. With respect to the hand controller 201 of the type seen in figure 2, this may be due to the grip portion 202 being knocked and accidentally removed from its cradle 206. When the grip portion 202 begins a falling motion, this results in changes to the positions of linkages 203 and angles of joints 207. It is acknowledged that the falling motion may not have an entirely vertically downward movement path, but will have a component of downward motion to its movement. The falling motion is a motion of the component which is as a result of a force having a downward component due to gravity. As previously described, each of the joints of the component are driven by a motor such as the motor 301 seen in figure 3. Since movement of one of the joints 207 (i.e. a change of joint angle) is usually driven by a rotation of the motor's rotor relative to its stator (in the driving mode), movement of the joint caused by a force that is not imparted by the motor has the effect of causing a rotation of the motor's rotor relative to its stator. In other words, at 502, movement of a driven joint which hasn't been driven by the motor causes the motor to rotate. As will be understood by the skilled person, due to the interaction of the magnetic fields of the rotor and stator when they rotate relative to one another, such rotation of the motor causes a current to be induced in the motor's windings. In other words, at 503, a back emf occurs in the motor and it acts as a generator. The kinetic energy due to the falling motion of the surgical robotic component is therefore converted into electrical energy in the control system. As previously mentioned, the timeline shown 500 in figure 5 will be observed in situations in which the drive circuit power source 306 is switched off or has otherwise failed. Due to the connection between the drive circuit 302 and the motor 301, at 504, the back emf generated in the motor due to the start of the falling motion of the component is fed back into the drive circuit 302. As previously mentioned, the voltage-controlled switch elements 308a-308f including body diodes seen in figure 3 act as a three-phase rectifier to convert the back emf to a DC supply voltage. Therefore when the drive circuit power source 306 is unavailable, the back emf can be held as a DC supply voltage on the DC link between the upper supply rail 309 and the lower supply rail 310 (in place of the drive circuit power source 306). As previously described, figure 3 illustrates a link 315 between the drive circuit 302 and the trigger circuit 304. At 505, the back emf DC supply voltage held in the drive circuit is used to charge capacitor 316 in the energy harvesting circuit of the trigger circuit 304 via the link 315. With respect to the trigger circuit seen in figure 4, the back emf DC supply voltage is directed from the drive circuit (not shown in this figure) towards the trigger circuit 304, entering the energy harvesting circuit 314 at rail 408. The back emf DC supply voltage is used to charge capacitor 316 of the energy harvesting circuit 314. According to some examples, the capacitor 316 may be partially charged before the component starts to exhibit a falling motion. Charging the capacitor 316 may therefore involve charging the capacitor from a completely uncharged state (0 C) or from a partially charged state. When the capacitor 316 is being charged, the voltage-controlled switching elements 312a, 312b, 312c are in an open position and the motor 301 exhibits a high voltage and low current as it spins due to rotation of the motor's rotor relative to its stator caused by a falling motion of a component of the surgical robotic system under gravity. When the capacitor 316 is sufficiently charged (as will be explained in more detail below) and the brake circuit is activated, the voltage-controlled switching elements 312a, 312b, 312c are closed and the rotation of the motor is braked. The motor therefore transitions from a quick rotation to a slow rotation (or no rotation for a complete braking action). At 506, the charge stored in the capacitor 316 is used to activate the brake circuit 303 and cause the control system 300 to operate in the braking mode as previously described. Specifically, the charge 314 is used to power the control circuit 311 to send a control signal 409 to activate the brake circuit 303 to form and maintain the closed current path containing the motor windings and voltage-controlled switching elements 312a, 312b, 312c as previously described. According to the example seen in figure 5, the charge is only used to activate the brake circuit once the capacitor 316 has been charged to a threshold voltage value. As will be appreciated by the skilled person, a voltage across a capacitor is as a result of charge having been stored in that capacitor. The amount of charge stored in a capacitor is a product of the voltage across it V and the capacitor's capacitance C. A threshold voltage value therefore corresponds to a threshold charge stored in the capacitor. Thus, a threshold charge value may be simply calculated from a threshold voltage value, and vice versa. The following explanation can therefore easily be applied to an example in which the threshold value is a threshold charge value. The threshold voltage value may be predetermined as a value which provides enough of a voltage to send the control signal to the brake circuit, close the voltage-controlled switching elements 312a, 312b, 312c and keep them