Power Tool Control Systems

This invention generally relates to control systems, and to related methods and computer program code, for controlling a power tool, in particular a portable power tool. Aspects of the invention also include a cutting tool adapted for use with the system and a controller for use with the system. The techniques we describe have applications in biomedical and do-it-yourself (DIY) fields among others.

Orthopaedic implants are frequently held in place using screws, and this involves drilling holes in bone. However if the bone is exposed to too high a temperature, for example greater than 55 ⁰ C the bone may be damaged. See, for example Journal of Materials Processing Technology 92-93 (1999) 302-308 "Temperature effects in the drilling of human and bovine bone", M.T. Hillery, L. Shuaib. There has been some research into the drill speed, geometry and the like in order to address this, see, for example, J Bone Joint Surg [Br] 2002; 84-B: 137-40, "Drilling efficiency and temperature elevation of three types of Kirschner-wire point", M. Piska, L. Yang, M. Reed, M. Saleh; Journal of Biomechanical Engineering - June 2003 - Volume 125, Issue 3, pp. 305-314 "Drilling in Bone: Modeling Heat Generation and Temperature Distribution", Sean R. H. Davidson and David F. James; and J Bone Joint Surg [Br] Volume 78-B(3). May 1996.357.362, "Orthopaedic Bone Drills - Can they be improved?: Temperature Changes near the Drilling Face", Natali, Colin; Ingle, Paul; Dowell, John. One commercially available drill, the MicroAire SmartDrive (MicroAire Surgical Instruments LLC, VA, USA) includes an irrigation system.

Similar problems occur in the DIY field, when material to be drilled may be temperature sensitive. Examples include some types of wood, plastic, and soft metals. Complex control systems are used when drilling for oil (see, for example, GB2, 333,793) but these are unsuitable for small scale use. Other publications are WO2006/066259 (US2006/159533).

We will describe techniques to address the above and related problems encountered especially in the fields of biomedical applications and DIY.

According to the present invention there is therefore provided a control system for a portable power tool, said power tool including a motor configured to drive a cutting tool, the control system comprising: a cutting tool for said power tool, said cutting tool having, in particular incorporating, a sensor to sense a characteristic of an environment local to said cutting tool; a signal communications system to communicate a signal from said sensor to a controller; and a controller configured to output a control signal for controlling said power tool responsive to said signal from said sensor.

Embodiments of the above described control system are particularly useful for sensing temperature and other environmental characteristics such as the nearby presence of method and/or an electrical field indicating the presence of electric cables. By controlling the power tools responsive to a signal from the sensor the control system can be arranged so as not to exceed a user-defined parameter envelope for the material being cut. For example in response to excessive temperature, or an excessive temperature for an excessive time, one or both of a feed rate and motor speed of the power tool, optionally in conjunction with an irrigation system, may be controlled to return the sensed local environment to within the desired parameter envelope. Incorporating a sensor in the cutting tool facilitates implementation of the control system for a portable power tool, where space is generally limited, as well as facilitating sensing environmental characteristics in the vicinity, of the cutting tool.

In some preferred embodiments the sensor comprises a temperature sensor for example a thermistor or thermocouple incorporated into the cutting tool. Alternatively the cutting tool may have an infrared sensor, which may be incorporated into the power tool (for example for, say, a drill bit, looking down the middle of the drill bit). Thus the sensed characteristic may comprise a temperature in the vicinity of the cutting tool, generally sensed indirectly by sensing the temperature of the tool itself. In some preferred embodiments the controller is configured to determine an estimated temperature of the local environment in response to a temperature sensed by said temperature sensor and to one or more of a past a temperature sensed by said

temperature sensor and data defining a thermal characteristic of the cutting tool. For example the sensed temperature may be adjusted in response to a calculated rate of change of temperature and/or using a weighting dependent on a thermal characteristic of the cutting tool such as a thermal mass and/or conductivity of the cutting tool. The controller may include means for a user to enter data defining this thermal characteristic either directly or, for example, by selecting from amongst one of a number of stored types or definitions of cutting tools.

In some preferred embodiments of the system the controller is able to control the motor speed to increase as well as decrease, dependent on a difference between the sensed temperature and the threshold temperature. More particularly the controller may be configured to increase the motor speed when the sensed temperature (which may be adjusted to more accurately represent the temperature of the local environment, as described above) is less than the threshold temperature. A number of different control algorithms may be employed, for example linear, more particularly proportional, control. Preferably, however, the control system is configured to avoid the sensed temperature increasing significantly above the threshold temperature so that, in embodiments, the threshold temperature acts as a ceiling.

