Background of the Invention Most commercial available shearing hand pieces are powered by electric motors via a mechanical transmission system. The mechanical transmission system typically includes a number of drive shafts connected together by universal joints. A proximal shaft is coupled to the hand piece to effect a reciprocating motion of a cutter of the hand piece. A distant drive shaft is typically coupled by a clutch or other coupling mechanism to a wheel driven by the electric motor. To turn the hand piece OFF, a user pulls on a cord to disengage the clutch or coupling so that torque from the wheel is decoupled from the drive shaft. By again pulling on the cord, the clutch or coupling is engaged so that torque is imparted from the wheel to the drive shaft.

The present invention was developed to provide an alternate way of controlling the shearing hand piece. However embodiments of the invention can be applied to other power driven tools, including hand tools with in-built motors such as electric drills, saws, planes etc.

Summary of the Invention According to a first aspect of the present invention there is provided an electronic control circuit for controlling the operation of a power means for powering a tool having a hand held body and a work piece retained by said body, said electronic control circuit

coupled to said hand held body and said power means and configured to enable said power means to deliver power to said work piece when said body is touched by a user, and to disable said power means from delivering power to said work piece when the body is not being touched by a user.

Preferably, said electronic control circuit includes timing means for timing a continuous period of time during which said body is being touched, said electrical control circuit configured to control said power means to deliver power to said work piece only after said period exceeds a predetermined period.

Preferably, said predetermined period is up to 1 second.

Preferably, said predetermined period is between 150-800 ms.

Preferably, said electronic control circuit further includes a sensor circuit coupled between said body and said timer means, said sensor circuit responsive to an electrical load applied to said body by virtue of a user touching said shearing hand piece.

Preferably, said sensor circuit includes a capacitive bridge.

Preferably, said capacitive bridge has a first arm in electrical contact with said body, a second arm provided with an adjustable capacitance, and an output tap between said first and second arms carrying a signal indicative of a difference in capacitive load between said first and second arms, and wherein said timing means commencing timing said period as a function of said signal.

According to a further aspect of the present invention there is provided a touch activated tool system including at least: a tool provided with a hand held body and a work piece retained by said body; and an electronic control circuit coupled to said body and a power means for providing operative power to work piece, said electronic control circuit configured to

control said power means to enable delivery of deliver power to said work piece when said body is being touched by a user and to prohibit delivery of power to said work piece when said body is not being touched by said user.

Brief Description of the Drawings Figure 1 is a block diagram of an electronic control circuit in accordance with the present invention; and Figure 2 is a circuit board for said electronic control circuit.

Detailed Description of Preferred Embodiment Figure 1 depicts broadly a touch activated shearing system including the main component blocks of the electronic control circuit 10 for controlling a hand tool in the form of shearing hand piece 12. The shearing hand piece includes a hand held body (not shown) and a"work piece"in the form of a cutter (not shown) retained by the body.

The circuit 10 is coupled to a power means or drive for the shearing hand piece 12 in the form of an electric motor 14, and the shearing hand piece 12. Drive is imparted from the motor 14 to the hand piece 12 via a drive system 13 of conventional construction which allows the handpiece to be manually manipulated relative to the motor 14. In general terms, the drive system 13 comprises a metallic sleeve which houses a flexible rotatable NYLON cord. There are two typical forms for the sleeve 15. The first is a flexible metallic outer tube which essentially runs the full distance from the motor 14 to the hand piece 12; the second being two separate rigid metallic shafts coupled together by an articulated elbow joint. The NYLON cord is rotated by the motor 14. This rotary motion is inverted to a reciprocating motion of the cutter (not shown) of the hand piece 12 by conventional means. The circuit 10 includes a capacitive bridge 16 having a first arm (or antenna arm) 18 electrically coupled with the shearing hand piece 12 via a down tube or drive shaft 13 attached to the hand piece 12; and, a second opposite arm 20 coupled to ground through an adjustable capacitor 22. A signal is tapped from a point 24 between the first and second arms 18 and 20 which is indicitive of a capacitive imbalance or difference between the arms 18 and 20. The signal is provided on line 26 and provided as an input to a comparator 28. Provided the signal on line 26 is above a

threshold voltage, the comparator effectively energises both an indicator LED 30 and a timing means, in the form of an on-delay timer 32. The on-delay timer 32 provides an output signal to close a relay 34 provided the on-delay timer 32 is maintained in an active (energised) state by the comparator 28 for a predetermined continuous period of time. After the predetermined continuous time has expired the on-delay timer 32 maintains the relay 34 in a closed state for as long as the comparator 28 maintains the on-delay timer in an active state. The relay 34 closes a circuit to energise the motor 14 providing drive to the shearing hand piece 12.

The capacitive bridge circuit 16 is balanced by adjustment of capacitor 22 to provide a zero or at least minimum voltage at the centre tap 24 when the user 36 is not holding the shearing hand piece 12. Electrical (capacitive) coupling is achieved between the user and arm 18 due to the body of the shearing hand piece being made from metal.

