Machine tool probe

[00002] This invention relates to probes for use on position determining apparatus such as coordinate measuring machines, measuring robots and in particular machine tools.

[00003] An example of such a probe is shown in U.S. Pat. No. 4,153,998. Probes intended for use on machine tools in which there is a wireless signal transmission between the probes and the controller of the machine tool are shown in European Patent Nos. 337669 and 337670.

[00004] When used on machine tools such probes are commonly battery-operated. These probes may transmit measurement signals optically to a receiver module however, the power consumption in sending these optical messages reduces the battery life.

[00005] The present invention provides a probe for position determining apparatus comprising:

• • [00006] signal generating means for generating a signal;
  • [00007] signal transmitting means for transmitting the signal generated by the signal generating means to a receiver module in the form of optical pulses;
  • [00008] characterised in that a pulse width modulator is provided to reduce the duration of each pulse.

[00009] Preferably the signal generating means includes a universal synchronous/asynchronous receiver transmitter. The pulse width modulator may control the timing of the universal synchronous/asynchronous receiver transmitter (USART).

[00010] Preferably the probe is battery powered. The optical pulses may be generated by LEDs.

[00017] Referring to FIG. 1, the probe 10 is mounted in the spindle 12 of a machine tool exchangeably with the normal cutting tools. The spindle 12 can move the probe in three dimensions x, y, z relative to a workpiece 14 clamped on a table or bed 22 of the machine tool. Measurements are made by contact between the stylus 11 of the probe and the workpiece. Measurement signals from the probe are transmitted optically as indicated by arrow 16 to a receiver module 19 mounted on a fixed structure 20 of the machine tool. The probe 10 is battery operated. The battery powers LEDs located within window 18 which are used to transmit the optical measurement signals.

[00018] The optical measurement signals indicated by arrow 16 in FIG. 1 comprise serially transmitted codewords. Each codeword is 8 bits long and comprises a sequence of on/off pulses. These codewords are generated by signal generating means, comprising a universal synchronous/asynchronous receiver transmitter (USART). A section of codeword formed in this way is shown in FIG. 2a. Typically each pulse is 8 microseconds long and a new codeword may be transmitted every 16 milliseconds.

[00019] Using the universal synchronous/asynchronous receiver transmitter (USART) alone to generate the codewords has a disadvantage that as each pulse is relatively long this results in high power consumption and thus reduced battery life.

[00020] To overcome this disadvantage the universal synchronous/asynchronous receiver transmitter (USART) is combined with a pulse width modulator. As shown in FIG. 2B, the pulse width modulator generates a regular series of pulses of shorter pulse length and with a shorter time interval between pulses than generated by the universal synchronous/asynchronous receiver transmitter (USART). The time between pulses is typically 8 microseconds and pulse length is typically 2 microseconds. The universal synchronous/asynchronous receiver transmitter (USART) output is combined with the pulse width modulator output to provide a chopped output for transmission. As shown in FIG. 2c the form of this resultant output is the same as the form of the output of the universal synchronous/asynchronous receiver transmitter (USART) shown in FIG. 2a, however the duration of each ON pulse has been reduced to equal that of the pulse width modulator. This reduces the duration of each LED flash and therefore reduces the power consumption of the system. Battery life is therefore enhanced.

[00021] Use of the pulse width modulator in combination with the universal synchronous/asynchronous receiver transmitter (USART) has a second advantage. In previous systems without the pulse width modulator the microprocessor must control both the timing and the sequence of the pulses, however when a pulse width modulator is incorporated into the system this may be used to set the timing of the universal synchronous/asynchronous receiver transmitter (USART), i.e. it acts as a serial clock for timing. The microprocessor is used less as it is only required to send on/off messages to the universal synchronous/asynchronous receiver transmitter (USART) and therefore has improved multi-tasking.

[00022] The pulse width modulator may already be incorporated into the probe for other uses such as motor control or digital/analogue conversion. The use of existing components in the probe for the system therefore reduces component cost.

[00023] FIG. 3 is a schematic diagram showing the formation of the serial codewords. A microprocessor 30 in the probe incorporates a pulse width modulator 32 and signal generating means in the form of a universal synchronous/asynchronous receiver transmitter (USART) 34. An output 36 from the pulse width modulator 32 controls the timing of the universal synchronous/asynchronous receiver transmitter (USART). Outputs 38,40 of the pulse width modulator 32 and the universal synchronous/asynchronous receiver transmitter (USART) 34 are combined at an AND gate 42 to form a serial codeword 44. This serial codeword is transmitted from an LED or other signal transmitting means to an external receiver.