The inventor has recognised that the temperature of the material will in general depend upon both the speed of the cutting tool and the feed pressure. This can be taken into account by dynamically controlling the drill speed, as described above, allowing the drill speed to increase when the temperature is below the threshold, for example when the feed pressure is reduced.

In some preferred embodiments the speed of the motor driving the cutting tool is controlled in response to the sensed temperature or, more preferably, the estimated temperature of the material being cut. For example if the temperature is greater than a threshold temperature the motor speed may be reduced to a fixed level or by a fixed step or in proportion to or dependent upon a difference between the sensed temperature and a threshold. Alternatively the motor may be stopped for a time interval in response to detection that a temperature parameter for the material has been exceeded.

In some preferred embodiments the controller has a limp mode in which the speed of the motor is reduced to a limp speed, the controller being configured to enter this limp mode in response to the sensed temperature being above the threshold temperature. Reducing the speed of the motor to a limp speed rather than stopping the motor entirely helps to prevent the cutting tool from sticking in the material being cut. This is particularly important in biomedical applications when, for example, cutting bone. The limp speed is preferably chosen so that it is sufficiently slow that little or no significant cutting takes place, but it is nonetheless non-zero so that the cutting tool does not become stuck. In embodiments a predetermined limp speed may be applied, for example of less than lOOrpm (revolutions per minute), preferably less than lOrpm, for example around lrpm.

In embodiments the controller is responsive to a combination of sensed temperature and time. This may comprise, for example, a duration for which the temperature exceeds a threshold temperature, or an (approximate) integral of a function of the sensed temperature over time. This is because some materials, for example bone, may be able to withstand brief periods at an elevated temperature. In embodiments the controller may control an irrigation system for the cutting tool additionally (or alternatively) to a motor driving the tool.

In embodiments the sensor comprises a sensing circuit which is substantially electrically isolated from the cutting tool. This allows for a single short to the cutting tool without the sensing circuit being substantially affected. Additionally, isolating the sensing circuit from the cutting tool can, in a wireless system, help to reduce antenna loading. The sensing circuit is preferably incorporated in the cutting tool but may alternatively be associated with or incorporated into the power tool.

In some embodiments of the system, particularly those intended for home/DIY applications, additionally or alternatively to a temperature sensor the sensor may include a metal detector, in which case the controller may warn an operator and/or slow or stop the motor when metal such as a pipe or cable is detected near the cutting tool. Additionally or alternatively an electric field detector may be incorporated within the cutting tool, for similar reasons. Additionally or alternatively the sensor for the sensing

circuit may comprise a pressure or touch sensor, preferably incorporated within the cutting tool although optionally (in particular in the case of a pressure sensor) incorporated into the power tool. Such a sensor enables, for example, controlled breakthrough of a drill into the cavity of a cavity wall. In a still further alternative such a breakthrough may be sensed indirectly, by sensing a change in the electrical characteristics, for example the dielectric constant, of the material being drilled, in particular the vicinity of the drill tip (or cutting tool cutting portion).

In embodiments in which the cutting tool is electrically conductive the sensor may comprise a sensing circuit which is electrically connected to the cutting tool (although in other embodiments the sensing circuit, as mentioned above, may be electrically isolated from the cutting tool). In particular, the sensor may comprise a sensing antenna coupled to the sensing circuit, and the sensing circuit may then use the cutting tool as this antenna. However in some preferred embodiments which employ a wireless signal communications system a transmit antenna of this system is preferably substantially electrically isolated from the cutting tool.

Depending upon the frequency of operation of the signal communications system, if a portion of the cutting tool is being used as an antenna then it is desirable to isolate this portion of the cutting tool, for example the tip of a drill, from the rest of the cutting tool, which is often grounded. This can be done by incorporating an insulating coupling or layer within the cutting tool. Alternatively if this is mechanically undesirable the frequency of the signal communications system can be increased so that one part of the cutting tool, for example a portion of a metal drill held by a chuck, can be grounded whilst the other part still acts as an effective antenna. Thus the frequency of the signal communications system may be greater than 0.5 GHz, 1 GHz, 2 GHz or more. More generally the frequency may be selected so that it is high enough that for a defined minimum length of cutting tool, this length is at least quarter of a wavelength or more preferably half a wavelength. (The defined minimum length may be, for example, 2 cm, 3 cm, 5 cm, 10 cm or more). The skilled person will know that a frequency of approximately 1 GHz corresponds to a wavelength of approximately 30 cm).

