WO1984000809A1

WO1984000809A1 - A probe for displaying surface deviations

Info

Publication number: WO1984000809A1
Application number: PCT/US1983/001279
Authority: WO
Inventors: Gary M Ackerman; Nicholas Edgington
Original assignee: Ait Corp
Priority date: 1982-08-19
Filing date: 1983-08-18
Publication date: 1984-03-01
Also published as: EP0116633A1

Abstract

An indicator for displaying measurement and surface deviation from initial reference measurement level which includes the probe needle (N) positionable against the surface being measured. The probe needle (N) provides a control signal representative of probe needle (N) position and there is provided display (Y) for displaying the relative surface deviation from the reference measurement level. The sensor from the probe needle (N) comprises a capacitive sensing layers comprising a fixed plate (22A) and a movable plate (22) with the probe needle (N) secured to the movable plate (22) whereby the movable plate moves with the probe needle (N) to cause a change in capacitance between the plates thereby generating the control signal.

Description

A Probe For Displaying Surface Deviations

Background of the Invention

The present invention relates in general to a digital indicator and pertains, more particularly, to a digital indicator for measuring surface deviation or part smoothness. The digital indicator of this invention is in particular meant as a replacement for a conventional dial gage, commonly used in measuring surface deviations such as in checking the smoothness of a surface.

The conventional dial gage is primarily used as an inspection device and is used both for in-process machine part inspection on a machine, and is also used for inspection in a qualified inspection room. Nearly every aspect of industrial manufacturing has the need for assuring that dimensions and their tolerances are satisfied and maintained either on in-process procedures or in inspection rooms. There are virtually an unlimited number of procedures in industry today that require one to know the accuracy with which these procedures can be preformed. The standard technology used today in carrying out these measurements has been limited primarily to that of mechanical means. The most common device used to mechanically make these measurements is the dial gage indicator.

Fig. 1 herein illustrates a conventional mechanical dial gage which comprises four basic components incluαing a movable needle 1 gage body 2, indicator dial 3, and the zero adjust knob 4. In present today conventional dial gages, the body 2 contains a complicated system of gears which cooperate in translating a deflection of the movable needle 1 into a deflection of the dial needle of the indicator 3. The dial needle of the indicator 3 is typically callibrated in some subdivision of inches. With this conventional dial indicator, there are undesired constraints on both the manufacturer and the user.

For the manufacturer, the accuracy of the gage depends on the accuracy of the gear system and also the accuracy of the construction of the movable needle, particularly its length. The gear system requires the use of very small gears that are required to be built and assembled very precisely. This makes for a painstaking and expensive operation. Also, the gear system determines the calibration of the device. That is, a given deflection of the movable needle 1 causes a certain movement of rotation in the gears resulting in the proper deflection of the dial indicator. However, as the gears wear and get sloppy with use, the calibration is lost and the device is rendered less or completely useless for the user. The calibration is carried out in the factory, but once the mechanics of the device change, calibration is lost without any simple way of correcting the matter.

In FIG. 1, the zero adjust knob 4 allows the user to make the dial read zero for any given position of the needle. With the zero adjust knob in the normal position, the dial reads zero when the needle and body are in line. This is shown in FIG. 3 with the needle in its solid position. As pressure is exerted in either direction on the probe as demonstrated in Fig. 1, the needle of the indicator 3 will correspondingly move away from the zero position thus indicating an accuracy of measurement.

Reference is also now made to our U.S. Patent No. 4,200,986 and to pertinent prior art references listed as citations and mentioned therein. This patent describes a digital indicator that interprets and displays digitally the measurement of surface deviation. The Invention

It is an object of the present invention to provide an improved digital indicator particularly adapted for replacement of a dial gage, now commonly in use. Accordingly, there is provided a solid state electronic circuit in place of the previous mechanical system.

Another object of the present invention is to provide an improved digital indicator that can be constructed much more simply and at reduced expense, particularly in comparison with the standard mechanical system.

