The present invention relates to a surface measurement probe. In particular the invention relates to a probe having a transducer which converts an electrical signal into a vibration, such that a stylus of the probe can thereby by vibrated. A change in the characteristic mode of the stylus vibration is used to determine whether the stylus is in contact with a surface. The surface measurement probe may be mounted on a coordinate positioning machine. In particular it is suitable for mounting on a manual coordinate positioning apparatus such as a manual coordinate positioning machine (CMM) or a manual articulating measuring arm.
British Patent Application No. GB 2006435 discloses a surface measurement probe with a workpiece contacting stylus. The probe is provided with a driving transducer and generating transducer which both comprise piezoelectric crystals. An alternating current is applied to the driving transducer to produce vibrations which are in turn transmitted to the stylus. Vibrations of the stylus excite the generating transducer. If the stylus makes contact with the surface, the vibrations are reduced. This reduction in vibration is sensed from a change in parameters of the generating transducer. Thus it may be determined when the stylus comes into contact with the surface.
U.S. Pat. No. 5,247,751 discloses a touch probe which is provided with an ultrasonic horn which has a piezoelectric element sandwiched between electrodes. The piezoelectric element converts an RF electrical signal into ultrasonic vibration. The probe is provided with a feeler which is brought into contact with an object to be measured. The horn is ultrasonically vibrated in accordance with the ultrasonic vibration of the piezoelectric element. The current between the electrodes is monitored and a change in the current value indicates a touch between the object to be measured and the feeler.
A first aspect of the present invention provides a method of determining drift for a surface measurement probe, the surface measurement probe having a housing, a surface contacting stylus, a vibration generator which causes vibration of the stylus, a sensing device for determining a parameter related to change in vibration of the stylus, and a comparator for determining the relationship of the parameter with a threshold, the method comprising the following steps in any suitable order:
The transition time is the time taken for the probe to detect a transition from the stylus contacting free space and a surface.
This method has the advantage that a change in parameters due to drift can be differentiated from a change in parameters due to contact of the stylus with a surface.
The parameter may comprise a phase change between drive voltage for the vibration generator and current passing through the generator. Alternatively, the parameter may comprise the following: The amplitude of the current passing through the piezos in a system which runs with constant voltage amplitude; the amplitude of the voltage developed across the piezos in a system which runs with constant current amplitude; the power dissipated by the piezos; or the power factor of the system supplying the piezos.
The vibration generator may comprise one or more piezoelectric elements.
Preferably the method includes a step for compensating for drift of the parameter. This step may include adjusting the drive frequency. Alternatively, the step may include adjusting the threshold.
A second aspect of the present invention provides a surface measurement probe comprising:
The processor may carry out the additional step of using the measure of drift to adjust the behaviour of the vibration generator in order to compensate for the effect of drift on the parameter.
A third aspect of the present invention provides a surface measurement probe, the surface measurement probe comprising:
In a preferred embodiment the vibration generator comprises one or more piezoelectric elements.
The vibration generator may be kept at a constant temperature by placing it within an oven or temperature controlled environment. This allows the effects of drift in vibration characteristics to be removed by maintaining the temperature of the key vibrating components at a constant value (either at ambient temperature or at a fixed temperature above the ambient temperature).
A third aspect of the invention provides a method of determining whether a surface measurement probe is providing reliable results, the surface measurement probe having a housing, a surface contacting stylus, a vibration generator which causes vibration of the stylus, a sensing device for determining a parameter related to change in vibration of the stylus, and a comparator for determining the relationship of the parameter with a threshold, the method comprising:
This method thereby determines whether the probe has stopped performing reliably due to receiving an acceleration above a threshold, due to being dropped or knocked for example.
The variable may comprise the parameter related to change in vibration of the stylus, for example a phase change between drive voltage for the vibration generator and current passing through the generator. The variable may comprise the voltage of the vibration generator or a force experienced by the probe.
The output may be a visual or audio signal. The output may be sent to a controller or PC via a communications link.
The method may include the step of resetting the probe in the event of an output, for example by performing a frequency sweep of the vibration generator. The frequency sweep may be completed automatically on receiving an output.
A fourth aspect of the present invention provides a surface measurement probe comprising:
A fifth aspect of the present invention provides a surface measurement probe comprising:
The heat source may provide cooling as well as heating. A temperature transducer may be provided to measure the temperature of the vibration generator. Temperature feedback may be provided from the temperature transducer to the temperature controller. Alternatively, the temperature controller may receive an input relating to the parameter, for example phase.