In some embodiments the power tool comprises a battery powered power tool. In this case the control signal may comprise a control signal for controlling a DC motor or where the control system is intended for use with a power tool in which access to the DC power is difficult, the control signal output by the controller may simply comprise a visible and/or audible warning signal to a user of the power tool. Additionally or alternatively the control system may be configured for use with a main powered power tool. In this latter case phase control of the mains power may be employed or, for a power tool in which phase control is already used, for example to implement variable speed and/or soft start, the control system may control the amplitude of a mains power supply to the power tool. Still further additionally or alternatively, particularly in the context of orthopaedic drills, the power tool may comprise an air powered power tool (the power tool in this case comprising the air powered head). In this case the controller may include an air power control system, such as an electrically controllable pneumatic valve. The skilled person will understand that in these embodiments the controller output may comprise an internal output to a power control system.

In some preferred embodiments the signal communications system comprises a wireless signal communications system. This facilitates operation with a detachable/interchangeable cutting tool. The wireless communications system may employ infrared communication systems, but preferably radio frequency communications, for example in an ISM (Industrial Scientific and Medical) band are employed. Thus the cutting tool may incorporate a transmitter of this system and preferably, for an RF system, the transmitter antenna is substantially electrically isolated from the cutting tool (although there may be some limited degree of capacitative coupling). For example the antenna may extend through an insulating sleeve above a surface of the tool.

In some preferred embodiments the communication system also includes a system for detecting errors in the communications. Conveniently this may be implemented at the controller end, and may include forward error correction at the transmit end. In embodiments, however, the error detection system may be implemented at the controller end only, which can save power and space at the transmit end. For example the error detection system may comprise a system to detect consistency of a signal from the

sensor with an expected signal, for example checking whether the signal is within an expected range and/or checking whether the signal is consistent with one or more previous measurements by, say, determining a rate of change of the signal and determining whether this is within an allowed range. Optionally forward error correction may be employed at the sensor end, for example using convolutional encoding, which is simple to implement at the transmit end.

In some preferred embodiments the controller has a fail-safe mode in which the controller outputs a control signal to stop the motor or to reduce a speed of the motor to a limp speed in response to detecting errors in the communications which persist for longer than a fail-safe time, for example greater than 1 second, 5 seconds or 10 seconds.

In some preferred embodiments the cutting tool has a cavity which holds the sensor, a wireless transmitter where implemented, and a power source for the sensor, such as one or more watch batteries. Preferably a power control switch for the power source is also included.

In some preferred embodiments the power tool comprises a drill and the cutting tool comprises a drill bit. However the above described control system may also be employed with other types of cutting tool including, but not limited to, a saw (circular or reciprocating) and a grinder. In some preferred embodiments the control system is configured to be retrofitted to an existing biomedical or home/DIY power tool.

DIY power tools with which embodiments of the control system may be employed include DIY professional power tools.

The invention also provides a combination of a power tool and a control system as described above.

The invention further provides a cutting tool, in particular a drill bit, for a control system as described above. This may be an insulated or electrically isolated section, for example towards the tip of a drill bit.

Thus in a further aspect the invention provides a cutting tool incorporating a sensor to sense a characteristic of an environment local to the cutting tool, and a signal communications system to communicate a signal from the sensor to a controller, in particular as described above.

In some preferred embodiments the cutting tool, for example a drill bit, has two portions which are substantially electrically separate from one another, a first portion to provide an antenna for the signal communications system, and a second portion to provide a reference connection, in particular a ground or earth connection, for the signal communications system. The first portion of the cutting tool may then also be employed to provide a sensing function, for example to detect metal and/or electrical magnet field and/or changes in dielectric material properties to provide, for example, a pipe detection and/or electric wire detection and/or wall cavity detection function. Preferably (but not essentially) the power tool is grounded when used with a cutting tool of this type, since this increases sensitivity and reliability of the system. The invention still further provides a controller for a control system as described above.

