Capacitive Angular Position Transducer

This invention relates to an angular position transducer, and more particularly to a capacitive angular position transducer.

In many applications, the angular position of a shaft must be sensed over a limited range, for example 60.degree. or .+-.30.degree., to produce a proportional analog signal. This is sometimes required as a position feedback signal to a servomechanism turning the shaft, or simply to provide an electrical output signal proportional to the position of a shaft which may be used, for example, on a chart recorder. It is sometimes important that the transducer not provide additional mechanical load, such as may occur due to friction in the case of a potentiometer being used for that purpose. It is also sometimes important that the transducer operate over a significant temperature range, for example -20.degree. to +180.degree. F., and that axial motion of the shaft not affect the performance of the transducer.

In accordance with the present invention, a printed circuit board is mounted perpendicular to a shaft, and a capacitor vane, preferably bifurcated, is clamped to the shaft, or vice versa. The capacitor vane embraces both sides of the printed circuit board on which are two pairs of symmetrical capacitor plates, one pair on each side, with the pair on one side directly opposite the pair on the other side. The corresponding (opposite) plates are connected together at a junction on the board, thereby to connect in parallel the variable capacitor formed by each with the plates of the bifurcated vane. As the vane is pivoted on the axis of the shaft, or vice versa, the capacitance value of one pair of plates will increase while the capacitance value of the other pair will decrease. Because the capacitance value of each pair is the sum of the capacitance between the vane and each plate of the pair, any axial movement of the shaft that affects the spacing between the vane and the pair of plates has only a negligible effect on that sum.

The junction of each of the two pairs of plates is connected to circuit means on the printed circuit board for controlling the ration of Mark and Space periods of a periodic squarewave in proportion to the ratio of the capacitances of the two pairs, and converting the squarewave into a dc signal which is proportional to that ratio, and therefore proportional to the angular position of the shaft which positions the capacitor vane.

The circuit means may also include further means for dividing the frequency of the Mark-Space signal by two, thus producing a symmetrical squarewave output signal. Upon converting that output signal to a dc signal proportional to its Mark-Space ratio, a reference signal is produced that is proportional to the midposition of the vane. By differencing the actual position dc signal with this midposition signal, a signal equal to the difference is obtained that is proportional to the deviation. Upon adding a dc offset, the final output signal will vary from zero to a maximum as the shaft rotates the vane from one extreme position to another.

The novel features of the invention are set forth with particularlity in the appended claims. The invention will best be understood from the following description when read in conjunction with the drawings.

FIG. 1 is an isometric view of shaft position transducer which utilizes a bifurcated vane with two pairs of capacitor plates on a printed circuit board, one pair on each side, with the plates of a pair directly opposite each other connected through the circuit board that is standing between the two sides of the bifurcated vane according to the present invention.

FIG. 2 is a plan view of one capacitor plate of each pair on one side of the printed circuit board, with the position of the bifurcated vane shown in phantom lines.

FIGS. 2a and 2b are front and side elevation, and 2c is a block diagram.

FIG. 3 is a side elevation of the bifurcated vane.

FIG. 4 is a plan view of the bifurcated vane with the printed circuit board positioned between the sides of the bifurcated vane.

FIG. 5 is a diagram of the circuit means used to convert the vane position over the pairs of capacitors to provide a signal proportioned to shaft position.

FIG. 6 is a waveform diagram useful in understanding the invention.

In the embodiment of FIG. 1, there is shown a transducer for producing a signal proportional to the angular position of a shaft 10 comprised of a vane 12 which is preferably bifurcated into plates 12a and 12b, and a base 12c.

This bufurcation is desirable because it increases the capacitances of the transducer. If this increased capacitance is not needed, the vane need not be bifurcated.

The vane is locked on the shaft by a screw 13 shown in FIGS. 3 and 4 so that it will turn with the shaft over the desired angle in either direction. The shaft 10 has journals 10a and 10b in an E-shaped bracket 14 for support in space relative to a printed circuit board 16 on which capacitor plates 17a and 18a are provided on one side of the board 16. Identical and directly opposed capacitor plates 17b and 18b are provided on the other side of the board as shown in FIG. 2a. A plan view shown in FIG. 2b shows the shape of the plates 17a, b, and 18a, b, relative to the bifurcated vane 12.

As the shaft 10 moves in angular position, the vane moves from the midposition shown in FIG. 2b in either direction over the capacitor plates 17a, b and 18a, b. This changes the relative capacitance of the vane with the plates 17a, b and 18a, b. Circuit means on the board 16 transforms this change in capacitance into a change in a dc position signal which may be used, for example, on a chart recorder.

Two pairs of capacitor plates 17a, b and 18a, b are used with the bifurcated vane in order that change in the axial position of the shaft will not cause any change in the angular position signal. This results in having four variable PG,5 capacitors electrically connected as shown schematically in FIG. 2c. The capacitors C.sub.1a and C.sub.1b are always equal to a value that depends on angular position, except to the extent that the spacing S.sub.1 and S.sub.2 may be unequal due to axial motion of the shaft 10. But since the space S.sub.1 decreases as the space S.sub.2 increases, and vice versa, to increase and decrease the respective capacitors C.sub.1a and C.sub.1b, the capacitance C.sub.1a may not always remain equal to the capacitance C.sub.1b. However, their ratio will always remain the same. The same applies to the other pair of capacitor plates 18a and 18b.