A further object of the present invention is to proide an electronic digital indicator that eliminates the wear of the internal parts that occurs in a mechanical system. Also, sloppiness is greatly reduced with the electronic system as there is no settling and shifting of the mechanical parts.

Still another object of the present invention is to provide an electronic digital indicator wherein the calibration process is greatly simplified and wherein the length of the movable needle plays a smaller role in determining overall system accuracy.

Still a further object of the present invention is to provide an improved electronic digital indicator wherein recalibration is accomplished easily and can be accomplished by the user rather than the necessity of factory correction. Furthermore, the recalibration can be carried out without requiring disassembly of the device.

Another object of the present invention is to provide a digital indicator characterized by a precise digital readout rather than the approximate readmgs from a dial gage.

To accomplish the foregoing and other objects of this invention there is provided a solid state electronic indicator meant to replace the previous mechanical systems and having accurate and high resolution digital display for providing measurements in surface deviation and out of round conditions. The indicator in accordance with the present invention includes a movable needle member which may also be referred to as a plunger or probe for contacting the surface that is to be measured. Preferably, means are provided for supporting this needle, plunger or probe, so that it is positioned in contact with the surface with at least a minimum predetermined pressure against the surface that is being measured. The basic measurement transducer in accordance with the present invention is of a capacitive type.

Brief Description of the Drawings

Numerous other objects, features and advantages of the invention should now become apparent upon a reading of the following detailed description taken in conjunction with the accompanying drawing, in which:

FIG. 1 is a diagram of a prior art dial gage;

FIG. 2 is a schematic illustration of the capacitive principle of this invention; FIG. 3 is a circuit diagram of the oscillator circuit used in the invention including associated waveforms;

FIG. 4 shows additional circuitry used in the invention including a counter chip;

FIG. 5 is a circuit block diagram including the microprocessor;

FIG. 6 illustrates the calibration technique of the invention;

FIGS. 7A and 7B show the respective plates of the compacitor sensor;

FIG. 8 is a cross sectional view through the compacitive transducer showing the different layers that are formed in making the transducer; and

FIG. 9 schematically illustrates the manner in which the plates move relative to each other for varying compacitance as to needle deflection.

Detailed Description

Hereinbefore, in FIG. 1, there is illustrated the conventional dial gage with its movable probe, body, reading dial, and zero adjust knob. The remaining drawings show the concepts of the present invention embodied in a capacitive technique for sensing or measuring surface deviation.

The capacitor is a passive electronic component which has the ability to aquire and store energy in the form of an electric field. physically, it comprises two plates P1 and P2 usually constructed of some type of a conductive material and separated by dielectric material D1 which is typically an insulating material. FIG. 2 shows these basic components of a capacitor with associated leads L1 and L2 extending from the respective plates P1 and P2.

The value of the capacitance associated with the capacitor is given by a known equation. The capacitance is directly proportional to the area of the plates and also directly proportional to the dielectric constant. On the other hand, the capacitance is inversely proportional to the distance between the plates P1 and P2. The dielectric constant is a predetermined number associated with the particular insulating dielectric material that is chosen. Assuming that the distance between the plates D1 and D2 is not varied and that the dielectric constant remains fixed, then the amount of capacitance associated with the capacitor is dependent solely on the area of the plates P1 and P2.

Thus, in accordance with the present invention, one plate of the capacitor, such as plate P1 is fixed, and the other plate such as plate P2 illustrated in FIG. 2 is attached to a movable needle. As the movable needle goes through some deflection, the plate that the movable needle is attached to, namely plate P2 also moves relative to plate P1. The result is a change in the aligned areas associated between the two plates therefore changing the capacitance associated with the plates. By placing this variable capacitor in an electronic circuit, it is possible to measure the change in capacitance very accurately. Consequently, the deflection of the needle is also measured very accurately. In accordance with the invention, there is also provided peripheral electronics to provide a digital readout of variations in surface deviation of the tooled piece from an ideal reference level.

