Sensor fault detection

Abstract

A power tool comprising: a sensor having, in use at the power tool, a normal operating range; and a controller configured to monitor an output from the sensor and determine a condition of the power tool based on the output from the sensor; wherein, if the output from the sensor is outside of the normal operating range, the controller is configured to further monitor the output from the sensor to determine whether the output from the sensor remains outside the normal operating range for a predefined period and, responsive to that determination, raise a fault in respect of the sensor if the output from the sensor remains outside the normal operating range for the predefined period.

Description

[0001] Sensor Fault Detection

[0002] TECHNICAL FIELD

[0003] This present disclosure relates to a power tool having a controller configured to monitor a signal from a sensor and detect a fault condition.

[0004] BACKGROUND

[0005] The present disclosure is particularly applicable to, but not limited to, power tools for use in construction projects, such as rammers and compactors. Power tools for construction projects can include rammers and plate compactors for compacting and preparing a surface, vibrators for vibrating and removing air pockets from concrete, and drilling equipment. Such tools have conventionally been powered by gasoline or petrol engines, or electrically powered via a cord connected to a generator or a mains electrical outlet. The present disclosure relates in particular to electric power tools, such as mains-powered tools and battery-powered tools.

[0006] Due to improvements in battery technology, battery-operated power tools are now available even for applications that demand high power levels. For example, rammers and compactors are typically large, heavy power tools that require a relatively high power input. Battery-powered rammers and compactors are now available. However, because of the environments in which such tools operate, the motor of a rammer or plate compactor is substantially sealed to prevent ingress of water and dirt. This generally means that there is no active cooling for the motor, which can therefore become hot when running for extended periods. This is a problem because, at high temperatures, an electric motor can become damaged and start to de-magnetise.

[0007] Other components of the tools may overheat under such conditions. For example, electric tools typically have an electronic motor control module comprising an inverter that provides the power to the motor. Particularly where brushless motors are employed, at high currents the inverter can become hot because the inverter switches are required to constantly switch on and off the high currents provided to the motor windings.

[0008] It is therefore important to provide one or more sensors at a power tool arranged to capture the temperature of one of more components of the power tool. This enables a controller arranged to monitor the sensor signals to, in response to detecting a high temperature, take steps to prevent components of the power tool overheating. For example, the controller could be configured to shut down the power tool or reduce the power at which the power tool can operate so as to allow the temperature of the components to return to acceptable levels.

[0009] Typically the motor controller in a power tool is located remotely from the motor. For example, the motor controller may be provided with a user interface and located on the power tool at a point convenient to the user - e.g. on the handle of a rammer - whereas it is typically mechanically advantageous to locate the motor close to the load being driven. For rammers and compactors, it is typically further advantageous to locate the heavy motor and its drive mechanism at a point as low as possible so as to improve the stability of the power tool during use.

[0010] Since the motor controller and the motor are generally located at different positions on a power tool, the two are connected via a wire connection which will carry any signals from temperature sensors located at the motor. The wire connection presents a potential source of failure at the power tool - especially in power tools for construction sites which can see extremely heavy use and experience very high vibrational forces. This is a particular issue for rammers and compactors where the handle at which the motor controller is located moves relative to the motor so the wires in the wire connection are constantly subject to bending forces. Even after taking steps to make the wire connections at a power tool as rugged as possible, the environment in which they operate can lead to failures of the wire connections.

[0011] Failure of a wire connection to the controller may result in a break in a wire carrying signals between a temperature sensor at the motor and the controller, leaving the controller unable to monitor the motor temperature. As a result, the controller cannot protect the motor from overheating. This issue may similarly arise for temperature sensors located at other components of a power tool which are being monitored by the controller.

[0012] Furthermore, the temperature sensors themselves may fail leading to unreliable outputs that prevent the controller from monitoring the temperature of the component.

[0013] SUMMARY

[0014] There is provided a power tool comprising: a sensor having, in use at the power tool, a normal operating range; and a controller configured to monitor an output from the sensor and determine a condition of the power tool based on the output from the sensor; wherein, if the output from the sensor is outside of the normal operating range, the controller is configured to further monitor the output from the sensor to determine whether the output from the sensor remains outside the normal operating range for a predefined period and, responsive to that determination, raise a fault in respect of the sensor if the output from the sensor remains outside the normal operating range for the predefined period.

[0015] The controller may be configured to, in response to raising the fault, permit operation of the power tool to continue and, optionally, limit one or more of the power tool's operating speed, power, torque, current, and operating time until shutdown.

[0016] The power tool may comprise an electric motor and the controller is configured to control one or more of the speed, power, current and torque of the electric motor. The power tool may be battery-powered.

[0017] The controller may be configured to raise the fault in respect of the sensor by one or more of: providing an indication of the fault at a user interface of the power tool; and writing a fault flag indicative of the fault to a memory of the power tool.

[0018] The indication may comprise turning on an indicator light indicative of a sensor fault

[0019] The memory may be a non-volatile memory and the controller may be configured to, at power-up of the power tool, check for the fault flag and, if the fault flag is present, provide an indication of the fault at a user interface of the power tool.

[0020] The sensor may be connected to the controller by a wired connection.

[0021] The sensor may be a passive component configured to receive power over the wired connection from the controller.

[0022] The sensor may exhibit an impedance that varies in dependence on the physical parameter measured by the sensor, the sensor forming part of a potential divider with an impedance provided at the controller.

[0023] The controller may be configured to, when the output from the sensor lies above the normal operating range for the predefined period, raise a fault indicating a broken connection in relation to the sensor.

[0024] The controller may be configured to, when the output from the sensor lies below the normal operating range for the predefined period, raise a fault indicating a short circuit in relation to the sensor.

[0025] The sensor may be a temperature sensor arranged to detect a temperature of a component of the power tool.

[0026] The component may be one or more of: a motor, at battery, and one or more inverters providing an alternating current to a motor of the power tool.

[0027] The sensor may be a thermistor.

[0028] The predefined period may be in the range 2 to 6 seconds.

[0029] The controller may be configured to further monitor the output from the sensor by, over the predefined period, periodically determining whether the output from the sensor remains outside the normal operating range. The controller may be further configured to further monitor the output from the sensor by, in response to each determination that the output from the sensor remains outside the normal operating range, incrementing a first counter.

[0030] The controller may be configured to determine that the output from the sensor remains outside the normal operating range for the predefined period when the first counter reaches a predefined value indicating that the predefined period has elapsed.

[0031] The first counter may be maintained at a non-volatile memory of the power tool.

