Digital Logic Circuit for Incremental Encoders

1. Field of the Invention

The present invention relates generally to control circuits for incremental positioning encoders and more particularly to a digital logic circuit particularly adapted for use with an incremental position type encoder.

2. Description of the Prior Art

Encoders are important components of motion control systems. For closed-loop control, encoders are used to feedback the actual position and/or speed of a device for comparison with a reference corresponding to a desired position and/or speed. Open-loop systems often have encoders coupled to their movable members, such a motor output shafts, for providing signals indicating direction of motion and the distance to the final desired position. Typically, encoders generate two quadrature pulse train signals to increase resolution and to decide the moving direction of, for example, an associated output shaft. More specifically, this happens in disk drives which utilize tracking error signals to generate quadrature signals to indicate the position of a read/write head with respect to a disk track. To convert the quadrature signals, generally two pulse trains, to position information, an electronic circuit incorporating digital logic components is used. The circuit accepts the two quadrature pulse trains and converts them to a counting CLOCK signal and an UP/DOWN count signal. By passing the signals through a counter circuit, which counts a net number of the count pulses, a position information signal can be obtained.

A patent of interest for its teaching in this art is U.S. Pat. No. 3,843,915 entitled "Servo-Motor Control System Including A Two Phase Digital Shaft Position Detector" by J. E. Helmbold.

Another patent of interest for its teachings in U.S. Pat. No. 4,481,569 entitled "Feedback System For Robot Controller" by S. A. Hoodbhoy.

The digital logic circuits used in the above referenced patents use an external clock and a flip/flop circuit to detect the edge of each pulse in a pair of pulse trains. After obtaining the edge signals for each of the two pulse trains, an UP/DOWN count signal and count pulse signals are generated. The UP/DOWN count signal and the counting pulse signals are then used as the input signals to a counter. The output signal from the counter is the position information signal which then can be used to activate, for example, a closed-loop servo controller.

In the present invention, a digital logic circuit functions to convert two input quadrature phase pulse train signals to a count CLOCK signal and an UP/DOWN count signal without the use of an external CLOCK signal. The two quadrature phase pulse train signal are each delayed by an associated NAND and OR gate. When the two input signals are each exclusively NOR'd with their delayed self the result is a pair of signals having edges corresponding to each of the pulses in their respective pulse trains. The two edge signals are then logically OR'd together to generate a counting CLOCK signal. The two input signals are also exclusively NOR'd to produce another pulse train signal which is exclusively NOR'd with the output of a flip/flop to create and UP/DOWN count signal. The input signals to the flip/flop are the pair of edge signals dervied from the two input quadrature phase signals. The count CLOCK signal is directed to a counter along with the UP/DOWN count signal such that the counter's output signals indicate the magnitude of the counted pulses.

From the foregoing it can be seen that it is a primary object of the present invention to provide an improved digital logic circuit for use with an incremental position type encoder.

It is another object of the present invention to provide a digital logic circuit which uses a small number of components to achieve a reduced response time.

It is a further object of the present invention to provide a simpler and more reliable digital logic circuit for use with an incremental encoder.

These and other objects of the present invention will become more apparent when taken in conjunction with the following description and drawings wherein like characters indicate like parts and which drawings form a part of the present disclosure.

FIGS. 1 (A-B) are a schematic diagram which illustrates the preferred circuit embodiment of the invention at a logic chip level.

FIGS. 2A and 2B are timing diagrams illustrating the timing relationship of signals taken from strategic locations of the preferred circuit embodiment shown in FIG. 1.

Referring to FIG. 1, the .phi..sub.1 signal (shown in FIGS. 2A and 2B) is applied to the inputs to a NAND gate U19B, an input to an Exclusive-NOR gate U13C, and an input to an Exclusive-NOR gate U13A. In a like manner, the quadrature phased signal .phi..sub.2 (shown in FIGS. 2A and 2B)) is applied to the inputs of a NAND gate U19C, and an Exclusive-NOR gate U13B, and to an input to the Exclusive-NOR gate U13C. In FIG. 2A the signal .phi..sub.1 is shown leading the signal .phi..sub.2. In FIG. 2B the signal .phi..sub.2 is shown leading the signal .phi..sub.1. Which signal leads is determined by the direction of rotation of linear motion from a reference position of the encoder producing the signals. The output signal from NAND gate U19B is directed through an OR gate U8A to an input of the Exclusive-NOR gate U13A. In a like manner, the output signal from NAND gate U19C is directed through an OR gate U8B to an input of the Exclusive-NOR gate U13B. Each of the quadrature phased pulse train signals .phi..sub.1 and .phi..sub.2 are delayed by the effects of their respective NAND and OR gates (which delay is well known in the digital logic circuit art). The output signals from the Exclusive-NOR gates, U13A and U13B, produce edge signals from the pulses making up the input pulse trains.

The output signal from the OR gate U8A, is the gate delayed signal .phi..sub.1, shown in FIGS. 2A and 2B as U13P2. The edge signal from the Exclusive-NOR gate U13A is shown in FIGS. 2A and 2B as U13P3. In a like manner the gate delayed signal .phi..sub.2 from the output of OR gate U8B is shown in FIGS. 2A and 2B as U13P5 and the exclusively-NOR'd signal for the quadrature signal .phi..sub.2 is shown in FIGS. 2A and 2B as U13P4.

The Exclusive-NOR gate U13C, upon receiving the quadrature phase signals .phi..sub.1 and .phi..sub.2, logically NORs them together to provide a signal to the inputs of a NOR gate U8D. From there the signal is passed to an input to an Exclusive-NOR gate U13D and is shown in FIGS. 2A and 2B as U13P11. The OR gate delay, provided by U8D produces a delay which corresponds to the delay caused by a D-type flip/flop U14A. An OR gate U8C, logically combines the signals from the Exclusive-NOR gates U13A and U13B to provide the CLOCK signal U8P10. The D-type flip/flop U14A receives the edge signals from the Exclusive-NOR gates U13A and U13B on its S/ and C/ inputs, respectively, to provide at its output the Q signal U14P12. The signal U13P12 and U13P13 are exclusively NOR'd by Exclusive-NOR gated U13D to provide the UP/DOWN signal U13P11.

A reset/clear switch 10 is connected to a +5 volt source to provide a RESET signal. A counter 20, comprised of four bit counter sections U3, U9, U15 and U21, forms a cascaded counter which receives the three signals, UP/DOWN, RESET and CLOCK, resulting from the previously described circuit. The counter 20 counts CLOCK pulses, either up or down, as a function of the logic level of the UP/DOWN signal. The output count from the bit counter section is directed to LED units U4, U10, U16 and U22 to give a visual indication of the count within their respective counter sections. The output count may be converted to an analog format by a D to A converter (not shown) for purposes of driving some desired apparatus or for providing a position signal indicative of the positioning of the device from which the quadrature phased encoding signals were generated.

While there has been shown what is considered to be the preferred embodiment of the invention, it will be manifest that many changes and modifications may be made therein without departing from the essential spirit of the invention. It is intended, therefore, in the annexed claims, to cover all such changes and modifications as may fall within the true scope of the invention.