FIG. 3 shows an oscillator circuit in accordance with the invention and associated therewith in FIG. 3 are also shown pertinent waveforms. In FIG. 3 there is included a time constant circuit of capacitor Cl and resistor Rl coupled between the input and output of inverter gate I. When the input of gate I reaches the voltage V_a, then its output switches to the voltage V_b. Alternatively, when the input of the gate I reaches the voltage V_b, then its output switches to the voltage V_a. The actual output voltage from the circuit of FIG. 3 is the square waveform depicted in FIG. 3. This waveform is an oscillating one that oscillates between the voltages _Va and Vb with a defined period T. The period T is a function of the rate at which the capacitor increases or decreases the amount of energy that it stores. This rate is dependent on the value of the capacitor. As the capacitance chances, so does the rate at which energy is stored or dissipated, and consequently so does the period T of the oscillator.

Thus, assuming that the value of the resistor Rl stays constant, then by determining a change in the value of the period T, a change in the value of the capacitance is derived which in turn is equivalent to a change in the location of the movable needle.

Reference is now made to FIG. which shows the oscillator circuit O responsive to movements of the movable needle N. The output of the oscillator O couples to the counter chip CC. The counter chip includes a switch X and counter CTR. The output of the oscillator is referred to as the clock-in signal. Each time this signal experiences a negative going transition from a high voltage level V_a to a low voltage level V_b, the counter increments by one count. There are also illustrated in FIG. two other control inputs associated with the counter. There is a reset input which is used to reset the counter to zero. There is also provided a clock enable input which can enable the counter to increment when the clock input goes through a transition or can essentially disable the clock input from reaching the counter. This is illustrated in FIG. schematically by the switch X which is essentially opened and closed under control of the clock enable signal. When the clock input is disabled, the counter retains the value it has reached even though the clock input is still experiencing transitions.

The control of the clock enable and reset lines illustrated in FIG. is accomplished with a microprocessor MP such as in the diagram of FIG. 5. The microprocessor also is usable in establishing tne device calibration and the zero adjustment.

The microprocessor MP initially resets the counter output to zero. It then enables the clock for a predetermined length of time such as say, X seconds. During this time the oscillator causes the counter to count up. After X seconds have passed, the microprocessor disables the clock by opening switch X schematically illustrated in FIG. 4. The microprocessor then reads the output of the counter and stores this count in the microprocessor memory. The microprocessor then resets the counter output to zero once again and re-enables the clock. During the time that the microprocessor is waiting for X seconds to expire, the microprocessor compares the counter output with two reference counts which were previously placed in memory during the calibration process. Depending on whether the output of the counter falls within this reference range, the microprocessor determines the deviation of the needle from its zero position and displays this on the digital output display Y. The microprocessor then waits again until the X seconds has elapsed and the process is repeated.

The calibration process comprises the following steps:

1. The device is switched into the calibration mode.

2. With the needle in its rest position the counter is enabled to count through X seconds and then the clock is disabled.

3. The output of the counter is stored in memory.

4. The needle is deflected by a known amount.

5. The output of the counter is reset to zero.

6. The clock is enabled and there is a count provided through the X seconds interval and then the clock is disabled.

7. The output of the counter is stored in the reference memory of the microprocessor in a second location.

It is noted in the operation of the electronic circuit of this invention that for different positions of the needle there is obtained different values of capacitance corresponding to different clock periods T resulting in different count outputs for the same clock enable time interval. By reading the variations in the count for the different needle locations, the device is calibrated.

In accordance with the present invention the device preferably has two modes of operation including a normal operating mode and a calibrate mode. A simple push button switch is usable by the user to decide which mode of operation to initiate. Depressing the switch puts the device in the calibration mode with the operation as follows.

The device is placed into a calibration test jig which consists of a rail to slide the gage along in a surface with a known step in it. Tne movable needle has some initial angle on it corresponding to some period T. It is then slid along over the step creating a new angle and a new period T. Reference is made to FIG. 6 which snows the device and associated step.

By way of example, it is assumed that the first angle corresponds to an oscillator frequency of 450 KHz and a second step to a frequency of 550 KHz. The microprocessor determines the difference between the two frequencies which for our example is 100 KHz. It is also assumed for this example that the known step is 0.001 inches. The microprocessor now knows that a step of 0.001 inches corresponds to a change in oscillator frequency of 100 KHz. This becomes the calibrated reference and is stored permanently until another calibration is desired. This storage is in a non-volatile memory location and the calibration process is completed.

In accordance with the present invention there is provided an incredibly simple calibration process not at all possible with the counterpart mechanical device. In a mechanical device the calibration is incorporated into the gear system design including the length of the needle. Once this design is complete, every device manufactured must be made precisely in accordance with the design. Any deviation from the design results in inaccuracy in the device calibration.

However with the electronic indicator, the operation is self-calibrating. Any difference from one device to the other such as parasitic capacitance or different needle lengths is automatically compensated for in the calibration process. Also, the strict quality control aspect of the manufacturing process is greatly reduced.

In the foregoing example, reference was made to differences in frequency. Actually, the microprocessor does not compute frequency, but rather counts the number of oscillator clock pulses over a given period of time namely, X seconds. It then performs its comparisons with these counted numbers. For easier descriptive purposes, oscillator frequencies have been used rather than counted numbers but it should be noted that the actual algorithm of operation is in accordance with counts rather than frequencies.

Now that the device has been calibrated, it is now capable of use in making an actual measurement. The device may be placed into a test jig. The needle is initially displaced from its rest position by some angle. Let this angle correspond to a frequency of say 325 KHz. The device zero button such as of the type illustrated in FIG. 1 is then depressed causing the microprocessor to store this frequency in a memory location. It also causes the digital display to read zero. The device is then slid along the surface and eventually hits a deviation in the surface smoothness creating a new angle between the needle and body. This new angle corresponds to a new oscillator frequency of, for example, an assumed frequency of 375 KHz. The microprocessor then does the following calculations to determine the change in height from the reference level.

1. Obtain a difference between the measured frequency and zero or reference frequency, namely, 375 KHz - 125 KHz = 50 KHz.

2. Divide the difference by calibrated frequency difference referred to hereinbefore, namely 50 KHz/100 KHz = 1/2.

3. Multiply the quotient by the calibrated height difference, namely, 1/2 (0.001 inch) = 0.0005 inch. 4. Display the result.

Thus, the digital display Y illustrated in FIG. 5 reads 0.005 inches rather than zero. Thus, the deviation of the surface from the ideal smoothness is 0.005 inches.

If it is assumed that the needle goes over this deviation and returns to the same angle it had initially, then the microprocessor is continuously updating the digital display and therefore it goes tnrough the same process again, but now returning to a reading corresponding to the initial reference reading. This in the assumed example would then display a zero on the digital readout. This indicates that the surface is back to its nominal height.

This simple four step algorithm is a result of the calibration process. From the calibration process the following ratios may be established:

Known change in frequency Measured change in frequency

Known distance = Unknown distance For our example, this calculates as:

550 KHz - 450 KHz 375 KHz - 325 KHz .001 inch ⁼ x inches x = 0.005 inches.

In accordance with the present invention the device is designed to measure small changes in distance by changes in capacitance. The basic sensor or transducer referred to hereinbefore preferably comprises a number of overlapping teeth made of a conductive mate rial deposited on a non-conductive substrate. The teeth are separated by a thin dielectric film which is also used as a bearing surface between the two plates. As one plate is moved in relation to the other, the overlapping areas change, thus changing the capacitance from a minimum intrinsic capacitance with no overlap, to a maximum capacitance with full overlap.

One of the problems encountered with a system as described herein, is that the variation in the dielectric constant and in the thickness of the dielectric will vary causing alterations in the readings taken. This is compensated for in accordance with the present invention by using a CMOS microprocessor that has a non-volatile memory which can store readings for a known calibration distance and use this information to calculate the actual distance moved regardless of the dielectric. Also, temperature and aging effects are compensated for by using a fixed capacitor which functions as a reference. The fixed capacitor is either used as the master microprocessor clock or as a second oscillator which is read during a measurement. Herein before, there has been a discussion of the campacitive sensor of this invention. In fact, this capacitive sensor is shown very schematically in Fig. 2. The details of a prefered embodiment are now illustrated in Figs. 7A, 7B, 8 and 9. Thus in Fig. 7A there is shown a first plate 20 which has dimensions of 1 3/4 inch by 7/8 inch. This plate is made of a nonconductive ceramic rectangle 22. By means of a thin film deposition process, a conductive material 24 is deposited on each plate creating a predetermined, well-defined pattern of conductive material. The pattern on the second plate is illustrated in Fig. 7B and may be considered as the negative or dual pattern to that snown in 7A. in Fig. 7B there is shown a ceramic plate 20A, consisting of a nonconductive ceramic rectangle 22A having deposited thereon by a thin film deposition process a conductive material 24A. Both of the plates shown in Figs 7A and 7B have the same dimensions of 1 3/4 inch by 7/8 inch.

With regard to Fig. 7A at the center of the edge 25 is a semicircle 26 with a diameter of 1/10 inch. From the edge of this semicircle eighteen fingerlike conductive strips 24 extend out terminating on the edge of the rectangle. In Fig. 7A all of these strips terminate on either edge 27 or edge 28 with the exception of the two longest fingers which terminate adjacent to the edge 30. Each of the fingers occupys a 5 degree arc. The first finger starts at 2.5 degrees and ends at 7.5 degrees. It is noted that these angles are each the angle between the sharp edge of the rectangle and the edge lines of the fingers. It is noted that the edge of the finger at say 7.5 degrees does not actually reach the edge 28 of the rectangle. This is due to the fact that the finger has a curved tip as clearly illustrated in Fig. 7A. This curvature is obtained by making a 5 degree arc which corresponds to a portion of the circle whose radius equals the length of the line at 2.5 degrees taken from the center of the semicircle 26.

The fingers are then repeated as illustrated in Fig. 7A with the curvature of each finger obtained by making a 5 degree arc which corresponds to a protion of the circle whose radius equals the length of the line which actually reaches the rectangle edge, this being either edge 27 or edge 28. Thus the length of the lines at 2.5 degrees 12.5 degrees, 22.5 degrees, etc. correspond to the circled radius needed to obtain the correct arc for that particular finger. For the two largest fingers the arcs are obtained as follows. An arc corresponding to a portion of a circle whose radius is 1 3/4 inch is swung along the far edge of the rectangle. The arc is tangent to the rectangular edge 30 at the center of the rectangular edge. The fingers then terminate on this arc as shown in Fig. 7A. The pattern in the plate illustrated in Fig. 7B is very similar to that shown in Fig. 7A for the first plate. In Fig. 7B the plate also contains a series of conductive fingers each of 5 degree width. This rectangular ceramic plate with the conductive fingers deposited thereon has edges 25A, 27A, 28A, and 30A. The fingers that are deposited may be deposited in the same fashion as with the plate shown in Fig. 7B. The only difference is that the fingers on the second plate shown in Fig. 7B essentially occupy the intermediate angles not occupied by the fingers in the first plate. This is why we refer to one plate as being considered as the negative of the other plate. By way of example, in Fig. 7A a conductive strip appears between 2.5 degrees and 7.5 degrees. A blank space occurs on this plate between 7.5 degrees and 12.5 degrees. In Fig. 7B there is a blank space between 2.5 degrees and 7.5 degrees and the finger instead appears between 7.5 degrees and 12.5 degrees. Accordingly, when both of the plates overlap there is essentially no overlap between the fingers but instead the fingers of one of the plates essentially fills the space between fingers of the other plate.

Fig. 7B also shows the needle N which is secured to the base of the plate. It is the needle N that is deflected from its midrange position which causes a shifting of the plate shown in Fig. 7B relative to the fixed plate shown in Fig. 7A. It is this shifting that causes a variance in capacitance in accordance with the invention.

Because the needle has a maximum deflection of ± 5 degrees from its center position, the plates in accordance with the present invention are chosen to experience a maximum change in common area for this amount of movement realtive to each other. Thus, there is a correlation between the maximum deflection that is desired with regard to one of the plates and the selection and the selection of finger width. Also, it is prefered that the amount of compacitance should be relatively large to minimize tne effect of parasitic or stray compacitance which might be present. Since one is looking for a large compacitance and a large changing compacitance one should try to maximize the area of the ceramic plate and choose a conductive pattern which optomizes the area on the plate. Also, the change in compacitance as a function of angle should oe linear and this is readily accomplished with the finger or compacitance arrangement of this invention as illustrated in the drawings.

Tne dial indicator of the present invention will preferably be constructed in a rectangular shape and therefore the choice of a rectangular ceramic plate is desired so as to optomize space considerations within the indicator itself. When the plates are places directly on top of each other with the needle in its rest position at zero degrees there is no common area and the compacitance is at its minimum. As the movable plate, the one to which the needle is attached, swings through some angle, the change in area causes a change in compacitance. This is a linear change which is a function of the angle. For a full 5 degree deflection, the common area of the two plates is maximized and is equal to approximately 1/2 the area of the entire plate. This provides for a relatively large utilization of the ceramic plate area along with creating a large amount of compacitance change for a small amount of change in angle.

Both of these ceramic plates have, in addition to the layers of conductive material, also have another layer deposited during the thin film deposition process. First, the conductive layer is deposited on to the ceramic. This is shown in Fig. 8 by the layers 24 and 24A on the respective ceramic layers 22 and 22A. Thereafter, in insulating material, specifically a dielectric, is deposited on top of the conductive material. This dielectric layer is shown as a single layer in Fig. 8 even though it is actually two separate layers. Fig. 8 identifies this as layer 23. This dielectric layer prevents the two plates from shorting out to each other along with determining the net amount of compacitance associated with the plates. Fig. 9 schematically illustrates the manner in which the fingers overlap for some predetermined rotation of the needle N.

What is claimed is :

Claims

1. An indicator for displaying measurement in surface deviation from an initial reference measurement level comprising; a probe means positionable against the surface being measured, means resposive to probe position for providing a control signal representative of probe position and display means for displaying the relative surface deviation from the reference measurement level, said means responsive to probe position comprising compacitive sensing means comprising a fixed plate and a movable plate with said probe means secured to said movable plate and wherein said movable plate moves with said probe means to cause a change in compacitance between said plates representative of said control signal.

2. An indicator as set forth in Claim 1 wherein said means for providing a control signal includes oscelater means having an output frequency as a control signal and corresponding to the interplate compacitance that is sensed.

3. An indicator as set forth in Claim 2 further comprising a counter means responsive to the output of said oscelator for providing a digital count representative of sensed compacitance which in turn is representative of surface and deviation.

4. An indicator as set forth in Claim 2 wherein said oscelator circuit comprises an inverter and associated R-C circuit.

5. An indicator as set forth in Claim 1 wherein each of said compacitive plates has a plurality of fmger-like conductive strips with the strips of one plate overlying the open area of strips of the other plate when the plates are aligned.

6. An indicator as set forth in Claim 5 wherein each finger strip is a circle segment of small width.

7. An indicator as set forth in Claim 6 wherein said segment width corresponds to the desired maximum deflection of said probe means.