[0032] The power tool may comprise one or more additional sensors each having, in use at the power tool, a respective normal operating range, and the controller is further configured to, on determining that the output from the sensor remains outside the normal operating range for the predefined period, check whether the output of any of the one or more additional sensors is outside their respective normal operating range, and: responsive to a determination that none of the one or more additional sensors has an output outside its respective normal operating range, proceed to raise the fault condition in respect of the sensor; and optionally, otherwise, extend the predefined period by a predefined extension period and not immediately raise the fault condition or shut down the tool.

[0033] The sensor and the one or more additional sensors may be related sensors configured to measure the same physical parameter.

[0034] The sensor and the one or more additional sensors may be related sensors arranged to measure the same physical parameter of the same component of the power tool.

[0035] The controller may be further configured to clear a fault raised in respect of the sensor if the output of the sensor returns to a restricted normal operating range for a second predefined period of time.

[0036] The controller may be further configured to clear the fault by one or more of: turning off an indication of the fault at a user interface of the power tool; and clearing a fault flag indicative of the fault in a memory of the power tool.

[0037] The restricted normal operating range of the sensor may be the normal operating range of the sensor with the upper bound of the normal operating range lowered by an upper hysteresis value and the lower bound of the normal operating range raised by a lower hysteresis value.

[0038] There is provided a controller for a power tool, the controller being arranged to receive an output from a sensor at the power tool, the controller being configured to: monitor an output from the sensor and determine a condition of the power tool based on the output from the sensor; and if the output from the sensor is outside of its normal operating range, further monitor the output from the sensor to determine whether the output from the sensor remains outside its normal operating range for a predefined period and, responsive to that determination, raise a fault in respect of the sensor if the output from the sensor remains outside the normal operating range for the predefined period.

[0039] There is provided a method of detecting a sensor fault at a power tool comprising: receiving an output from a sensor at the power tool, the sensor having a normal operating range when in use at the power tool; monitoring an output from the sensor; if the output from the sensor is outside of the normal operating range, determining whether the output from the sensor remains outside the normal operating range for a predefined period; and responsive to that determination, raising a fault in respect of the sensor if the output from the sensor remains outside the normal operating range for the predefined period.

[0040] There is provided a power tool comprising: a sensor having, in use at the power tool, a normal operating range; and a controller configured to: monitor an output from the sensor; based on the output from the sensor, determine a fault in respect of the sensor; and in response to the determination of the fault in respect of the sensor, provide a user warning indication and allow the power tool to operate.

[0041] The controller may be configured to determine a fault in respect of the sensor if the output from the sensor is outside of the normal operating range for a predefined period.

[0042] The controller may be configured to, in response to determining the fault, permit operation of the power tool to continue.

[0043] The controller may be further configured to limit one or more of its operating parameters such as speed, power, torque, current, and / or operating time until shutdown.

[0044] The power tool may comprise an electric motor and the controller is configured to control one or more of the speed, power, current and torque of the electric motor.

[0045] The sensor may be a temperature sensor arranged to detect a temperature of a component of the power tool.

[0046] The component may be one or more of: a motor, at battery, and one or more inverters providing an alternating current to a motor of the power tool.

[0047] There is provided a power tool comprising: a first sensor; and a controller configured to monitor an output from the first sensor and determine a condition of the power tool based on the output from the first sensor; wherein the controller is configured to monitor the output from the first sensor to determine whether the output from the first sensor is substantially constant for a predefined period and, responsive to that determination, raise a fault in respect of the first sensor if the output from the first sensor remains substantially constant for the predefined period.

[0048] The controller may be configured to: monitor an output from the second sensor; determine whether said substantially constant output from the first sensor is abnormal based on the monitored output from the second sensor; and proceed to raise the fault condition in respect of the first sensor if said abnormal determination is made.

[0049] The controller may be further configured to: determine an operating state of the power tool; determine whether said substantially constant output from the first sensor is abnormal based on the operating state of the power tool; and proceed to raise the fault condition in respect of the first sensor if said abnormal determination is made.

[0050] The power tool may be a soil compaction apparatus such as a rammer or a plate compactor.

[0051] BRIEF DESCRIPTION OF THE DRAWINGS

[0052] The present invention is described by way of example with reference to the accompanying drawings. In the drawings: Figure 1 shows a battery-powered rammer;

[0053] Figure 2 shows a battery-powered plate compactor;

[0054] Figure 3 is a sectional view of a drive mechanism for use in the rammer of Figure 1 ;

[0055] Figure 4 is a schematic view of the mechanism of the rammer of Figure 1 ;

[0056] Figure 5 is a schematic diagram of a control system for an electric power tool, such as the rammer of

[0057] Figure 1 and plate compactor of Figure 2;

[0058] Figure 6 is a schematic diagram of a temperature sensor located at the motor and a controller located remotely to the motor for monitoring the motor temperature;

[0059] Figure 7 illustrates the voltage over time of the signal received by the controller from the temperature sensor which are indicative of fault conditions;

[0060] Figure 8 is a flow chart illustrating an algorithm for detecting a fault condition in the connection to the temperature sensor.

[0061] Figure 9 is a flow chart illustrating an algorithm for detecting a fault condition in the connection to the temperature sensor.

[0062] DETAILED DESCRIPTION

[0063] The following description is presented by way of example to enable a person skilled in the art to make and use the invention. The present invention is not limited to the embodiments described herein and various modifications to the disclosed embodiments will be apparent to those skilled in the art. Figure 1 illustrates a battery-powered rammer 100 to which the principles described herein may be applied so as to detect a fault condition at a connection between a controller and a sensor. The sensor may be a temperature sensor configured to capture the temperature of a component of the rammer. In the examples described herein, the temperature sensor is configured to capture the temperature of the electric motor of the rammer, but in general there could be one or more temperature sensors arranged to capture the temperature of one or more components of the rammer, the connection to each of the one or more temperature sensors being monitored according to the principles described herein.

[0064] A rammer 100 may be used to perform compacting tasks such as compacting soil, asphalt or hardcore. As is described in more detail below, the rammer comprises a reciprocating foot 612 which impacts and flattens the surface to be compacted. The reciprocating foot 612 is driven by an electric motor 104 under the control of a controller, which is located in controller housing 103. The controller is connected to the motor and other components of the power tool by a wired connection 105. A rammer may also be known as a tamper, a jumping jack compactor, or a jumping jack tamper.

[0065] Figure 2 illustrates a battery-powered plate compactor 200 to which the principles described herein may be applied so as to detect a fault condition at a connection between a controller and a sensor. The sensor may be a temperature sensor configured to capture the temperature of a component of the plate compactor. In the examples described herein, the temperature sensor is configured to capture the temperature of the electric motor of the plate compactor, but in general there could be one or more temperature sensors arranged to capture the temperature of one or more components of the plate compactor, the connection to each of the one or more temperature sensors being monitored according to the principles described herein.

