A major difficulty encountered on many sites where hand-held powered tools are used is that when the tool is left unattended, even for a short time, it is likely to be stolen.

The usual way of attempting to overcome this problem is to ensure that the tool is stored in a safe location when unattended. However, occasions will arise when a workman's attention will be distracted (perhaps by an urgent call to another part of the site) and he will leave the tool unattended.

Again, it is often desirable to be able to control access to the use of an electrical machine, be it a hand-held power tool or some other type of machine.

An object of the present invention is to ensure that an electrical machine cannot be used by unauthorised personnel when removed from a site.

According to the invention an electrical machine which comprises a plurality of parts all of which require to be energised to enable the machine to operate includes control circuitry comprising a plurality of latch means each having a SET state and a RESET state and each associated with an individual one of the parts of the machine, connections between each latch means and its associated part arranged in a coded pattern so that some of the latch means complete a connection to their associated parts when in the SET state and other of the latch means complete a connection to their

associated parts when in the RESET state, and key means having a coded pattern of connections operable to SET some of the latch means whilst the machine is connected to a power supply and to leave other of the latch means in the RESET state, whereby only when the coded patterns of the latch means and the key means coincide are connections completed to said parts of the machine to enable it to operate.

In one preferred embodiment the key means is physically separable from the machine and means are provided for ensuring that if the key means are so separated whilst the machine is connected to a power supply then the states of the latch means remain unaffected until the machine is disconnected from its power supply.

The latch means may comprise relay devices each having a relay coil and each drivable from a RESET to a SET state when its relay coil is energised and some of which relay devices are driven to a SET state by operation of said key means and are held in that state thereafter whilst the machine is connected to an electrical power supply.

Each relay device may have a SET output and a RESET output and connectors may be provided from each relay device to its associated part, the connectors being connected to either the SET outputs or the RESET outputs in said coded pattern. Each relay device may have a pair of ganged relays one of which relays provides SET and RESET outputs and the other of which relays, when the device is in its SET state, is coupled to a power supply to hold the relay device SET.

Preferably an ON/OFF power switch is included which is so connected that when the switch is in its OFF state the SET and RESET states of the latch means remain unaffected.

The machine may comprising a portable power tool and may include detachable plug means for connection to a power supply so that detachment of the plug means from the power supply causes all the latch means to revert to their RESET state.

It will be appreciated from the foregoing that an electrical machine that embodies the invention cannot be used without its key once it is disconnected from an electrical power source. In this way theft of the machine will be deterred since the thief will need to have access to the key to ensure the power tool can thereafter be operated.

An embodiment of the invention will now be described with reference to the accompanying drawings in which:- Figure 1 is a schematic circuit diagram of a power tool showing the interconnection of its field coils and its armature, Figure 2 is a circuit diagram of circuitry now proposed for use in controlling operation of the power tool, and Figure 3 is a circuit diagram of an individual latch forming part of the circuitry of Figure 2.

An electrical power tool, for example a drill, has a rotor or armature 10, upper field windings or coils 12,14 and lower field coils 16,18. The upper field coils 12,14 are connectable to an electrical power supply line 20 via a switch 22. The lower field coils 16,18 are connectable to a neutral line 24 of the electrical power supply. The power supply may be the mains electrical supply or the output of a transformer connected to a mains electrical supply.

Figure 2 shows the additional circuitry now proposed and the parts of the power tool shown in Fig. 1 are also shown in Fig. 2 and have similar reference numbers therein.

In Fig. 2 there are shown a plurality 30 of latches (30a, 30b, 30c... 30h) respectively connected to the ends of the coils 12,14,16 and 18 and to corresponding ones of a plurality 32 of connectors (32A, 32B, 32C... 32H). Each of the connectors 32 has two input pins (e. g. 32Ail, 32Ai2 etc) and two output pins (e. g. 32Aol and 32Ao2).

A selected one of the two input pins in each connector is hardwired to the corresponding one of its output pins (e. g. pin 32Ai2 is wired to pin 32Ao2). One side of armature 10 is connected to both output pins of connectors 32A and 32B and the opposite side of armature 10 is connected to both output pins of connectors 30C and 30D.

Operating electrical power for the power tool is provided from a flexible power cable having a live power line 20 and a neutral return line 24. An ON/OFF switch 22 for the tool is provided in line 20. Its load terminal is connected to an additional latch 30i one output pin of which is connected to both output pins of connectors 32E and 32F.

The neutral 24 of the electrical power supply is connected to both output pins of connectors 32G and 32H. Electrical live power line 20 and neutral line 24 are also directly connected to the input of a 24VDC output power supply unit 34. It will be noted that the connection to unit 34 is taken from the live side of switch 22.