closed to maintain the closed current path. If the threshold value is too low, the voltage due to the amount of charge stored in the capacitor when it reaches the threshold voltage value may not be enough to close the switching elements in the braking circuit in order to form a closed current path with the windings and keep the switching elements closed to maintain the closed current path, meaning that braking of the motor will not occur. According to one example, at least 5V is needed to implement the closed current path in the brake circuit. In this case, the threshold voltage has a value that is greater than 5V. On the other hand, the threshold voltage value may be limited by the maximum voltage that can be handled by the brake circuit and / or other components of the control system. The threshold voltage value may also be limited by the maximum charge that can be stored in the capacitor 316. According to one example, the maximum voltage which can be conducted by the brake circuit is 20V. In this case, the threshold voltage has a value that is less than 20V. The exact limits on the voltage of the threshold voltage value will depend on the exact components e.g. the capacitor selected for use in the trigger circuit. The threshold voltage value may also be selected based on the particular application. The threshold voltage value may be predetermined as a value which enables the brake circuit to be activated before a collision of the component occurs. The threshold value should be low enough that collisions of the component with the ground and / or other elements of the surgical robotic system are prevented. In other words, the threshold voltage value must be low enough that the braking mode is activated quickly enough that the falling motion has not yet finished. If the threshold value is too high, the component may have already completed its fall before the braking mode is activated. On the other hand, as previously mentioned, if the threshold voltage value is too low, the voltage across the capacitor when it reaches the threshold voltage value may not be enough to activate the brake circuit to brake the motor, resulting in the collision. The threshold voltage value may therefore be determined as a voltage value across the capacitor which is large enough to activate the brake circuit as well as being a voltage that the capacitor can reach in an amount of time less than the duration of a falling motion. Depending on the type and location of the component undergoing the falling motion, the falling motioning may have a duration of the order of tens of milliseconds, for example 10ms, 20ms, 30ms, 40ms or 50ms. Thus in order to active the braking mode soon enough to prevent a collision of the component, the threshold voltage value is one which can be reached in less time that the duration of the fall. According to one specific example, the time taken for grip portion 202 of the hand controller to complete a falling motion from cradle 206 is approximately 80ms. It may be preferable for the braking mode to be activated for at least half of the duration of the falling motion. Therefore, in this case, the time taken for the capacitor to reach the threshold voltage value may be between 5ms and 40ms. In other words, the threshold value for the voltage across the capacitor 316 may be chosen as a value which the capacitor can reach in between 5ms and 40ms. The threshold voltage value chosen will therefore depend on the particular component of the surgical robotic system and the duration of a potential falling motion of that component. With reference to figure 4, when the capacitor 316 in the energy harvesting circuit has been charged to a threshold charge associated with the threshold voltage value, this voltage is used to power the control circuit 311. According to one example, when the threshold charge stored in the capacitor 316 is reached, the control circuit 311 is powered such that it sends the control signal 409 to the brake circuit. In other words, the stored charge is used to activate the brake circuit. The control signal 409 instructs the brake circuit to close the voltage-controlled switching elements 312a, 312b, 312c. Activating the brake circuit may therefore involve sending a control signal to the brake circuit. According to one example, the control system holds off sending the control signal 409 to activate the brake circuit during the normal driving mode when the drive circuit power source 306 is available. However, when power is lost (e.g. the voltage from the drive circuit power source drops below a threshold value), and the control circuit 311 receives signal 410 indicative of this power loss, the control signal 409 is sent to the brake circuit. As discussed above, a minimum voltage in capacitor 316 is required in order for the control signal 409 to be sent to the brake circuit and in order for the voltage-controlled switching elements 312a, 312b, 312c to be closed and held closed. According to another example, the trigger circuit may be configured to activate the brake circuit in response to a different trigger. For example, the trigger circuit may be configured to activate the brake circuit after a predetermined amount of time has passed since the start of the falling motion. According to the example in which the motor 301 is used to drive the joints of hand controller 201 and the grip portion 202 of the hand controller 201 is accidentally released from the cradle 206, the time between the