In embodiments the controller may be implemented using processor control code and/or data for controlling a processor to implement the above described controller functions, or defining hardware for implementing the controller. Thus the invention further provides a carrier carrying such processor control code.

In a still further aspect the invention provides a method of controlling a portable power tool including a motor configured to drive a cutting tool, the cutting tool incorporating a sensor to sense a characteristic of an environment local to the cutting tool, the method comprising sending a signal from the cutting tool incoiporating the sensor to a controller, and using the controller to output control signals for controlling the power tool responsive to a signal from the sensor.

In some preferred embodiments the controller controls the motor in response to a sensed temperature, which may comprise an estimated temperature of the local environment of the cutting tool. This estimated temperature may be determined, in embodiments of the method, using one or more of a past sensed temperature and data defining a thermal

characteristic of the cutting tool. Additionally or alternatively in embodiments of the method the sensor may sense metal and/or an electric field and/or a cavity in a cavity wall in the vicinity of the cutting tool. Any or all of the above-described features of the control system may be included in embodiments of the method.

These and other aspects of the invention will now be further described, by way of example only, with reference to the accompanying figures in which:

Figure 1 shows a first embodiment of a drill control system according to the invention;

Figure 2 shows a second embodiment of a drill control system according to the invention;

Figure 3 shows an embodiment of a drill tip according to the invention;

Figure 4 shows a longitudinal cross section through the drill tip of Figure 3 and a sensor and communications system incorporated into the drill tip of Figure 3;

Figure 5 shows a longitudinal cross section through a second embodiment of a drill tip according to the invention;

Figure 6 shows a flow diagram of a procedure implemented by a system incorporated into an embodiment of a drill tip according to the invention;

Figure 7 shows a flow diagram of a procedure implemented in a receiver and controller of a control system according to an embodiment of the invention;

Figure 8 shows a further part of the procedure of Figure 7;

Figure 9 shows an example of a drill tip according to an embodiment of the invention;

Figure 10 shows CAD drawings of an embodiment of a drill tip according to the invention;

Figure 11 shows a longitudinal cross section view of parts of the drill tip shown in Figure 10; and

Figure 12 shows a longitudinal cross section view through the drill tip of an alternative embodiment of the invention.

Broadly we will describe a system comprising a drilling unit (battery powered or AC powered), a modified drill tip, a control system which may be incorporated into the drilling unit or an external "add on" between the drill and the energy source, and a control algorithm implemented within the control system. The system aims at controlling dynamically the rotating speed of the drill tip using the temperature (or other measured parameters) of the drill tip as one of the main controlling variables.

One embodiment comprises a portable (hand held) drilling system with a motor, a drill tip with wireless sensing capability (stand alone or in conjunction with the drill chuck), a control unit which uses the information coming from the sensor to control dynamically the speed of the motor in order to keep/control the drill tip within some predefined set characteristics. The characteristics may comprise temperature for general DIY use or temperature versus time for biomedical applications, depending on the type of material drilled. The system may control dynamically the speed of the motor in order to avoid drilling into some metal surface (water pipe, electric cable) or to stop when a cavity of a cavity wall is reached.

For orthopaedic applications where the temperature needs to be controlled much more accurately, the control unit may measure the time the temperature is above a preset value. The control action may then depend not only on the temperature measured (and estimated) but also on the duration of the procedure. The control unit may also control an "irrigation" system, normally manually controlled by the surgeon to "wet" the bone surface, keeping the area clean and reducing the temperature.

In general the control system takes into consideration the kind of material that is being "drilled" - human bone for example - before establishing the control action to perform on the drill motor.

Drill system

Thus, referring to Figure 1 this shows a first embodiment of a drill control system according to the invention. The component parts of the system are as follows:

10 Drill

11 Drill Tip

12 Drill Tip Antenna

13 External Controller with Input Device and Display

14 Wall Plug

15 Optional Irrigation System

16 Tube with solution from irrigation system

Figure 2 shows a second embodiment of a drill control system according to the invention. The component parts of the system are as follows:

101 Drill (Battery / Electric Main / Compressed Air)

111 Drill Tip

121 Drill Tip Antenna

131 Integrated Controller with Input Device and optional Display

141 Wall Plug (for Main operated drill) or Air Line Plug (Compressed Air operated)

151 Optional Irrigation System (as Figure 1)

161 Tube with solution from irrigation system

Figure 3 shows an embodiment of a drill tip according to the invention. The component parts of the drill tip are as follows:

30 Drill Tip

31 Antenna (electrically isolated from drill)

32 ON/OFF Switch (electrically isolated from drill)

Figure 4 shows a longitudinal cross section through the drill tip of Figure 3 and a sensor and communications system incorporated into the drill tip of Figure 3. The component parts are as follows:

40 Sensor

41 Electronics / RF Transmitter (electrically isolated from drill)

42 Batteries

43 ON/OFF Switch

44 Antenna (electrically isolated from drill) 301 Hollow Drill Tip: Cross Section

311 Antenna Through Hole

321 ON/OFF Switch Access Point

Drill Tip -Transmitter

Figure 5 shows a longitudinal cross section through a second embodiment of a drill tip according to the invention. The component parts are as follows:

50 Hollow Drill Tip: Cross Section

51 Sensor (it could be located on the end of the tip for sensing temperature for example or near the electronic (41) and connected to the metal of the drill if sensing metal or electric fields)

52 Electronics / RF Transmitter (electrically isolated from drill)

53 Batteries

54 ON/OFF Switch

55 Antenna (electrically isolated from drill)

In more detail, the drill tip (50) has a cavity which contains the batteries (53), switch (54, electronics and RF transmitter (52) and antenna (55). Depending on the application the sensor can be located near the cutting edge of the drill tip (51) or in the main hollow compartment where the electronics is located.

Figure 6 shows a flow diagram of a procedure inside the drill tip (1000), implemented by a system incorporated into an embodiment of a drill tip according to the invention.

At step 1001a measured parameter MP is input (for example temperature or distance from a metal object). Step 1002 performs Amplification, Conditioning and Filtering. Step 1003 performs Analogue to Digital conversion, for example using a clock with Frequency Modulation (FM) or Pulse Width Modulation (PWM). Step 1004 then in response to this transmits a High Frequency RF (eg AM or FM) signal to antenna 1005.

Figures 7 and 8 show a flow diagram of a Receiver and Controller Cycle (2000) procedure implemented in a receiver and controller of a control system according to an embodiment of the invention. The steps are as follows:

2001: Receive signal at RF Receiver (Demodulator)

2002: Measure Frequency (Fm) or measure Duty Cycle (for PWM)

2003: Is Fm within expected Range? If not Transmission Error Detected (step 2004), else:

2005: Time Filter (Rolling Average) to get a stable reading of Fm. Filtered Fm is now called FFm

2006: Convert FFm into Measured Parameter MP

2007: Calculate the Estimated Parameter EP (eg. Temperature of the surface being drilled, or the proximity of the drill from a metal object)

2008: Read the user defined Control Parameter CP (max temperature allowed or minimum distance allowed by user), optionally Warning CP WCP and Tdmax from

Input Device

2009: Measure Time Drill Is ON (TDon) with EP > preset value Warning CP (WCP)

2010: Compare EP with the user defined CP and exercise the control action.

If EP >= CP or (EP >= WCP and TDon > Tdmax) Then Reduce Drill Speed to Min (or

Zero)

Else IfEP > CP - Threshold Then Reduce Speed Drill proportionately to the difference

(CP - EP)

2011 : Control the Drill Speed. Depending on the type of Motor this can be done by:

For Pneumatic Motors, by reducing the pressure/volume of air from the feeding tube via a proportional valve connected to a stepper motor For a DC motor, changing the PWM duty cycle voltage to the motor For AC motor with "soft-start" and electronics speed control, either by commanding the electronics (internal control) or (external control) by changing the amplitude of the main supply.

For AC motor without soft-start, by controlling the phase of the main supply which is fed to the motor via a TRJAC

2012: Optional Control of the Irrigation System. Depending on the type of Irrigation used this can be done by reducing the quantity of liquid which is feed via an electronic controlled hydraulic valve. 2013: Repeat All Receiver Cycle Again (loop back to 2002).

