Geometrical instruments – Gauge – Movable contact probe – per se
Reissue Patent
1997-07-25
2001-01-30
Fulton, Christopher W. (Department: 2859)
Geometrical instruments
Gauge
Movable contact probe, per se
C033S561000
Reissue Patent
active
RE037030
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a touch probe, used on a coordinate positioning machine such as a machine tool or coordinate measuring machine to enable the inspection of, for example, the dimensions of machined components, and to a signal processing circuit which acts as an interface between the probe and the control of the machine on which the probe is used. Coordinate positioning machines typically comprise an arm to which the probe is mounted, and which is movable relative to a table on which a component or workpiece to be inspected is supported. The machine includes one or more transducers which measure displacement of the arm from a reference position which is usually fixed relative to the table.
2. Description of Related Art
A known probe includes a fixed structure such as a housing by which the probe is mounted on the arm of the machine, and a stylus supporting member supported relative to the housing in a repeatable rest position, from which the supporting member may be displaced when a deflecting force is applied thereto, and to which it may return when the deflecting force has been removed. Measurements are taken by operating the machine to move the arm until a stylus connected to the supporting member comes into contact with the surface of the part to be inspected, and, upon detecting such contact, taking readings from the transducers of the machine to determine the position of the movable arm relative to the reference position. The probe includes one or more analogue sensors which emit signals indicating contact between the stylus and the surface whose position is to be measured. These sensors may sense displacement of the stylus supporting member relative to the housing, or, in high accuracy probes, the sensors may sense deformation of the stylus and/or the stylus supporting member which takes place before such a displacement occurs. Probes employing displacement or deformation sensors are shown in U.S. Pat. No. 4,153,998 or U.S. Pat. No. 4,177,568 respectively.
Signal processing circuitry, which acts as an interface between the probe and the machine control, emits a step-change, or “trigger” signal when the analogue signal from the sensor inside the probe has attained a predetermined threshold set in the interface. The trigger signal instructs the machine control to determine the position of the movable arm and arrest motion of the arm in order to prevent damage to the machine. A small scale movement of the arm relative to the part to be inspected after contact between the stylus and the surface has occurred, known as “overtravel”, is accommodated by the ability of the stylus supporting member to deflect relative to the housing of the probe.
SUMMARY OF INVENTION
A first aspect of the present invention relates to the optimum location of deformation sensors in such a probe. According to a first aspect of the present invention, a touch probe includes a fixed structure, by which the probe may be supported on the movable arm of a coordinate positioning machine, and a stylus supporting member biased into a repeatable rest position relative to the first structure, from which the supporting member is displaceable when a deflecting force is applied thereto, and to which it may return when said deflecting force is removed, the direction of biasing action defining a probe axis, the fixed structure and stylus supporting member forming at least part of a load path between a stylus securable to said stylus supporting member and said movable arm, wherein the probe comprises 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, each said sensor having an axis of maximum sensitivity to tension and compression, wherein said sensors are provided in said load path and are grouped into pairs, the axis of maximum sensitivity of each sensor in a pair of 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 the pair.
In one embodiment, the sensors are provided between two parts of said stylus supporting member. In a further embodiment, the sensors are provided on the fixed structure, and are pre-stressed by the weight of, and/or biasing action on the stylus supporting member.
The deformation sensors may, for example, be provided by strain gauges (and associated load cells), or by piezoelectric sensors. The output from such sensors increases with increased deformation of the stylus and/or the stylus supporting member, up to a maximum value at which displacement of the stylus supporting member relative to the housing occurs. A further independent aspect of the present invention relates to an advantageous configuration of piezoelectric sensor which provides good signal to noise characteristics. According to a further aspect of the present invention, a piezoelectric sensor for sensing tension and compression between two conducting surfaces comprises first and second piezoelectric elements provided between said surfaces, each of which generates a polarisation of electric charge in a direction extending between said surfaces upon tension or compression thereof in said direction, wherein said elements are stacked one upon another and have oppositely directed polarities, the sensor further comprising means for insulating each of said surfaces from said elements, means for equalising the electric potential of said surfaces, a first electrode connected to one of said elements at a point adjacent one of said surfaces, and a second electrode connected to said one element at a point adjacent its abutment with the other of said elements.
This arrangement enables a larger manifestation of the electric charge polarisation, created by e.g. compression of a stack of elements, in the form of a voltage, by minimising the effect of stray capacitance between the electrodes and the surfaces.
A problem which occurs particularly with probes which have analogue sensors indicating deformation of the stylus and/or stylus supporting member, is that vibration of the machine during movement of the movable arm causes the sensors to emit signals whose value exceed the threshold set in the interface, causing the interface to emit a “false” trigger signal (i.e. a trigger when no contact between the stylus and a surface has occurred). To overcome this problem, interfaces have been provided which generate an initial “latching” signal when the signal level from the probe reaches a predetermined threshold, and a subsequent confirmation signal some time later if the signal level from the sensor is still above the threshold. The latching signal causes the machine control to register the position of the movable arm; the confirmation signal validates the “latched” position reading and is also used to arrest movement of the movable arm. An interface of this type is described in U.S. Pat. No. 4,177,568. Machine vibrations causing isolated increases in the analogue signal level above the predetermined threshold thus fail to cause false trigger because the signal output from the analogue sensor does not correspond to the trigger signature required by this interface. However, a machine vibration resulting in the generation of a latching signal may occur sufficiently close to a genuine contact between the stylus and a surface for the confirmation signal emitted by the interface in respect of the genuine measurement event to confirm the validity of the position measurements latched in respect of the analogue signal resulting from the machine vibration; the resulting measurement will thus be the position of the movable arm at which the machine vibration occurred, rather than the position at which the stylus contacted the surface (an event which occurred a very short space of time afterwards).
To overcome this problem, a third independent aspect of the present invention provides an interface for connecting a measuring probe to a control for a machine on which said probe is used, the inter
Hellier Peter K.
Lloyd Peter G.
McMurtry David R.
Fulton Christopher W.
Oliff & Berridge PLC.
Renishaw plc
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