The vane 12, having plates 12a and 12b forming the variable capacitors C.sub.1a, C.sub.1b and C.sub.2a, C.sub.2b, is grounded through the shaft, which in turn is electrically connected to the bracket 14. The board 16 is also referenced to the same circuit ground by connection of a ground conductor on the board to the bracket which supports it, as indicated schematically in FIG. 2c.

FIG. 3 illustrates the bifurcated vane in elevation. A slot 12d cut at an acute angle with respect to the axis of the base 12c receives the shaft. To lock the bifurcated vane on the shaft so that it will turn with the shaft, a set screw 13 is used in a threaded hole 12e to press the shaft firmly against the back of the slot 12d.

FIG. 4 is a top view of the bifurcated vane. Shown between the plates 12a and 12b is the printed circuit board 16 with the capacitor plates on opposite sides. The spacings S.sub.1 and S.sub.2 shown exaggerated in FIG. 2a are shown here more realistically.

Referring now to FIG. 5, the circuit on the board 16 is comprised of a relaxation oscillator 21, such as of the type commonly referred to as an astable multivibrator (MV), with two timing capacitors, which may be of the type more commonly known as multivibrators (MV). These oscillators utilize the parallel connected variable capacitors C.sub.1a, C.sub.1b and C.sub.2a, C.sub.2b for RC timing of Mark-Space periods of a square waveform shown in FIG. 6. When the two pairs of parallel connected capacitors are of equal capacitance, the square waveform is symmetrical, as shown in waveform A of FIG. 6. If they become unequal in one direction, the Space period increases, and the Mark period decreases by the same amount, as shown in waveform B. If they become unequal in the opposite direction, the Space period decreases, and the Mark period increases by the same amount, as shown in waveform C. If this square waveform of variable Mark-Space periods is passed through a low-pass filter 23, it is apparent that the change in the average dc level of the signal will be proportional to the change in position of the vane 12 from its midposition.

The Mark and Space periods of the square waveform are produced in the following way. Assume that the bifurcated vane 12 is in the midposition, and the power supply to the circuit of FIG. 5 has been turned on for some time. When the RC timing circuit on the left of the relaxation oscillator 21 times out, it shifts to the other state and sets a flip-flop 24 via a threshold detector 25 to initiate a Mark period. When its RC timing circuit on the right times out, it shifts the multivibrator 21 back to its first state and resets the flip-flop 24 through a threshold detector 26 to initiate a Space period. If the vane changes angular position clockwise in FIG. 1 from the midposition, so as to decrease the capacitors C.sub.1a and C.sub.1b, the Space period decreases and the Mark space increases, but the total Mark-Space waveform period remains the same.

The Mark-Space square waveform output is fed to the low-pass filter 23 amd a noninverting amplifier 27, and also to a divide-by-two circuit (D-type flip-flop) 28 that triggers on the leading edge of the Space period (or Mark period, whichever is chosen). This produces a symmetrical square waveform regardless of any difference in the Mark and Space periods, as shown by waveform D in FIG. 6. A low-pass filter 29 provides a signal, the average dc level of which will be proportional to the vane midposition. The difference between the output signals of the low-pass filters 23 and 29, computed by an operational amplifier 30 connected to amplify that difference, is then proportional to the deviation of the vane position from its midposition. A potentiometer 32 is connected to the summing junction of the operational amplifier 30 (connected as a differencing amplifier) to add a dc offset signal that will shift the output of the amplifier 30 to provide zero volts at one extreme position and a maximum dc voltage level at the other extreme position.

The symmetry of the circuit, which includes capacitors C.sub.1a, C.sub.1b and C.sub.2a, C.sub.2b, and the fact that the ratio, and not their absolute value, is important, means that most interfering parameters cancel out. The frequency of the squarewave is not important.

Although particular embodiments of the invention have been described and illustrated herein, it is recognized that modifications and variations may readily occur to those skilled in the art. For example, in the event that the absolute dc average is desired, rather than the change in the dc output with respect to a change in vane position, it would be possible to disconnect the offset voltage potentiometer and the amplifier 30. The output voltage at the junction 33 is then given by the equation V.sub.out =V.sub.s (C.sub.1 /C.sub.1 +C.sub.2), where V.sub.s is the power supply voltage and C.sub.1 and C.sub.2 are the paired capacitors C.sub.1a and C.sub.1b and C.sub.2a and C.sub.2b, and if the same output signal is desired with an offset, the offset potentiometer 32 could be reconnected. Since the primary purpose for having the paired capacitor plates is to increase the capacitance, it is evident that even greater capacitance may be achieved by using a bifurcated vane and plates on a circuit board to the rear of the arrangement shown. In fact, four bifurcated vanes could be provided on the same base 12c, each with its own paired plates on a circuit board. Consequently, it is intended that the claims be interpreted to cover such modifications and variations.