Figures 9 to 11 show an example of a drill tip according to an embodiment of the invention and CAD drawings of an embodiment of a drill tip according to the invention;

Figure 12 shows a longitudinal cross section view through the drill tip of an alternative embodiment of the invention. The component parts are as follows:

60 Drill Tip Cutting End (conductive)

61 Insulated materials

62 Drill Tip Main Body (conductive)

63 ON/OFF Switch (electrically isolated from drill)

64 Oscillator circuitry with battery connected to Cutting End, Main Body and Switch

Drill tip with temperature control capability

Referring again to Figure 5, if for example we are considering a drill tip with temperature control capability, then the temperature sensor (which could be a thermocouple, a thermistor or an electronic sensor) is located near the cutting edge of the drill tip (51) and is mechanically held in position (for example with thermally conductive bounding compound).

The sensor is connected to an electronic circuit that amplifies, filters and conditions the signal. Then the conditioned signal can be used to modulate (via a Voltage Controlled Oscillator - VCO) the frequency of a digital clock signal (50% duty cycle) or to change the duty cycle of a digital clock at fixed frequency (PWM control). The digital signal is now ready to be transmitted via a short wave RF Transmitter (for example in the 433MHz band) which uses, for example, either AM or FM modulation to transmit the information via an antenna (55). The entire circuit can be turned off via a control switch located, for example, at the end of the drill tip (54).

The metal case of the drill tip is preferably electrically isolated from the entire circuit for a number of reasons:

a) if an electric contact is present between an internal wire and the metal of the drill, due to a fault in the manufacturing process for example, then the circuitry would still work, only a double fault would generate a failure. b) The electric loading of the metal of the drill tip around the antenna is lower if the metal is not connected to the circuitry. c) However for particular applications one may need to use the drill tip itself as an active part of the circuitry (see below).

Drill tip with metal / electric field sensing capability

If for example we are considering a drill tip with metal sensing capability, useful for a DIY application when drilling a hole in floors or walls where electric cables or water pipes could be present, then the sensor is actually the drill tip itself.

The drill tip is electrically connected inside to an electronic circuit. For example this could be either:

an oscillator, whose frequency characteristics depends on the value of the impedance characteristic of an electric load of which the drill tip is an integral part. In this scenario the drill tip acts as a capacitive antenna, and any changes of

the electric characteristics of this antenna, which happens has it approaches a metal object, is seen as a change in the frequency of the oscillator.

a Wheastone bridge, where the antenna is part of an impedance of variable electrical characteristic within one of the 4 branches of the bridge. The measuring points of the bridge can be connected to an amplifier and filter before driving a VCO, thus any change in the impedance will be amplified and reflected as a change in the frequency of the oscillator.

The digital signal coming out from the above circuitry is now ready to be transmitted via a short wave RF Transmitter (for example in the 433MHz (ISM) band) which uses either AM or FM modulation to transmit the information. The entire circuit can be turned off via a control switch located, for example, at the end of the drill tip.

In one constructed embodiment the drill bit had a shank with a diameter of approximately 1 lmm in which a 5mm cavity was formed to hold the sensor and transmitter electronics and 2 watch batteries to provide a power source. The antenna extended for approximately 2 cm out of the surface of the drilled shank. Although the drill shank had an 1 lmm diameter, the drill bit was of the type in which the diameter changes from a large diameter where the drill bit fits into the chuck to a smaller diameter at the tip.

Alternatively, as shown in Figure 12, the drill bit may be electrically divided into two sections, the main section (the section with the cutting edge) being isolated from the drill chunk and used as antenna connected to an oscillator inside the drill tip. The oscillator has two connections; on one end it is connected to the section of the drill bit with the cutting edge (antenna) and on the other end to the section of the drill tip in contact with drill chunk (electric ground). Any changes of the electric characteristics of this antenna, which occur as it approaches a conductive/metal object, are seen as a change in the electric characteristics of the oscillator. This change creates a modulation and depending on the type of the oscillator this may be an AM or FM modulation. This modulation is directly affected by the presence of conductive objects in the vicinity of the drill tip and therefore it can be directly transmitted without the need for a separate

antenna, and received by the (AM or FM for example) receiver-controller. To make the system more sensitive the drill preferably has a ground / earth connection.

In addition or alternatively the described embodiments may be used to detect changes in the dielectric characteristics of the area or region surrounding the drill tip, which can occur when, for example, the drill tip is cutting into a cavity wall and it reaches the cavity area. An alternative method to detect when the cavity inside the cavity wall has been reached is to locate a pressure sensor inside the drill tip, connected to the wireless transmitter. Information on the pressure created between the drill tip and the wall been drilled is therefore transmitted and can be used to stop the drill tip if a sudden lack/change of pressure is detected (which happens when the drill reaches (breaches into) the cavity of a cavity wall).