However a similar coupling could be achieved by forming only a portion of the body from metal or even by having a plastics material body but with an electrically conductive plate below the surface of the body and coupled to the arm 18. When a user 36 picks up or touches the shearing hand piece 12, the capacitive load on arm 18 in increased providing an imbalance in the capacitive load between arms 18 and 20 thereby producing an increased voltage at the centre tap 24 which is delivered along the line 26 to the comparator 28. Provided the signal 26 above a threshold level at a positive input of the comparator 28 the comparator 28 activates or energises the LED 30 and the on- delay timer 32. As previously mentioned, provided the on-delay timer 26 is maintained ON for a predetermined period of time it then acts to close the relay 34 energising the motor 14 thereby driving the shearing hand piece 12 which is mechanically coupled thereto via a plurality of drive shafts and universal joints (not shown).

Figure 2 depicts the circuit 10 in greater detail. The circuit 10 is energised by a 24 volt DC supply 36 via a conventional power supply filtering circuit 38. The bridge circuit 16 includes an oscillator 40 and a transformer 42. The oscillator 40 oscillates at a nominal frequency of 23kHz which drives the transformer 42. The oscillator 40 is of conventional configuration and includes a transistor 44 having its emitter coupled to ground through a resistor 46 and its collector coupled between the transformer 42 and

series connected capacitors 48 and 50 to ground. A short circuit 52 extends from between the emitter and resistor 46 to between the capacitors 48 and 50.

The transformer 42 has a primary winding 54 coupled to the collector of the transistor 44 and two identical secondary windings 56 and 58. The winding 56 is part of the first arm 18 (or antenna) and is coupled through capacitor 60 to the sleeve 15 and shearing hand piece 12. The winding 58 is coupled via a parallel combination of the adjustable capacitor 22 and a further fixed capacitor 62 to ground. Line 26 extends from the centre tap 24 between the windings 56 and 58 to a negative input of the comparator 28. The signal derived at the centre tap 24 is an AC signal. This is rectified through a rectifier circuit 64 comprising a diode 66 in the line 26 and a resistor 68 coupled between the diode and ground. Thus, the signal on line 26 presented to the negative input of comparator 28 is a rectified AC signal, ie a DC signal.

The positive input to comparator 28 is coupled to a variable resistor 70. Thus, the voltage presented to the positive input of comparator 28 can be varied for the purposes of setting a threshold level against which the rectified voltage on line 26 is compared by the comparators 28. In effect, the resistor 70 provides a sensitivity adjustment mechanism for the circuit 10. When the rectified voltage on line 26 presented to the negative input of comparator 28 is greater than the voltage presented to the positive input of comparator 28, the output of the comparator on line 72 goes high, energising the LED 30 and turning ON a transistor 74. The transistor 74 acts as a switch to energise the on-delay timer 32. The on-delay timer 32 comprises a standard chip LM4541 with is time delay set by an RC circuit comprising variable resistor 76 and capacitor 78. The on-delay timer 72 produces a high on output line 80 provide that the on-delay timer is supplied with power via the transistor 74 for a period greater than the time determined by the RC circuit 76,78. In this particular embodiment, the time delay can be varied between 150-800 ms although again by replacement of the resistor 76 or capacitor 78 this delay can be changed outside of this range and may be, for example, up to 1 second or more.

An off-the-shelf LM7815 regulator chip 79 provides a stable 15V DC voltage to the

comparator 28, LED 30 and on-delay timer 32.

The output line 80 is coupled to a switching circuit 82 which operates a relay coil 84 of the relay 34. Energising the coil 84 causes relay switch 34 to close, providing power to the motor 14 (not shown in Figure 2) used to drive the shearing hand piece 12.

Now that an embodiment of the present invention has been described in detail in will be apparent to those skilled in the relevant arts that numerous modifications and variations may be made without departing from the basic inventive concepts. In particular, it wll be recognised that many of the specific components and circuit blocks within the circuit 10 can be replaced with other components or blocks of different configuration and make up which perform the same function. Further, the preferred embodiment is described in relation to a shearing hand piece which is powered by a remotely located electric motor via a mechanical drive system transmission. In addition to the drive systems described, a further drive system comprising one or more metallic rigid tubes coupled together by universal joints or similar coupling can be used. Further, embodiments of the invention may easily be incorporated into hand held tools having integrated electric motors (i. e. electric motors within the tool body) such as electric drills, saws, planes, routers etc.

Also it is not necessary that the body have an exposed surface made of an electrically conductive material for connection to the arm 18. Rather a conductive plate (for example) could be disposed under a plastic material body shell, the plate being connected to the arm 18. In the example of an electric drill the body of drill is its exterior casing and the"work piece"is drill bit and/or chuck for holding the drill bit.

All such modifications and variations are deemed to be within the scope of the present invention the nature of which is to be determined from the above description and the appended claims.