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Shaft Position and Direction Encoder

IP.com Disclosure Number: IPCOM000074734D
Original Publication Date: 1971-Jun-01
Included in the Prior Art Database: 2005-Feb-23
Document File: 3 page(s) / 37K

Publishing Venue

IBM

Related People

Bailey, DL: AUTHOR [+2]

Abstract

The logic shown in Fig. 1 operates on tachometer signals from a shaft to indicate the direction of rotation of the shaft and to further indicate the position of the shaft as monitored by counter 10. The tachometer signals are derived by illuminating a tachometer disk attached to the shaft, monitoring the reflected light from the disk with two photosensitive diodes, and using a squaring circuit for the output of each diode. The result is the two tachometer signals A and B shown in Fig. 2. The input to the logic in Fig. 1 is: the A and B signals or the inverted A and B signals -- as indicated by the bar over the A or the B.

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Shaft Position and Direction Encoder

The logic shown in Fig. 1 operates on tachometer signals from a shaft to indicate the direction of rotation of the shaft and to further indicate the position of the shaft as monitored by counter 10. The tachometer signals are derived by illuminating a tachometer disk attached to the shaft, monitoring the reflected light from the disk with two photosensitive diodes, and using a squaring circuit for the output of each diode. The result is the two tachometer signals A and B shown in Fig. 2. The input to the logic in Fig. 1 is: the A and B signals or the inverted A and B signals -- as indicated by the bar over the A or the B.

In operation, assume first that at the start both A and B equal 0 (down level). This condition will produce an output from AND gate 11 which will set latch 12. If the next state is A = 0 and B = 1 (up level for B), latch 14 is set because AND gate 16 is enabled and has an output. Latch 14 indicates the direction of rotation of the shaft. With latch 14 set, the direction of rotation indicated is clockwise. When latch 14 is reset, the direction of rotation indicated is counterclockwise. With this logic, the latch 14 will indicate a clockwise rotation only on the positive transition of the B tachometer signal.

During counterclockwise rotation, latch 12 will be reset by AND gate 18. Immediately thereafter, when the A tachometer signal drops and the B tachometer signal stays up, AND gate 20 is satisfied and resets latch 14 to indicate counterclockwise rotation. Therefore, latch 14 will indicate counterclockwise rotation only if A and B are both up, followed by the A tachometer signal going down.

The method of tracking position of the shaft is to count counter 10 up or down depending upon the direction of rotation. The inputs to the counter 10 are derived from latch 14, a set condition in latch 22, and an output from either gates 11 or 18.

Latch 22 is set by an output from AND gate 24 and reset by an output from AND gate 26. AND gate 24 has an output when the tachometer signal combination is B and not A, while AND gate 26 has an output when the tachometer signal combination is A and not...