In the above described embodiments which sense metal and/or electric field, the sensing circuit may be nulled initially, for example by touching the drill bit to the surface to be drilled, in order to allow an offset caused by an adjacent metal object and/or electric field to be detected with greater sensitivity.

Receiver - Controller

The controller receives the RF signal, and in the first stage the signal is demodulated (AM or FM for example) (2001) and then the frequency (Fm) associated to the information been transferred is extracted (2002- for example by counting the number of pulses of a square wave at a high and fixed frequency which are "gated" by the receiving signal, which is a lower frequency ON-OFF square wave. In this way the counting of pulses becomes a direct measure of the length of the period of the incoming Fm and therefore the frequency Fm can be precisely determined).

A check (2003), if the value of the received frequency Fm is within range, will tell if there has been a transmission error, given that Fm should be within a well-defined range of values. If a transmission error occurs (2004) this can be signalled (acoustically -

visually) and optionally if this condition persists, the system can drive the drill motor speed at a predefined low safety speed.

If the incoming signal is within the correct range, then a time filter over a time window stabilizes the measured Fm (2005). This filtered value FFm is converted in a "physical" parameter MP (2006), which represents, in the case of a temperature control system, the measured temperature at the cutting edge of the Drill tip. This can be done via a map (each value of FFM has an associated value for the "physical" parameter MP) or via an equation.

This measured temperature MP represent the temperature of the drill tip, with associated thermal delays due to the thermal characteristics of the drill tip itself (dimensions, material). To achieve greater precision and sensitivity, it is preferable to calculate what is the Estimated Temperature (EP), based on the current measured temperature value and the rate of change (or gradient) (2007). A simple formula which can be used is (2007):

Estimated_Temperature = Measured_ Temperature + K * Measured_Rate_of_Change

Where K is a constant whose value depends on the type of drill used (user selectable).

Based on the value of the Estimated_Temperature and the value of the Set_Temperature (CP), which is user defined via an input device (2008) like a potentiometer for example, the software can then exercise the control action based on the following set of rules (2010):

If EP >= CP Then Reduce Drill Speed to Min (or Zero)

Else IfEP > CP - Threshold Then Reduce Speed Drill proportionately to the difference (CP - EP)

In orthopaedic embodiments drill speeds of between 50 rpm and 2000 rpm may be employed, for example the drill speed being reduced to below, in order of preference, 750 rpm, 400 rpm, and 250 ipm, if the drill speed is not to be reduced to 0.

An additional refinement can be achieved by estimating (2009) the time TDon the drill is at a temperature above a predefined (user defined) value WCP (2008). If this time is greater than a user defined maximum Tdmax (which would depend on the material which is been drilled) then an other control condition can be added, with the objective to avoid the material been drilled to have a temperature above WCP for longer than Tdmax (2010):

If EP >= CP or (EP >= WCP and TDon > Tdmax) Then Reduce Drill Speed to Min (or Zero)

Else IfEP > CP - Threshold Then Reduce Speed Drill proportionately to the difference (CP - EP) Else Increase Speed to maximum required

Once the control action has been decided, the electronics will control the motor of the drill accordingly (in a proportional - linear way). Depending on the type of motor there are several scenarios possible (2011):

Control of the Drill Speed

Depending on the type of Motor this can be done by: a) For Pneumatic Motors, by reducing the pressure/volume of air from the feeding air-pressure (or vacuum) tube via a proportional valve connected to a stepper motor controlled proportionally by the electronics. b) For a DC motor, changing the PWM duty cycle of the voltage to the motor c) For AC motor with "soft-start" electronics, either by commanding the electronics which provides the soft-start (in the case of internal control) or (external control) by changing the amplitude of the main supply. d) For AC motor without soft-start, by controlling the "ON" phase of the main which is feed to the motor via a bidirectional TRIAC.

If a more accurate temperature control is required (for bone implants for example) then an optional Control of the Irrigation System could be added (2012). Depending on the type of Irrigation used this can be done by reducing the quantity of liquid which is fed via an electronic controlled hydraulic valve piloted by, for example, a stepper motor controlled proportionally by the Control electronics.

The receiver cycle preferably then repeats itself (2013).

No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto.