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Introduction
It is quite common in electro-mechanical applications to attach a position encoder to the shaft of a DC motor. Such devices typically consist of a wheel marked with two concentric patterns as shown below, and two optical sensors.
In some cases, there are more alternating black and white segments in each ring giving a higher angular precision, and also there can be a third ring containing just one black segment to indicate the zero position.
When the motor spins, the black and white marks pass the two photo-sensors and this results in two square wave patterns at the encoder outputs, as shown below on the top two waveforms. In this case, the motor is rotating anti-clockwise, a black mark generates a logic 1 and a white mark generates a logic 0.
The clever part is shown in what happens when the motor rotates in the opposite direction (clockwise), as shown below:
If you study these two diagrams carefully, you will see that in the first case, rising edges of Q1 occur when Q2 is low, but in the second case, rising edges of Q1 occur when Q2 is high. This means that a device (typically a microcontroller) attached to the Q1 and Q2 outputs can determine both the rotational speed and the absolute angular position of the motor shaft, even if it rotates both clockwise and anti-clockwise.
The presence of the index pulse (IDX) which is generated only once per revolution provides a way to identify the zero position of the shaft.
Theory
The MOTOR-ENCODER model is identical to the DC Motor Model save for the addition of a DLL based model that tracks the angular position value at the output of the DC motor model schematic, and generates the appropriate output pulses. This model is parameterized in terms of the number of pulses per revolution (PPR), and uses some sophisticated interpolation techniques to generate the digital pulses at the correct times without unduly limiting the speed of the simulation.
Note also:
The index (home position) pulse is generated in phase with the Q2 output and has the same width as a single Q2 pulse.
Gearing arrangements between the motor and the encoder can be modelled by specifying a fractional value for the pulses per revolution.
The digital outputs (Q1, Q2, IDX) are modelled as being TTL compatible and the MOTOR-ENCODER model therefore has two hidden power pins (VCC/GND) which provide notional power for this