grip portion starting its fall and colliding with the ground is approximately 80ms. In this case, it would be desirable to active the brake circuit after a time between 5ms and 40ms. The brake circuit may be activated 10ms after the start of the falling motion. In other words, the grip portion may be allowed to fall for 10ms enabling the capacitor to build up a charge during the falling motion after which the braking mode is activated so that the grip portion's collision with the floor is prevented, thereby avoiding damage to the component. As previously mentioned, the amount of charge Q which can be stored in a capacitor and the associated voltage across the capacitor is dependent on the capacitor's capacitance C. The time taken to charge a capacitor is also dependent on the capacitance C of the capacitor as well as the resistance R of the charging circuit. A capacitor may therefore be selected for use in the control system which has a capacitance C suitable for charging to a desired threshold charge or voltage value and / or for a desired predetermined amount of time. The control system of the present invention may therefore be adapted for use in preventing motion of a variety of different components of a robotic surgical system. As an example, for a component positioned close to the ground (with a falling motion having a short duration), a capacitor may be selected which has a lower capacitance such that the time taken to charge the capacitor and activate the braking mode is shorter and happens quickly enough during the free fall to prevent the component hitting the ground. A capacitor with a larger capacitance C may be used for surgical components located further from the ground. The capacitor may be selected based on the weight of the component to which the system is applying the braking action. For example, for a heavy component for example with a heavy linkage, the capacitor may be chosen as a capacitor with a larger capacitance as a greater voltage may be required to activate the brake circuit and arrest the falling motion of the linkage by inducing braking of the motor. The capacitor may also be selected based on the position of the component of the system, for example how close it positioned to the ground or another component with which it could collide (and therefore how long a potential falling motion could last).Returning to figure 5, at 506, the voltage across the capacitor 316 as a result of the charge stored in it is used to send control signal 409 to activate the brake circuit thereby creating a closed current path containing the windings of the motor. As previously described, when the brake circuit is activated and switches 312a-c are closed, a closed current path is formed between the motor windings 305a-c and the common rail 313. Thus, at step 507, the motor is passively braked and rotation of the joint of the component is prevented. Therefore, the falling motion of the component is arrested. In other words, the free fall is halted. As discussed, the voltage stored in the trigger circuit is used to activate the brake circuit to close the voltage-controlled switching elements 312a, 312b, 312c creating the closed current path containing the motor windings. Once the closed current path has been created, the voltage stored in the trigger circuit is used to keep the voltage-controlled switching elements 312a, 312b, 312c closed and maintain the passive motor braking. As previously described, any rotation of the rotor relative to the stator causes a current to flow in the windings in a direction so as to generate a magnetic field that retards the relative motion of the rotor in accordance with Lenz's law such that a braking torque acts on the motor. At step 508, since the current path containing the motor windings remains closed, the passive braking of the motor continues to prevent further falling motion of the component such that the component does not engage in a collision. As described above, once motion of the rotor has been arrested, the closed current path may further function as a holding brake to resist any further rotation of the rotor caused by an external force. The braking torque resulting from the passive motor braking described above will oppose the driving torque ensuring that the motor remains braked. The braking torque therefore opposes any further torque at the joint resulting from gravitational forces. The examples described herein relate to a control system which includes a brake circuit configured to passively brake the motor by forming a closed current path containing the motor windings. However, it will be understood that the trigger circuit described and its energy harvesting functionality could be used in conjunction with a control system configured to actively brake the motor, for example using a mechanical brake or by another means. It is also to be understood that the robot arm described herein could be for purposes other than surgery. For example, the surgical robot arm could be controlled for manipulating tissue, which is not part of a patient, e.g. for manipulating tissue of a cadaver or of any other object. As another example, the robot arm could control a viewing instrument for viewing inside a manufactured article such as a car engine. The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that aspects of the present 5 invention may consist of any such individual feature or combination of features. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.