The invention will now be described, by way of example, with reference to the accompanying drawings in which:
The piezoelectric stack is mechanically attached to the stylus of the probe, causing it to vibrate. By varying the frequency of the drive voltage, the frequency at which the stylus vibrates can be varied.
As in the stack PZ1,PZ2, the reference signal ‘Ref. sine’, is input to the zero crossing detector 19. The differential signals developed across the single piezoelectric element PZ are input to an instrumentation amplifier 61. Its output, ‘Piezo sine’, is input to the other input of the zero-crossing detector 19, as in
The advantages of using a single element are that the probe will be cheaper to produce and its length can be reduced. The disadvantages are that more electronic components are required and the element would require insulating from the probe body.
When the vibrating stylus contacts a surface, the characteristic vibration mode of the stack oscillation changes and a measurable phase difference results.
If the measured phase difference is above the threshold value, the stylus tip is not in contact with the surface. In
The calculation of the phase difference between the ‘Ref In’ and ‘Piezo In’ signals will now be described in more detail. The FPGA (reference number 17 in
A counter in the FPGA is set to 0 on the rising edge of the ‘Ref In’ signal and increments on each master clock tick until the falling edge of the ‘Piezo In’ signal, when the count is latched. The count represents a phase difference in clock cycles, which is called the ‘phase count’. This method enables both phase advance and phase delay to be accurately measured.
As can be seen from
Other aspects of the probe are described in more detail in UK Patent applications GB0608998 and GB0609022. The contents of these applications are incorporated herein by reference.
Temperature variation of the probe can cause changes in the curve illustrated in the graph in
The change in measured phase difference caused by temperature shift is a slow change wherein the change in measured phase difference due to contact of the stylus with a surface is a fast change. The difference in rate of change can be used to determine whether the change in measured phase difference is due to temperature drift or contact with a surface, as described below.
In a first step regular measurements are taken of the phase difference. The measured phase differences determined when the stylus is not in contact with the surface are averaged. The difference between the expected phase difference (i.e. as originally tuned) and the phase difference now measured (i.e. averaged over a long period compared to a surface detection measurement cycle when not in contact with the surface) is determined. A growing error between these two values shows long term drift.
By this method the temperature effect can be tracked and compensated for by increasing or decreasing the excitation frequency. For example an increase in temperature causes the curve to move to the left resulting in an increase in the measured phase difference. To maintain the drive frequency of the steepest point of the curve, the drive frequency is decreased by a small amount. For a decrease in temperature the opposite is true.
One measurement cycle of the probe typically takes about 40 μs. The thermal temperature compensation loop may take 65,000 measurements. Thus if the probe remains off the surface during these 65,000 measurement (i.e. 2.6 seconds), thermal compensation will occur. As the thermal compensation loop is much greater then one measurement cycle, the change in phase difference due to surface contact will only have a small effect (particularly as the thermal compensation loop stops when the probe contacts the surface). As soon as the probe looses touch with the surface, the thermal compensation loop will re-start and any increase in phase difference due to the surface contact will be reduced. This example is for illustrative purposes and other values may be used.
As one measuring cycle is typically 40 μs, the time to detect that the probe is off the surface is equal to one measuring cycle, i.e. 40 μs. However, the time taken to detect that the probe is on surface is longer, it is 16 sequential measuring cycles, 16×40 μs=640 μs. By using 16 sequential measuring cycles, the number of false triggers is reduced. (Of course, another multiple of the measuring cycles may be used).
As an alternative to adjusting the drive frequency, other parameters may be adjusted for temperature compensation. For example the threshold value could be varied to maintain the phase relationship set at the tuned resonant frequency. For example, the threshold value may be kept at 4° from the long term value of the phase difference.
In an alternative embodiment an analogue system may be arranged in place of a digital system for compensation. Analogue elements may be connected in parallel or in series with the piezoelectric stack via a switching network to compensate for the changing electrical characteristics caused by temperature variation. These elements may have variable capacitance, inductance, and/or resistance which are used to change the component values in the circuit.
Another method of temperature compensation uses a digital phase advance/delay to compensate for the phase changes. This comprises mathematically compensating for the long term drift. For example for a phase change of 2°, a timer is started relative to the reference wave either 2° earlier or later to compensate for the drift. The timer measures the time between the reference wave and the measured wave.