[0066] A plate compactor 200 may be used to perform compacting tasks such as compacting soil, asphalt or hardcore. As is described in more detail below, the plate compactor comprises a reciprocating plate 202 which impacts and flattens the surface to be compacted. The reciprocating plate 202 is driven by an electric motor, which is located in motor housing 107 under the control of a controller, which is located in controller housing 103. The controller is connected to the motor and other components of the power tool by a wired connection 105 (not visible in figure 2 but is located between the motor and controller).

[0067] In use, rammers and plate compactors generate high vibrational forces which, over time, can lead to failure of wired connections between components - for example, connection 105 in Figures 1 and 2 between the controller housing 103 and motor 104. In particular, in the rammer shown in Figure 1 , the handle 106 at which the controller 103 is supported moves in use relative to the main body 602 of the rammer comprising the motor 104. This results in repetitive stresses on the connection 105 between the controller and motor as the reciprocating foot 612 is driven up and down by the motor. Similarly, in the plate compactor shown in Figure 2, the controller housing 103 moves in use relative to the motor housing 107of the plate compactor, subjecting the connection 105 (not visible in Figure 2 but located between the controller and motor) to repetitive stresses as the plate 202 vibrates.

[0068] The principles described herein may be applied to detecting a fault at a connection to a sensor in any kind of power tool. The sensor could be any kind of sensor, for example a temperature sensor - in particular, a Negative Temperature Coefficient (NTC) temperature sensor. In the examples provided herein, the principles are described with respect to a rammer having an NTC at its motor for monitoring the temperature of the motor.

[0069] Figure 3 illustrates the reciprocating mechanism 700 of the rammer 100 of Figure 1. The rammer includes a main body 602 and a reciprocating leg portion 610 coupled to a compacting foot 612. The reciprocating mechanism 700 can include a connecting rod 716 which is connected between an eccentric drive wheel 736 which is driven by electric motor 104 (not shown in the figure). The connecting rod 716 can be configured to move a reciprocating piston 732 between a retracted position where a first end 720 of the reciprocating piston 732 is moved towards the main body 602 and an extended position where the first end 720 of the reciprocating piston 732 is moved away from the main body 602.

[0070] The reciprocating piston 732 is arranged to move along a longitudinal axis within a piston cylinder 728. The piston cylinder 728 receives and guides the movement of the reciprocating piston 732. The distal end 718 of the piston cylinder 728, located away from the main body 602, is connected to the compacting foot 612 such that movement of the reciprocating mechanism 700 results in movement of the compacting foot 612.

[0071] The reciprocating piston 732 may be coupled to a spring assembly comprising a first spring 724 and a second spring 726. When the rammer 600 is not operational, the reciprocating mechanism 700 rests in the position as shown in Figure 3. This position is dependent on the weight of the rammer and the balance of the upper and lower springs 724 and 726 of the spring assembly. The first spring 724 acts in opposition to movement of the reciprocating piston 732 away from the compacting foot 612 and towards the retracted position. In this way, the first spring 724 urges the reciprocating piston 732 to towards the compacting foot 612 and the extended position. The second spring 726 acts in opposition to movement of the reciprocating piston 732 towards the compacting foot 612 and towards the extended position. In this way, the second spring 726 urges the reciprocating piston 732 away from the compacting foot 612 and towards the retracted position.

[0072] The up-and-down movement of the reciprocating piston 732 due to rotation of the electric motor driving the eccentric drive wheel 736 causes the first and second springs 724, 726 alternately expand and compress. Accordingly, the compacting foot 612 reciprocates up and down so as to provide a compacting force to the surface to which the rammer is applied. The reciprocating leg portion 610 (comprising the piston cylinder 728 and spring assembly) and the reciprocating foot 612 form a lower mass assembly 750 which reciprocates with respect to an upper mass assembly 760. The upper mass assembly 760 may comprise the components of the rammer 600 supported at the main body 602 (e.g., its motor, eccentric wheel drive, battery pack, etc.). In other words, the lower mass assembly 750 may include those parts of the rammer connected to end 710 of the connecting rod 716 and which therefore move in a reciprocal motion relative to the upper mass assembly 760.

[0073] Figure 4 is a schematic illustration of the operation of the drive mechanism 400 of the rammer. As shown in Figures 3 and 4, the connecting rod 716 is connected between the reciprocating piston 732 and the eccentric drive wheel 736. The eccentric drive wheel 736 is part of a drive mechanism 400 which is shown in cross-section in Figure 4. The drive mechanism 400 is arranged to generate the reciprocating movement of the lower mass assembly 750 with respect to the upper mass assembly 760. The drive mechanism 400 is rotatably coupled to a drive shaft of an electric motor 104. In the example shown in Figure 4, motor 104 is directly coupled to the eccentric drive wheel 736. In other examples, a transmission (e.g., one or more gears) may be provided between the electric motor 104 and the eccentric drive wheel 736.

[0074] Figure 5 is a schematic diagram of a control system 500 for detecting a fault in a sensor connection, for example in the rammer shown in Figure 1 . The control system 500 includes a controller 503 and a sensor 506 - in this example, an NTC temperature sensor located to monitor the temperature of the power tool motor 104. The control system may further comprise one or more of a battery 502 for powering the power tool (e.g. its motor and the controller), a memory 505 (e.g. an EEPROM) as will be discussed further below, and a user interface 501 . The user interface may comprise one or more controls that enable a user to control the power tool - for example, an on / off switch, speed control, and one or more lights and / or displays to indicate the status of the power tool (e.g. battery level, fault conditions, current operating speed). The user interface is preferably provided at a location on the power tool convenient for the user - e.g. at the handle 106 of the rammer.

[0075] The controller 503 may be configured to control the motor 104, e.g. through control of one or more of the current, frequency, voltage, and relative phases of alternating currents provided to the windings of the motor. For example, the controller may comprise or control one more inverters arranged to receive direct current (DC) from the battery and provide suitable alternating currents (AC) to the windings of the motor. Typically the motor 104 of a power tool would be a brushless direct current (BLDC) motor. The controller 503 may be a microprocessor or microcontroller.

[0076] In Figure 5, the sensor is a negative temperature coefficient (NTC) thermistor 506. An NTC sensor provides an output voltage that corresponds to the temperature the sensor is arranged to measure - in this case a temperature of the motor 104. An exemplary arrangement 600 of an NTC sensor for measuring the temperature of motor 104 is illustrated in Figure 6. NTC sensor 506 is provided at an auxiliary circuit board 610 located at the motor 104, with the NTC being suitable positioned to appropriately capture a temperature of the motor during operation. A signal from the NTC sensor is carried over a signal line 608 and received by the controller 503. The signal line may be carried in the connection 105 between the controller and sensor located at the component of the power tool whose temperature is to be monitored (e.g. motor 104). In the example shown, the controller includes a Motor Controller Unit (MCU) 602 which itself includes an analogue-to-digital converter (ADC) 604 that is arranged to sample the voltage on the signal line 608 received from the NTC sensor 506. The MCU and potentially other parts of the controller may then may use of the sensor voltage measured by the ADC.

[0077] As it well known in the art, the voltage on the signal line 608 represents the temperature at the NTC because the resistance presented by the NTC is a function of its temperature. In the present example, a resistor 606 of a known value is provided (e.g. at the controller but in general it could be anywhere suitable) and connected to the NTC sensor by the signal line 608. A voltage Vdd (e.g. 3.3 V) is applied across the arrangement of the NTC 506 and resistor 606 relative to a suitable ground such that the NTC and resistor form a potential divider whose midpoint voltage at the signal line 608 represents the temperature of the NTC.

[0078] The controller may be configured to convert the sampled voltage measured by the ADC into a temperature (e.g. in degrees Celsius) using a known relationship between resistance and temperature for the particular NTC in use. Such a temperature could be made available to the user, e.g. at a display on user interface 501. However, it is not necessary to convert the measured voltage into a temperature in order to perform the fault detection algorithms described herein.

[0079] Figure 7 illustrates the signal voltages received by the controller from the temperature sensor which are indicative of two fault conditions. In normal operation, the voltage on the signal line 608 will be in the central region of the voltage range, as indicated in the figure. In this region, variations in temperature will lead to variations in the resistance of the NTC and hence variations in the voltage dropped over the NTC, which is the voltage at the signal line that is measured by the ADC of the controller.

[0080] A first fault condition may occur when the signal line 608 carrying the signal from the sensor is broken. It will be appreciated from Figure 5 that, under the first fault condition, the voltage measured at the signal wire as sampled by the ADC will be at or close to Vdd. This is represented by the upper region in the figure marked "broken-wire detection”.

[0081] A second fault condition may occur when there is a short circuit between the signal line 608 and ground. For example, as is shown in the arrangement of Figure 6, the ground for the NTC may be exported over a ground line from the controller (e.g. carried in the same connection 105 as the signal line 608) such that, in the event of damage to the signal line 608 it is possible for a short to develop between the signal and ground lines. In some examples, a short may occur between the signal line 608 and a ground at any point of the power tool - e.g. metal parts of the power tool will often be connected to a local ground for the apparatus and in the event of damage to the signal connection a short could develop between the signal line 608 and ground. It will be appreciated from Figure 5 that, under the second fault condition, the voltage measured at the signal wire as sampled by the ADC will be at or close to ground, i.e. 0 V. This is represented by the lower region in the figure marked "short-circuit detection”.

[0082] A third fault condition may occur when the sensor itself develops a fault. Depending on the particular type of sensor, this may result in the voltage on the signal line being above its normal range (e.g. in the "broken-wire detection” region of Figure 7) or below its normal range (e.g. in the "short-circuit detection” region of Figure 7).

[0083] Using the voltage on the signal line it is therefore possible to discriminate between normal operation and the fault conditions described above. However, it is possible for sensor readings to be outside the normal operating range in the absence of a persistent sensor fault - for example, due to transitory extremes in temperature or moisture in the power tool which may resolve during use of the power tool.

[0084] An exemplary algorithm for detecting a fault condition relating to a sensor will now be described with respect to the flow chart shown in Figure 8. The algorithm may be performed by the controller 503 of the power tool, e.g. at its MCU 602 when the sensor is a temperature sensor arranged to monitor the temperature of motor 104. Step 800 represents the measurement of the voltage UNTC at the signal line 608 - e.g. as sampled by an ADC 604 of the controller 503. Checks 802 and 804 are performed on the captured voltage: these checks may be performed in any order. Step 802 checks whether the measured voltage UNTC is less than a minimum voltage UMIN below which the voltage is indicative of a short-circuit fault condition, as shown in Figure 7. For NTC sensors, a typical value for UMIN is around 10% of Vdd. Step 804 checks whether the measured voltage UNTC exceeds a maximum voltage UMAX above which the voltage is indicative of a broken-wire fault condition, as shown in Figure 7. For NTC sensors, a typical value for UMAX is around 95% of Vdd.

[0085] If either of the checks 802 and 804 is satisfied then a fault condition is possible. However, the power tool is not powered down and the controller continues to allow the tool to run to determine how the sensor reading changes over time. If either of the checks 802 and 804 is satisfied, the algorithm takes the left-most branch shown in Figure 8 in order to verify whether a persistent fault exists. Firstly, a check 806 may be performed to check that the measured voltage UNTC is approximately the same as a previously measured voltage UNTC_OLD - e.g. the last voltage measurement, a voltage measurement captured a predefined length of time ago, or an average of two or more of the last voltage measurements. In some examples, the voltage may be checked to ensure that the previously measured voltage is within a predefined absolute or relative value (e.g. within 5%, 2% or 1 % of the measured voltage). Each measured voltage may be stored in a memory 505 (e.g. an EEPROM) accessible to the controller so that it has access to the previously measured voltage UNTC_OLD.

[0086] In some examples, the check at 806 may be performed merely to ensure that the previously measured voltage was indicative of the same fault condition: i.e. if UNTC > UMAX then UNTC_OLD > UMAX; or if UNTC < UMIN then UNTC_OLD < UMIN. If step 806 is satisfied, the algorithm proceeds to step 808; otherwise the fault detection process is aborted and the controller returns to taking its next temperature measurement at the appropriate time - e.g. according to the sampling frequency at which the ADC is operating and / or the frequency at which controller is configured to perform the fault detection algorithm set out in Figure 8 (in some examples, not all the measurements taken by the ADC need be tested according to the fault detection algorithm which might operate at a lower frequency and / or on an average of two or more measurements).

[0087] A first counter is incremented at step 808 and then a check 810 is performed to determine whether or not the counter exceeds a threshold value X. The threshold X represents the time shown in Figure 7 over which fault detection is performed so as to avoid false fault detections. Typically, the threshold X may correspond to a time period of 2 to 6 seconds - this is particularly suitable if the measurements are of motor temperature at a rammer or plate compactor. The sampling period of the sensor may be, for example, 10 ms, in which case a time period of 2 seconds would correspond to a counter threshold X of 200, and a time period of 6 seconds would correspond to a counter threshold X of 600.

[0088] In some examples, a plurality of sensor samples are averaged to form each measurement used according to the principles described herein - e.g. to form each measurement UNTC in Figure 8. The counter may be configured to count the number of "raw” sensor samples or the number of averaged measurements, with the counter threshold X being selected appropriately.

[0089] Until the counter reaches threshold value X, step 810 will not be satisfied and the controller will return to taking the next temperature measurement at step 800. However, the value of the first counter is maintained in memory 505 such that it is available for incrementation the next time step 808 is reached. In some examples, it can be advantageous to allow the counter value to be maintained even after the power tool has been switched off so as to enable the fault detection process to continue when the power tool is next used. The counter value can be maintained through power off by writing the value to a non-volatile memory, such as an EEPROM.

[0090] When a persistent fault is indicated, the algorithm will repeatedly reach step 808 until, when the first counter exceeds threshold X, step 810 is satisfied, causing the algorithm to proceed to step 812 and set a warning indicator ON in the user interface 501 to indicate to the user that there is a fault in the sensor line and / or set a flag in memory 505 to maintain an indication that a persistent fault has been detected. Preferably the flag is held in non-volatile memory (e.g. an EEPROM) such that it is available to the controller when the power tool is next powered up. This enables the controller to, for example, set the warning indicator to ON in the user interface as soon as the power tool is switched on and / or allows the controller to enforce modified behaviour of the power tool after a power cycle without having to again perform fault detection.

[0091] A warning indicator may be provided at the user interface 501 in any suitable manner - e.g. as a sensor fault light (e.g. an LED), as an icon and / or message on a display screen, or a change to a light on the user interface, such as a light indicating the power tool is on turning a different colour (e.g. green to red) and / or flashing (e.g. rather than solid). In some examples, a different warning indicator may be provided for each sensor, group of related sensors (e.g. motor temperature sensors), or type of sensor (e.g. temperature, current, voltage sensors, etc.). In some examples, a single warning indicator may be provided to indicate a fault with any of the sensors monitored at the power tool, such that a fault on any one of the monitored sensors is indicated by displaying the single warning indicator to the user.

[0092] In response to identifying a fault relating to a sensor, the controller may allow the tool to run under normal operating conditions or impose limitations on the operation of the power tool. Since the issue is likely with the sensor itself or the signal line to the sensor, rather than a mechanical problem with the power tool itself, the power tool is preferably permitted to keep operating - although preferably with a warning being presented to the user (e.g. a sensor fault light), as is discussed herein.

[0093] Once a fault has been identified with a sensor, the controller may be configured to limit one or more of operating speed, power, and operating time so as to minimise the risk of damage to the power tool while the controller is unable to receive valid measurements from the faulty sensor. For example, the controller may limit the operating speed of the motor so as to avoid a risk of the motor overheating whilst there is a fault preventing the controller from monitoring the motor temperature. The algorithm can next reset 814 the first counter and return to taking the next sensor measurement. In other examples, the power tool may be powered off in response to a sensor fault.

[0094] The algorithm described herein enables the signal from a sensor to be monitored over a period of time so as to avoid detecting false faults due to transient conditions at the power tool and ensure that the controller can be confident of a fault in the connection to the sensor.

[0095] In normal operating conditions, it can be expected that check steps 802 and 804 both indicate that the sensor line voltage is in its normal operating range and the algorithm then takes the right-hand branch and performs step 816, which checks whether a flag is set in memory 505 to indicate a fault condition with the sensor line being monitored in accordance with the principles of the present disclosure. In normal operating conditions, the flag will not be set in which case the algorithm returns to the starting step 800.

[0096] In some circumstances, a fault which has previously been detected on a signal line may clear. This might be because, for example, a cable (e.g. connector 105) carrying the signal line that had worked loose is reconnected, an exceptionally low or high environmental temperature encountered by the power tool has returned to the operating range for the power tool, or the sensor has been replaced. The right-hand branch in Figure 8 provides an exemplary algorithm to clear a previously-detected fault condition. If a fault has been detected then a flag will be set in memory 505 to indicate a fault condition relating to the sensor and at step 816 the algorithm will proceed to step 818. Step 818 checks whether the previously measured voltage UNTC_OLD lies within the expected operating range, i.e. between UMIN and U MAX. The previously measured voltage UNTC_OLD may be, for example, the last voltage measurement, a voltage measurement captured a predefined length of time ago, or an average of two or more of the last voltage measurements. Each measured voltage may be stored in a memory 505 (e.g. an EEPROM) accessible to the controller so that it has access to the previously measured voltage UNTC_OLD.

[0097] If the previously measured voltage does not lie within the operating range, then the branch is aborted and the controller returns to step 800, ready to take the next measurement. If the previously measured voltage does also lie within the operating range, then this is an indication that the fault may have cleared and a second counter is incremented 820. The second counter is checked at step 822 to determine whether or not its value exceeds a threshold Y. The threshold Y represents a time (e.g. 3 minutes) over which signal line verification is performed in order to clear a fault condition relating to the sensor. It is advantageous to select a value Y which ensures to a high level of probability that the fault has indeed cleared but which is as short as possible so as to allow the tool to return to its normal operating mode as quickly as possible (and, e.g., to allow the user to operate the power tool at higher speeds which the controller may prevent the user from selecting when a fault condition exists).

[0098] Until the second counter reaches threshold value Y, step 822 will not be satisfied and the controller will return to taking the next temperature measurement at step 800. However, the value of the second counter is maintained in memory 505 such that it is available for incrementation the next time step 820 is reached. In some examples, it can be advantageous to allow the second counter value to be maintained even after the power tool has been switched off so as to enable the fault clearing process to continue when the power tool is next used. The second counter value can be maintained through power off by writing the value to a non-volatile memory, such as an EEPROM.

[0099] When the fault has indeed cleared, the algorithm will repeatedly reach step 822 until, when the second counter exceeds threshold Y, step 822 is satisfied, causing the algorithm to proceed to step 824 and turn off the warning indicator in the user interface 501 that indicates to the user that there is a fault in the sensor line and / or clear the flag in memory 505 that maintains an indication that a persistent fault has been detected. The algorithm can next reset 826 the second counter and return to taking the next sensor measurement 800. Thus, by continuing to monitor a signal line once a fault has been logged, the controller may establish over a period of time that a fault has cleared.

[0100] In order to clear a fault condition for a sensor, it can be advantageous to configure the controller to require the measured voltage signal line to exceed the minimum voltage UMIN by a hysteresis value UHYSI and / or to be below the maximum voltage UMAX by a hysteresis value UHYS2. This can help to minimise the likelihood that fault conditions are cleared in error - e.g. due to a faulty sensor happening to give a sequence of readings that lie close to the upper and lower voltage bounds - whilst permitting the sensor to express its full operating range during normal operation of the sensor (i.e. when a fault has not been detected). Thus, in some examples, a fault may be cleared only if the sensor signal has returned to the region of normal operation, outside the hysteresis bands indicated in Figure 7. In some examples, the use of the hysteresis values may be implemented by the addition of a further step in the algorithm shown in Figure 8 between steps 816 and 818: in the event that the fault flag is set, the controller may further check that the measured voltage UNTC is between the hysteresis bounds UMIN + UHYSI and UMAX - UHYS2 (i.e. a more stringent test than that enforced by steps 802 and 804). At step 818, the controller may then check that the previously measured voltage UNTC_OLD is also between the hysteresis bounds UMIN + UHYSI and UMAX - UHYS2.

[0101] It can be advantageous to additionally or alternatively perform an algorithm to check for stagnation of a sensor signal. Stagnation occurs when a sensor signal does not change under conditions when it would be expected that the sensor signal should change because the parameter being measured by the sensor is expected to change (e.g. the motor of a power tool is expected to heat up after start-up or when the load increases). Stagnation can be an indication of a fault with the sensor.

[0102] An exemplary stagnation algorithm for detecting a fault condition relating to a sensor will now be described with respect to the flow chart shown in Figure 9. The algorithm may be performed by the controller 503 of the power tool, e.g. at its MCU 602 when the sensor is a temperature sensor arranged to monitor the temperature of motor 104. Step 900 represents the measurement of the voltage UNTC at the signal line 608 - e.g. as sampled by an ADC 604 of the controller 503. A check 902 is then performed to check whether the measured voltage UNTC is the same as a previously measured voltage UNTC_OLD - e.g. the last voltage measurement, a voltage measurement captured a predefined length of time ago, or an average of two or more of the last voltage measurements. In some examples, the voltage may be checked to ensure that the previously measured voltage is within a predefined absolute or relative value (e.g. within 2% or 1 % of the measured voltage). Each measured voltage may be stored in a memory 505 (e.g. an EEPROM) accessible to the controller so that it has access to the previously measured voltage UNTC_OLD.

[0103] If step 902 is satisfied, the algorithm proceeds to step 904; otherwise the fault detection process is aborted and the controller returns to taking its next temperature measurement at the appropriate time - e.g. according to the sampling frequency at which the ADC is operating and / or the frequency at which controller is configured to perform the fault detection algorithm set out in Figure 9 (in some examples, not all the measurements taken by the ADC need be tested according to the fault detection algorithm which might operate at a lower frequency and / or on an average of two or more measurements).

[0104] A counter is incremented at step 904 and then a check 906 is performed to determine whether or not the counter exceeds a threshold value Y. The threshold Y represents a time over which fault detection is performed so as to avoid false fault detections. Typically, the threshold Y may correspond to a time period of 2 to 6 minutes - this is particularly suitable if the measurements are of motor temperature at a rammer or plate compactor. In other examples, the threshold Y may correspond to a shorter time period - e.g. where the sensor is a pressure sensor in a dust extractor, the time period may be only a few seconds. In some examples, a plurality of sensor samples are averaged to form each measurement used according to the principles described herein - e.g. to form each measurement UNTC in Figure 9. The counter may be configured to count the number of "raw” sensor samples or the number of averaged measurements, with the counter threshold Y being selected appropriately.

[0105] Until the counter reaches threshold value Y, step 906 will not be satisfied and the controller will return to taking the next temperature measurement at step 900. However, the value of the first counter is maintained in memory 505 such that it is available for incrementation the next time step 902 is reached. In some examples, it can be advantageous to allow the counter value to be maintained even after the power tool has been switched off so as to enable the fault detection process to continue when the power tool is next used. The counter value can be maintained through power off by writing the value to a non-volatile memory, such as an EEPROM.

[0106] In some examples, the fault detection algorithms described herein with respect to both figures 8 and 9 may be used together. In some examples, the algorithms may share common steps: e.g. one or more of steps 800, 806, 808, 810, 812, 814 may be one and the same as their respective steps 900-906 and 912-916. The first counter referred to in Figure 8 may be the same as the counter referred to in Figure 9. The threshold X in Figure 8 may be the same as the threshold Y in Figure 9. Thus, both fault detection algorithms may be performed such that faults detected on either path may lead to a fault flag being set.

[0107] When a persistent fault is indicated, the algorithm will repeatedly reach step 906 until, when the counter exceeds threshold Y, step 906 is satisfied, causing the algorithm to proceed to one or more of the check steps 908 and 910. At step 908, a check may be performed to determine whether the counter was started at startup of the power tool - in other words, a determination is made as to whether the sensor output has been constant since the power tool started. Such a check is particularly appropriate where the sensor is a temperature of the power tool, since it can be expected that the temperature of the power tool (e.g. its motor, battery, etc.) will increase as the tool warms up with use. More generally, step 908 may be performed to determine an operating state of the power tool during the counter period - the operating state may be startup as described, or other states such as a diagnosis mode, high speed / power mode, etc.

[0108] At step 910, a check may be performed to determine whether the load on the power tool has varied during the counter period (i.e. since the counter was started). Such a check is particularly appropriate where the sensor is a temperature of the power tool, since it can be expected that the temperature of the power tool will vary with changes in load - e.g. a high load will require a high current which will increase the battery and motor temperatures, whereas a lower load will require a lower current, allowing the battery and motor temperatures to cool. Step 910 may be performed in any suitable manner. In some examples, one or more second sensors are provided to measure the load of the power tool (e.g. a current level from the battery / to the motor, and / or a mechanical load on the motor). A variation in the load measured by the one or more second sensors may be logged and stored (e.g. at an EEPROM) so that it is available for check step 910. Typically, check step 910 may check that the load variation is greater than a predefined minimum (e.g. at least 20%, 30%, 40% or 50% of the possible load range or some average measure of the load over the counter period). The stored load variation data may be cleared at steps 916, 914 when the counter is reset. To ensure that load variations are significant enough that a change in the measured parameter (e.g. temperature) can be expected, it is advantageous to arrange that only load variations which apply for at least some predefined minimum time (e.g. at least 30 seconds, 45 seconds or 1 minute) are logged or otherwise used at step 910. One way of achieving this is to store a measure of the time-weighted deviation in the measured load from some predefined value (e.g. a minimum, maximum or midpoint of the possible load range, or an average of the measure load - for instance, a running mean or median).

[0109] Steps 908 and 910 may be performed in either order and in some examples, only one may be performed. These steps help to distinguish between normal and abnormal operation of a sensor. For example, in some cases a constant temperature could be a legitimate output of a sensor, e.g. when the tool has been operating for some time and the motor temperature has reached a steady-state temperature.

[0110] If the determination at either of the check steps 908 and 910 indicates that the sensor output is unlikely to represent the true physical parameter that the sensor is arranged to measure, then the algorithm proceeds to step 912 at which a warning indicator may be set to ON in the user interface 501 to indicate to the user that there is a fault in the sensor line and / or a flag is set in memory 505 to maintain an indication that a persistent fault has been detected. Preferably the flag is held in non-volatile memory (e.g. an EEPROM) such that it is available to the controller when the power tool is next powered up. This enables the controller to, for example, set the warning indicator to ON in the user interface as soon as the power tool is switched on and / or allows the controller to enforce modified behaviour of the power tool after a power cycle without having to again perform fault detection. The algorithm can next reset 914 the counter and return to taking the next sensor measurement. In other examples, the power tool may be powered off in response to a sensor fault. Note that the fault detection algorithm of Figure 9 trigger a sensor fault even when the sensor readings are within the zone of normal operation indicated in Figure 7.

[0111] If the check steps 908 and 910 suggest that the sensor output might represent the true physical parameter that the sensor is arranged to measure, then the counter is reset 916 and the algorithm returns to taking the next sensor measurement.

[0112] In order to clear a fault flag set in accordance with the algorithm described with respect to Figure 9, the power tool may perform the fault clearance algorithm described above with respect to Figure 8 (e.g. steps 816-826 in Figure 8).

[0113] The fault detection and clearance methods described herein may be generally applied to any kind of sensor at a power tool that provides an electrical output having a normal operating range and one or more regions in which the output is indicative of a fault condition relating to the sensor - e.g. with the sensor itself or with a signal line carrying the output from the sensor. The sensor may be adapted to measure any physical parameter of the power tool: e.g. temperature of a component of the power tool, noise level, vibration level, power consumption, torque, current, voltage, etc. The electrical output may be one or more of, for example, voltage, current, frequency of an alternating current. It will be appreciated that the exemplary algorithms may be applied to sensors other than NTC temperature sensors through appropriate selection of the test parameters. For example, for a sensor that provides an output current indicative of the physical parameter being measured, UMAX and UMIN may be replaced with suitable maximum and / or minimum currents that define the limits of normal operating current range of the sensor.

[0114] In some examples, a plurality of sensors may be provided at the power tool and the controller 503 is configured to operate a fault detection algorithm on the sensor line for each sensor in accordance with the principles described herein. It can be advantageous to configure the controller to, prior to setting a fault flag and / or turning on a warning indicator for a sensor, determine whether the sensor readings from any related sensors are in their normal range (i.e. lie between their respective UMIN and UMAX). If the sensor readings from the related sensors are in their normal range then it is determined that there is a fault with the subject sensor and the algorithm can proceed to set the respective fault flag and / or turn on a warning indicator to indicate to the user that there is a fault with the sensor.

[0115] For example, a power tool may comprise a plurality of temperature sensors - e.g. at the battery, at the motor, at inverters that generate alternating currents to drive the motor, at its casing to measure environmental temperature etc. One or more (optionally all) of the sensors may be monitored according to the principles described herein. All of the plurality of sensors are related in that they measure a temperature of the power tool or its environment. This can allow a fault detected at a sensor to be verified. For example, in exceptionally cold weather, two or more sensors of the power tool may measure a temperature which is below the normal operating range (e.g. below UMIN) and therefore, considered individually, it may be determined in the manner set out in Figure 8 that there is a fault with a sensor. However, by checking whether any other sensors are also reading a value outside the normal operating range, the controller can determine that the reason for the temperature reading outside the normal operating range is likely due to environmental conditions rather than an actual fault with the sensor or sensor line.

[0116] Thus, for example, the exemplary algorithm in Figure 8 may comprise a further step between 810 and 812 of checking whether any other sensors are reporting values outside of their normal operating range. In response to reaching step 810 and then identifying that other sensors are operating outside their normal range, the controller may be configured to increase the threshold X of the first counter and then return to step 800 so as to provide a longer period of time for the power tool to return to its normal operating temperature (e.g. to allow the heat generated in use by the components of the power tool to warm up the power tool). In response to reaching step 810 and then identifying that other sensors are operating within their normal range, the controller proceeds to step 812 in the manner described above. In other examples, the set of related sensors may be: a set of temperature sensors arranged to measure temperatures of the same component or set of components (e.g. a motor, battery, inverters); a set of current sensors each arranged to measure a phase current provided to a motor winding; a set of voltage sensors each arranged to measure an alternating voltage provided to a motor winding; a set of frequency sensors each arranged to measure the frequency of an alternating current provided to a motor winding; a set of torque sensors arranged to measure torque at a plurality of points along the drivetrain of the power tool; a set of noise or vibration sensors arranged at a plurality of locations over the power tool to measure noise / vibration generated by the power tool; one or more pressure sensors in a dust extractor to measure the airflow rate. 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. 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

CLAIMS1 . A power tool comprising: a sensor having, in use at the power tool, a normal operating range; and a controller configured to monitor an output from the sensor and determine a condition of the power tool based on the output from the sensor; wherein, if the output from the sensor is outside of the normal operating range, the controller is configured to further monitor the output from the sensor to determine whether the output from the sensor remains outside the normal operating range for a predefined period and, responsive to that determination, raise a fault in respect of the sensor if the output from the sensor remains outside the normal operating range for the predefined period.

2. The power tool of claim 1, wherein the controller is configured to, in response to raising the fault, permit operation of the power tool to continue and, optionally, limit one or more of the power tool's operating speed, power, torque, current, and operating time until shutdown.

3. The power tool of claim 1 or 2, wherein the power tool comprises an electric motor and the controller is configured to control one or more of the speed, power, current and torque of the electric motor.

4. The power tool of any preceding claim, wherein the power tool is battery-powered.

5. The power tool of any preceding claim, wherein the controller is configured to raise the fault in respect of the sensor by one or more of: providing an indication of the fault at a user interface of the power tool; and writing a fault flag indicative of the fault to a memory of the power tool.

6. The power tool of claim 5, wherein the indication comprises turning on an indicator light indicative of a sensor fault7. The power tool of claim 5, wherein the memory is a non-volatile memory and the controller is configured to, at power-up of the power tool, check for the fault flag and, if the fault flag is present, provide an indication of the fault at a user interface of the power tool.

8. The power tool of any preceding claim, wherein the sensor is connected to the controller by a wired connection.

9. The power tool of claim 8, wherein the sensor is a passive component configured to receive power over the wired connection from the controller.

10. The power tool of claim 9, wherein the sensor exhibits an impedance that varies in dependence on the physical parameter measured by the sensor, the sensor forming part of a potential divider with an impedance provided at the controller.11 . The power tool of any preceding claim, wherein the controller is configured to, when the output from the sensor lies above the normal operating range for the predefined period, raise a fault indicating a broken connection in relation to the sensor.

12. The power tool of any preceding claim, wherein the controller is configured to, when the output from the sensor lies below the normal operating range for the predefined period, raise a fault indicating a short circuit in relation to the sensor.

13. The power tool of any preceding claim, wherein the sensor is a temperature sensor arranged to detect a temperature of a component of the power tool.

14. The power tool of claim 13, wherein the component is one or more of: a motor, at battery, and one or more inverters providing an alternating current to a motor of the power tool.

15. The power tool of any preceding claim, wherein the sensor is a thermistor.

16. The power tool of any preceding claim, wherein the predefined period is in the range 2 to 6 seconds.

17. The power tool of any preceding claim, wherein the controller is configured to further monitor the output from the sensor by, over the predefined period, periodically determining whether the output from the sensor remains outside the normal operating range.

18. The power tool of claim 17, wherein the controller is further configured to further monitor the output from the sensor by, in response to each determination that the output from the sensor remains outside the normal operating range, incrementing a first counter.

19. The power tool of claim 18, wherein the controller is configured to determine that the output from the sensor remains outside the normal operating range for the predefined period when the first counter reaches a predefined value indicating that the predefined period has elapsed.

20. The power tool of claim 18 or 19, wherein the first counter is maintained at a non-volatile memory of the power tool.

21. The power tool of any preceding claim, wherein the power tool comprises one or more additional sensors each having, in use at the power tool, a respective normal operating range, and the controller is further configured to, on determining that the output from the sensor remains outside the normal operating range for the predefined period, check whether the output of any of the one or more additional sensors is outside their respective normal operating range, and: responsive to a determination that none of the one or more additional sensors has an output outside its respective normal operating range, proceed to raise the fault condition in respect of the sensor; and optionally, otherwise, extend the predefined period by a predefined extension period and not immediately raise the fault condition or shut down the tool.

22. The power tool of claim 21 , wherein the sensor and the one or more additional sensors are related sensors configured to measure the same physical parameter.

23. The power tool of claim 21 or 22, wherein the sensor and the one or more additional sensors are related sensors arranged to measure the same physical parameter of the same component of the power tool.

24. The power tool of any preceding claim, wherein the controller is further configured to clear a fault raised in respect of the sensor if the output of the sensor returns to a restricted normal operating range for a second predefined period of time.

25. The power tool of claim 24, wherein the controller is further configured to clear the fault by one or more of: turning off an indication of the fault at a user interface of the power tool; and clearing a fault flag indicative of the fault in a memory of the power tool.

26. The power tool of claim 24 or 25, wherein the restricted normal operating range of the sensor is the normal operating range of the sensor with the upper bound of the normal operating range lowered by an upper hysteresis value and the lower bound of the normal operating range raised by a lower hysteresis value.

27. A controller for a power tool, the controller being arranged to receive an output from a sensor at the power tool, the controller being configured to: monitor an output from the sensor and determine a condition of the power tool based on the output from the sensor; and if the output from the sensor is outside of its normal operating range, further monitor the output from the sensor to determine whether the output from the sensor remains outside its normal operating range for a predefined period and, responsive to that determination, raise a fault in respect of the sensor if the output from the sensor remains outside the normal operating range for the predefined period.

28. A method of detecting a sensor fault at a power tool comprising: receiving an output from a sensor at the power tool, the sensor having a normal operating range when in use at the power tool; monitoring an output from the sensor; if the output from the sensor is outside of the normal operating range, determining whether the output from the sensor remains outside the normal operating range for a predefined period; and responsive to that determination, raising a fault in respect of the sensor if the output from the sensor remains outside the normal operating range for the predefined period.

29. A power tool comprising: a sensor having, in use at the power tool, a normal operating range; and a controller configured to: monitor an output from the sensor; based on the output from the sensor, determine a fault in respect of the sensor; and in response to the determination of the fault in respect of the sensor, provide a user warning indication and allow the power tool to operate.

30. The power tool of claim 29, wherein the controller is configured to determine a fault in respect of the sensor if the output from the sensor is outside of the normal operating range for a predefined period.

31. The power tool of claim 29 or 30, wherein the controller is configured to, in response to determining the fault, permit operation of the power tool to continue.

32. The power tool of claim 31, wherein the controller is further configured to limit one or more of its operating parameters such as speed, power, torque, current, and / or operating time until shutdown.

33. The power tool of any of claims 29 to 32, wherein the power tool comprises an electric motor and the controller is configured to control one or more of the speed, power, current and torque of the electric motor.

34. The power tool of any of claims 29 to 33, wherein the sensor is a temperature sensor arranged to detect a temperature of a component of the power tool.

35. The power tool of claim 34, wherein the component is one or more of: a motor, at battery, and one or more inverters providing an alternating current to a motor of the power tool.

36. A power tool comprising: a first sensor; anda controller configured to monitor an output from the first sensor and determine a condition of the power tool based on the output from the first sensor; wherein the controller is configured to monitor the output from the first sensor to determine whether the output from the first sensor is substantially constant for a predefined period and, responsive to that determination, raise a fault in respect of the first sensor if the output from the first sensor remains substantially constant for the predefined period.

37. The power tool of claim 36, further comprising a second sensor, wherein the controller is configured to: monitor an output from the second sensor; determine whether said substantially constant output from the first sensor is abnormal based on the monitored output from the second sensor; and proceed to raise the fault condition in respect of the first sensor if said abnormal determination is made.

38. The power tool of claim 36 or 37, wherein the controller is further configured to: determine an operating state of the power tool; determine whether said substantially constant output from the first sensor is abnormal based on the operating state of the power tool; and proceed to raise the fault condition in respect of the first sensor if said abnormal determination is made.

39. The power tool of any of claims 1 to 26 or 29 to 38, wherein the power tool is a soil compaction apparatus such as a rammer or a plate compactor.

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