Claims

1. A control system for a component of a surgical robotic system, the component comprising a driven joint and a linkage, the driven joint being attached to and configured to alter a position of the linkage, the system comprising:a motor for driving the driven joint of the component of the surgical robotic system,a drive circuit configured to drive the motor;a brake circuit configured to induce braking of the motor; anda trigger circuit configured to:in response to a falling motion under gravity of the linkage, store in the trigger circuit a charge generated by the falling motion; andactivate the brake circuit using the stored charge to arrest the falling motion of the linkage by inducing braking of the motor.

2. The control system of claim 1, wherein the brake circuit is configured to induce passive braking of the motor.

3. The control system of claims 1 or 2, wherein the motor comprises a rotor, a stator and three windings mounted on one of the rotor and the stator for acting on the other of the rotor and the stator.

4. The control system of claim 3, wherein the drive circuit is configured to drive the motor by dynamically energising the three windings.

5. The control system of claims 3 or 4, wherein activating the brake circuit causes the brake circuit to connect the three windings so as to form a closed current path and induce braking of the motor.

6. The control system of any preceding claim, wherein the trigger circuit is configured to activate the brake circuit in response to the charge stored in the trigger circuit reaching a threshold charge value.

7. The control system of any preceding claim, wherein the trigger circuit is configured to activate the brake circuit at a predetermined time after the start of the falling motion under gravity of the driven joint.

8. The control system of any preceding claim, wherein the trigger circuit comprises a capacitor and the stored charge is stored in the capacitor.

9. The control system of claim 8, wherein the trigger circuit is configured to activate the brake circuit in response to a voltage across the capacitor as a result of the charge stored in the capacitor, reaching a threshold voltage value.

10. The control system of any preceding claim, wherein the control system is configured to arrest the falling motion of the linkage before the component engages in a collision.

11. The control system of claim 8 as dependent on claim 6, wherein the time taken to charge the capacitor to the threshold charge value is less than the time between the start of the falling motion of the linkage and the time at which the component would engage in a collision.

12. The control system of any of claims 3 to 11, wherein the drive circuit comprises:a drive circuit power source;a plurality of voltage-controlled switching elements configured to selectively couple the power source to the three windings of the motor; anda diode connected across each of the plurality of voltage-controlled switching elements,wherein the drive circuit is configured to act as a three-phase rectifier.

13. The control system of any of claims 3 to 12, wherein the system is configured such that the falling motion under gravity of the linkage drives rotation of the rotor relative to the stator such that the motor generates a back emf.

14. The control system of claim 13, wherein the drive circuit is configured to rectify the back emf generated by the motor and store the rectified voltage as the charge in the triggercircuit.

15. The control system of any preceding claim, wherein the control system is configured to arrest the falling motion of the linkage when the drive-circuit power source of the drive circuit is in a powered-off state.

16. The control system of any preceding claim, wherein the charge stored in the trigger circuit at the start of the falling motion due to gravity of the driven joint is zero.

17. The control system of any preceding claim, wherein the brake circuit comprises a plurality of voltage-controlled switching elements connected to the three windings of the motor and activating the brake circuit causes the three switching elements to close to create a closed current path between the three windings.

18. The control system of claim 11 or claim 17, wherein the voltage-controlled switching elements are MOSFETs.

19. The control system of claim 5,17 or 18, wherein the closed current path includes a common rail and / or a power source.

20. The control system of any preceding claim, wherein the surgical robotic system comprises a surgical robot arm and the component is a part of the surgical robot arm.

21. The control system of any preceding claim, wherein the surgical robotic system comprises a set of hand controllers and the component is a hand controller.

22. A surgical robotic system comprising a surgical robot arm, a set of hand controllers for controlling motion of the surgical robot arm and the control system of any preceding claim.

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