It will be appreciated that the set of connectors 32 act as a coding device. The actual pattern that is coded is represented by the wiring pattern. Connector 32A (in which pin 32Ai2 is connected to pin 32Ao2) could represent digital'1'whilst connector 32B (in which pin 32Bil is connected pin 32Bol) could represent a digital'0'. In the example illustrated in Fig. 2 the eight connectors shown are internally connected to represent the eight binary digit code '10101010'. Of course the connectors could be wired in alternative combinations to represent any one of the 28 (=256) possible eight binary digit combinations.

Each latch 30 is of the form shown in detail in Figure 3. Each latch 30 comprises a pair of ganged relays 40,42 controlled by a relay coil 44. The lower (as viewed in the Figure) relay 42 is connected to ground via coil 44 and coil 44 is shunted by a diode 46. The pair of relays 40,42 are normally in the RESET state shown with the lower relay 42 contacting an output pin 42-1 whilst the upper relay 40 contacts an output pin 40-1. If power (from the 24VDC power supply 34) is applied to input terminal 48 of the latch then current will flow though coil 44 with the result that the two relays are driven to a SET state. In the SET state relay 40 contacts output pin 40-2 and relay 42 contacts output pin 42-2. It is to be noted that contact 42-2 is coupled to the 24VDC power supply via a link 50. Each pair of output pins 40-1 and 40-2 of latches 30a, 30b ... 30h are connected to the pair of input pins of corresponding connectors 32A, 32B ... 32H.

As a result of this particular configuration removal of the DC power to input 48 after a relay has been switched from its RESET state to its SET state will have no effect on the latch and the latch will stay in the SET state as long as power from the 24VDC power supply is applied to link 50, that is to say for as long as the power tool remains coupled to the electrical power supply.

The output of the 24VDC power supply 34 is applied to inputs 48 of selected latches 30. Selection of which latches are energised is determined by a coded key 54. Key 54 is, in accordance with one embodiment of the invention, physically insertable in the power tool and on being inserted is coupled via a link 56 to the 24VDC power supply 34. Key 54 has a plurality of outputs 54a, 54b... 54h. Each output from key 54 is connected to the corresponding input 48 of relay latches 30a, 30b... 30h respectively.

Only some of the outputs 54a, 54b... 54h of key 54 are supplied from the 24VDC power supply 34 and only those that are so supplied will cause the associated ones of the latches 30a, 30b... 30h to be driven to a SET state. The others of the latches 30 will remain RESET.

The pattern of supplied and unsupplied contacts 54a, 54b... 54h of key 54 can also be represented by a binary code. If a supplied contact is designated as'1'and an unsupplied contact is designated'0'then the exemplary connection pattern of contacts 54a, 54b... 54h that is illustrated in Fig. 2 can be represented by the eight digit binary code'10101010'. On energisation of supply line 20, as by plugging in the power tool of Fig. 1 to a mains supply, 24V DC supply unit 34 becomes live and if key 54 is inserted then the latches which are SET are those connected to a key contact designated by a'1'while the latches connected to a key contact designated by a'0' remain in their RESET state.

If the pattern of SET and RESET latches 30 corresponds to the code pattern of the hard-wired connectors 32 (which in the example of Fig. 2 is also represented by the binary code'10101010') then the field coils 12,14,16 and 18 and the armature 10 are all connected between the load side of switch 22 in mains power line 20 and to the neutral return line 24 in the correct sequence. The power tool can now be operated.

Because of the way latches 30 are configured those that are SET will remain SET for as long as the power tool is connected to the electrical supply. Thus key 54 can be removed from the power tool and placed in for example the pocket of a person using the tool and use of the tool can be continued. Furthermore switch 22, which could be a trigger-operated switch of the power tool, can be repeatedly turned OFF and ON without requiring re-insertion of the key.

If, however, the tool is at any time disconnected from the electrical power supply (for example by a thief) latches 30 will all RESET and it will not thereafter be possible to begin using the machine without insertion of key 54.

Figure 2 illustrates an implementation of a pattern of eight connectors 32 which is represented by the eight binary digit number'10101010'. This pattern is replicated in the outputs 54a, 54b... 54h of key 54. It will be appreciated that other, different, patterns may equally well be used for both the connectors 32 and key 54. The total number of different possible patterns is 2 (=256), although two of them ('00000000' and '11111111') may not be practical. It is possible to double the number of coded patterns available to 29 (=512) by having a ninth connection from key 54 which is taken to latch 30i.

It will be appreciated also that embodiments in accordance with the invention may replace the key 54 which is physically removable from the tool by some other means of inputting the desired pattern for example a keypad. It will also be appreciated that embodiments of the invention cover arrangements in which the pattern of the connectors 30 may be pre-set by a user (using a keypad for example) in order that the particular sequence of numbers to be input to enable operation of the tool can be personally chosen.

Although described with reference to a hand-held power tool (and deterring the theft thereof) it will be appreciated that the described arrangements have utility in other areas, e. g. controlling access to the use of any electrical machine.

Finally, it will be appreciated that the arrangements described are not limited to rotating electrical machines, and are usable in any electrical machine in which case the latches are connected to various parts of the machine to enable them to be connected one to another when selected one of the latches are SET.