Touch probe and signal processing circuit therefor

Abstract

A touch trigger probe incorporates piezoelectric sensors 50 , whose outputs are processed by an interface circuit. The interface circuit discriminates between signals generated from the piezoelectric sensors 50 as a result of machine vibration and those generated as a result of a genuine measurement event, by the use of a timing circuit 90 . The timing circuit 90 compares the time intervals (t 1 -t 2 );(t 2 -t 3 ) between attainment of first 1 1 and second 1 2 , and second 1 2 and third 1 3 output signal levels from the sensor 50 , and upon the basis of this comparison validates (or rejects) measurements made with the probe. Additionally, the interface determines whether measurements made with the probe are taken upon the basis of outputs generated by the sensors 50 due to a shock wave in the stylus 24 of the probe, or as a result of strain in the stylus 24 ; as an alternative, measurements may be made only on the basis of strain.

We claim:  
     
       1. A touch probe having a fixed structure and a stylus supporting member biased into a repeatable rest position relative to the fixed structure, from which the supporting member is displaceable when a deflecting force is applied thereto, and to which the supporting member may return when said deflecting force is removed, a direction of biasing action defining an axis, the probe further comprising a plurality of sensors sensitive to tension and compression, for sensing force applied to said supporting member prior to displacement thereof from said rest position, said sensors being clamped between first and second parts of said stylus supporting member and each of said sensors having an axis of maximum sensitivity to tension and compression, said sensors being grouped into pairs, the axis of maximum sensitivity of each of said sensors being inclined (a) with respect to the probe axis and (b) with respect to the axis of maximum sensitivity of the other sensor in each of said pairs, one of said first and second parts of said stylus supporting member comprising a plurality of V-shaped rides extending radially with resect to said probe axis, and each of said pairs including a sensor located on opening faces of said ridges. 
     
     
       2. An interface circuit for connecting a touch probe, having at least one analogue sensor for generating an analogue signal, to a control of a coordinate positioning machine, the interface comprising: 
       means defining first, second, and third analogue signal levels;  
       means for comparing the time interval between attainment of said first and second, and said second and third signal levels, and determining upon the basis of said comparison whether said analogue signal conforms to a predetermined trigger signature; and  
       means for generating a trigger output responsive to said analogue signal conforming to said predetermined trigger signature.  
     
     
       3. An interface circuit according to claim  2  further comprising means for generating a latching signal, instructing the machine control to store the position of the movable arm, when said analogue signal attains said first level. 
     
     
       4. A method of processing an analogue output signal from a touch probe, the method comprising the steps of: 
       defining first, second and third analogue signal levels;  
       determining the magnitude of a first time interval between attainment of said first and second analogue signal levels, and determining the magnitude of a second time interval between attainment of said second and third analogue signal levels;  
       comparing the magnitude of said first and second time intervals;  
       on the basis of the result of said comparison, generating a trigger output signal.  
     
     
       5. An interface circuit for connecting a touch probe to a control of a coordinate positioning machine, the interface comprising: 
       latching means for generating a latching signal, for instructing the machine control to record a measurement, in response to an amplitude of at least one analogue signal generated by said interface circuit attaining a predetermined amplitude threshold, said latching means being responsive to said at least one analogue signal having a frequency above or below a predetermined frequency threshold; and  
       means for determining whether said latching signal was generated in response to said at least one analogue signal having a frequency above said predetermined frequency threshold, and for generating a flag signal accordingly.  
     
     
       6. An interface circuit according to claim  5  wherein said predetermined threshold of frequency is 500 Hz. 
     
     
       7. An interface according to claim  5 , further comprising means for determining whether said at least one analogue signal conforms to a predetermined trigger signature, and for generating a confirmation signal accordingly. 
     
     
       8. An interface according to claim  7 , comprising means for determining whether, at a given instant of time after the emission of said latching signal, the amplitude of said at least one analogue signal exceeds said predetermined amplitude threshold. 
     
     
       9. An interface for connecting a touch probe to a control of a coordinate positioning machine, the probe having at least one sensor for generating at least one analogue signal, the interface comprising: 
       latching means for generating a first latching signal, for instructing the machine control to record a measurement, in response to an amplitude of said at least one analogue signal attaining a predetermined amplitude threshold, said latching means being responsive to said at least one analogue signal having a frequency above or below a predetermined frequency threshold;  
       discriminatingdetermining means for determining whether said latching signal was generated in response to said at least one analogue signal having a frequency above said predetermined frequency threshold, and for generating a flag signal accordingly; and  
       suppressing means for suppressing the emission of said  a second latching signal in the event that said discriminating means determines that said at least one  which is generated in response to said at least one analogue signal attaining said  having a frequency above the predetermined amplitude threshold has a frequency in excess of said predetermined frequency threshold , said suppressing means being selectively operable.  
     
     
       10. An interface circuit according to claim  9  wherein said predetermined frequency threshold is 500 Hz. 
     
     
       11. An interface circuit for connecting a probe to a control of a machine, the probe having at least one analogue sensor, the interface comprising: 
       at least one input channel, responsive to signals above and below a predetermined frequency threshold, which receives an input signal from said at least one analogue sensor, said at least one input channel comprising a comparison circuit which generates an output signal when an amplitude of the input signal exceeds a predetermined amplitude threshold; and  
       a frequency discriminator which determines whether the output signal from the comparison circuit occurred in response to an input signal having a frequency greater than said predetermined frequency threshold, and which generates a flag signal accordingly.  
     
     
       12. An interface according to claim  11 , further comprising a trigger signal discriminator which determines whether said at least one analogue signal corresponds to a predetermined trigger signature, and which generates a confirmation signal accordingly. 
     
     
       13. An interface according to claim  12 , wherein said trigger signal discriminator includes a comparator circuit which generates a step output signal in the event that an amplitude of said at least one analogue signal exceeds a predetermined threshold at a given instant of time after the emission of said latching signal. 
     
     
       14. An interface circuit for connecting a probe to a control of a machine, the probe having at least one analogue sensor, the interface comprising: 
       at least one input channel, responsive to signals above and below a predetermined frequency threshold, which receives an  a first input signal from said at least one analogue sensor, said at least one input channel comprising a comparison circuit which generates an  a first output signal when an amplitude of the input signal exceeds a predetermined amplitude threshold; and  
       a frequency suppressor which suppressesmeans for suppressing the emission of saida second output signal in the event that saidwhich is generated in response to a second input signal hashaving a frequency greater than said predetermined frequency threshold; and  
       a selector enabling selective operation of said frequency suppressor , the suppressing means being selectively operable.  
     
     
       15. An interface circuit for connecting a probe to a control of a machine, the probe having at least one analogue sensor, the interface comprising: 
       at least one input channel which receives an input signal from said at least one analogue sensor, said at least one input channel comprising first, second, and third comparators which generate first, second and third output signals when the input signal exceeds first, second and third predetermined thresholds, respectively; and  
       a timing circuit which receives said first, second, and third output signals, and determines whether the time intervals between the occurrence of the first and second input signals and the occurrence of the second and third input conforms to a trigger signature of said probe, the timing circuit further generating a trigger output signal on the basis of the determination.  
     
     
       16. An interface circuit for processing signals generated by at least one analogue sensor in a touch probe, and providing output signals which are useable by a control of a coordinate positioning machine, the interface comprising: 
       
         at least one input channel for receiving signals from the analogue sensor,  
       
       
         three threshold detectors, each of which generates an output signal when the analogue signal from the sensor reaches a given threshold amplitude,  
       
       
         a discriminating circuit which is connected to each of the threshold detectors and which receives their output signals, the discriminating circuit determining whether time intervals between signal emissions from the detectors are representative of a genuine measurement event. 
       
     
     
       17. An interface according to claim  16 , having two input channels for receiving the analogue signals, each input channel having a filter, and the two filters transmitting analogue signals of different frequencies, wherein one of the channels has at least two threshold detectors. 
     
     
       18. An interface according to claim  17 , further including a third input channel which receives a resistance signal corresponding to the resistance of an electrical contact within the probe, the third input channel having a further threshold detector which generates an output signal when the resistance of the electrical contact reaches a given threshold. 
     
     
       19. An interface according to claim  17 , further including means for producing an output that is indicative of which channel a first occurring output signal was generated. 
     
     
       20. An interface for connecting analogue sensors in a touch probe to a control of a coordinate positioning machine, the interface having: 
       
         a pair of input channels, one for relatively high frequency sensor signals and one for relatively lower frequency sensor signals;  
       
       
         at least two comparators in one of the input channels, each of which is set to respond to a different amplitude of sensor signal, and a further comparator in the other input channel; and  
       
       
         discriminating circuitry connected to the at least two comparators and the further comparator which determines, from time intervals separating occurrences of output signals from the comparators, whether the output signals represent a genuine measurement event. 
       
     
     
       21. An interface according to claim  20 , having three such comparators in said one of the input channels. 
     
     
       22. An interface according to claim  20 , further including means for producing an output that is indicative of which of the two input channels a first occurring comparator output signal was generated. 
     
     
       23. An interface according to claim  22 , further including a third input channel which receives a resistance signal corresponding to the electrical resistance of an electrical contact in the probe, the third input channel having a further comparator which is set to respond to the resistance of the electrical contact reaching a threshold level. 
     
     
       24. An interface for connecting analogue sensors in a touch probe to a control of a coordinate positioning machine, the interface having: 
       
         two input channels which are connected to receive sensor signals from the sensors, each channel having a filter wherein the two filters pass sensor signals of a different frequency, and each of the two channels having at least one threshold detector connected to the corresponding filter, which generates a threshold output when the sensor signal exceeds a threshold amplitude; and a circuit connected to the outputs of each of the threshold detectors, which produces an output that is indicative of the channel in which a first occurring threshold output was generated. 
       
     
     
       25. An interface according to claim  24 , further including a discriminating circuit which is connected to each of the threshold detectors and which receives their output signals, the discriminating circuit determining whether time intervals between signal emissions from the detectors are representative of a genuine measurement event. 
     
     
       26. An interface according to claim  25 , having at least two threshold detectors in one of the input channels. 
     
     
       27. An interface according to claim  26 , further including a third input channel which receives a resistance signal corresponding to the resistance of an electrical contact within the probe, the third input channel having a further threshold detector which generates an output signal when the resistance of the electrical contact reaches a given threshold. 
     
     
       28. An interface for connecting analogue sensors in a touch probe to a control of a coordinate positioning machine, the interface having: 
       
         at least three comparators connected to the analogue sensors, which receive signals from the sensors, and which are each set to respond to a different amplitude of signal; and  
       
       
         discriminating means to which the comparators are connected, for receiving output signals from the comparators, and determining on the basis of time intervals between outputs of each of the comparators whether the outputs represent a genuine measurement event. 
       
     
     
       29. An interface according to claim  28 , having two input channels, a first of which has a filter transmitting relatively high frequency sensor signals, and a second of which has a filter transmitting relatively low frequency sensor signals. 
     
     
       30. An interface according to claim  29 , further including a circuit connected to the comparators, which produces an output that is indicative of the channel in which a first occurring comparator output was generated. 
     
     
       31. Apparatus for use on a coordinate positioning machine including: 
       
         a touch probe having a housing and a stylus connected to a supporting member within the housing, the probe including at least one analogue sensor sensitive to tension and compression which generates an analogue signal; and  
       
       
         an interface circuit which connects the probe to a control of a coordinate positioning machine, the interface circuit having:  
       
       
         a high frequency input channel connected to the analogue sensor, and having a filter which transmits signals from the analogue sensor generated as a result of shock in the stylus when the stylus contacts a workpiece, the high frequency input channel also having a first comparator for generating a first output signal when the amplitude of the analogue signal at the first comparator exceeds a predetermined threshold;  
       
       
         a low frequency input channel connected to the analogue sensor, and having a filter which transmits signals from the analogue sensor generated as a result of strain in the stylus when the stylus contacts a workpiece, the low frequency input channel also having a second comparator for generating a second output signal when the amplitude of the analogue signal at the second comparator exceeds a predetermined threshold; and  
       
       
         means for determining whether, when the second comparator generates an output signal, an output signal has been generated from the first comparator.