The need for temperature compensating the vibration generator may be removed by keeping it at a constant temperature (the target temperature) by placing it within temperature controlled environment, such as an oven. This allows the effects of drift in vibration characteristics to be removed by maintaining the temperature of the key vibrating components at a constant value (either at ambient temperature or at a fixed temperature above the ambient temperature). A straightforward means for achieving this can be implemented by the addition of heating or cooling elements—for example power resistors (resistive elements that can safely dissipate electrical power as heat) or a Peltier device in intimate contact with the generator, and one of at least two alternative control regimes.
The first control regime requires the temperature to be measured by attaching a thermistor, thermocouple or other temperature transducer to the key vibrating components. A servo system can then be implemented to control the current through the heating or cooling element in order to maintain a measured temperature close to the target temperature. In the case where a heating element is attached, the target temperature would have to be above normal ambient temperature as no cooling capacity is available. The choice of a temperature above ambient means that the heating current can be increased or reduced to compensate for heat input from the vibration mechanism and also changes in the amount of heat going into the device from the surroundings e.g. from the operator handling the probe or from changes in the working environment. If a Peltier device is used heating or cooling is possible. The temperature at which the probe is initialised can therefore be selected as the target temperature, meaning no warm-up time is required for the probe.
The second control regime uses the measurement of phase counts to establish whether the generator vibration characteristics are drifting, instead of using a thermistor or similar to measure temperature. The drift is compensated for by having a low bandwidth current control loop (with a time constant of the same order of the thermal time constant of the generator) which uses the difference between the measured phase count and target phase count as the error signal, Phase count error, allowing the generator to cool when the phase count is too high or low and warming it up when the phase count is too low or high (the sense of the change depending upon which side of resonance the operating point is chosen to be). It is important that a low bandwidth controller is used as this type of controller can not fully compensate for rapid changes, only gradual ones. In this case, the effects of temperature drift are gradual, and the effects of the stylus contacting the surface are rapid. So a low bandwidth controller can fully compensate for temperature induced changes, but only compensates for changes caused by stylus contacts very slowly. A change in the phase count which is greater than a particular threshold indicates that the stylus is touching a surface, in which case the current feedback is held at the value prior to the large phase count change. This ensures that thermal run-away does not occur due to the current servo system trying to correct thermally what is not a thermal drift (but what is in fact due to the stylus contacting the surface) when the probe is in constant use. This method of maintaining stable operation has the advantage that the quantity of interest (the phase count when not on the surface) is that being directly controlled, and the temperature control of the generator is a side effect rather than the temperature being controlled to try and maintain a stable phase count. Most straightforwardly, the heating and cooling can be achieved in the same way as in the first control method, using a Peltier device. Where a resistive heating element is used there is no direct measure of temperature, so a warm up time is required where a base current is applied to the power resistor for a period of time before the probe can be tuned and used. This application of a known current for a known period of time, into a known thermal inertia will raise the temperature by a reasonably well defined range (depending upon variations in thermal losses), which will be within the operating temperature range of the probe.
These methods which do not adjust the value of the drive frequency have the disadvantage that enough long term drift can cause the drive frequency to no longer correspond to the steepest part of the curve. In this case the measurements become unreliable and the probe should be re-tuned. The probe may output a signal to indicate the probe needs re-tuning.
The present invention provides some crash protection for the probe. If the probe suffers a hard knock, the generator may start to vibrate in a different mode. In this state, reliable measurements cannot be obtained from the probe.
Empirical observations indicate that a large change in phase is measured over a very short period of time (e.g. microseconds) when the stylus is subjected to a hard knock; the change is much larger than could be produced from a normal surface touch on any material and far quicker than temperature drift could produce. Thus the normal measuring process can detect such an event.
Experimentation has also shown that the piezo-electric elements may be returned to their normal mode of vibration by performing a frequency sweep following a knock. This frequency sweep may be done very quickly by performing the sweep over a short range, for example over the frequency range which contains the expected highest gradient. The short sweep has the advantage of taking only a fraction of a second, whereas a full sweep would typically take a few seconds. Thus the short sweep can be performed in the time it takes for an operator to pick up the probe.
A hard knock can be detected by monitoring the generator's output. When piezoelectric elements are subjected to a force, large voltages can be generated for a short period of time. By monitoring this voltage, a knock can be sensed and reported.
Alternatively, an accelerometer or other device that measures a change in force can be used to detect and report a hard knock.
A hard knock can also be detected by monitoring the phase difference.
Alternatively, the probe can automatically re-tune itself if it detects a crash. It may be set to either do a full frequency sweep or a short sweep.
Number | Date | Country | Kind |
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0608999.9 | May 2006 | GB | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/GB2007/001667 | 5/8/2007 | WO | 00 | 10/27/2008 |