micropython-samples/encoders/ENCODERS.md

Incremental encoders

There are three technologies that I am aware of:

1. Optical.
2. Magnetic.
3. Mechanical (using switch contacts).

All produce quadrature signals looking like this:

consequently the same code may be used regardless of encoder type.

They have two primary applications:

1. Shaft position and speed measurements on machines.
2. Control knobs for user input. For user input a mechanical device, being inexpensive, usually suffices. See this Adafruit product.

In applications such as NC machines longevity and reliability are paramount: this normally rules out mechanical devices. Rotational speed is also likely to be too high. In machine tools it is vital to maintain perfect accuracy over very long periods. This may impact the electronic design of the interface between the encoder and the host. High precision comes at no cost in code, but there may be issues in devices with high interrupt latency such as ESP32, especially with SPIRAM.

The ideal host, especially for precison applications, is a Pyboard. This is because Pyboard timers can decode in hardware, as shown in this script from Dave Hylands. Hardware decoding eliminates all concerns over interrupt latency or input pulse rates.

Basic encoder script

This comes from encoder_portable.py in this repo. It uses the simplest and fastest algorithm I know. It should run on any MicrPython platform, but please read the following notes as there are potential issues.

from machine import Pin

class Encoder:
    def __init__(self, pin_x, pin_y, scale=1):
        self.scale = scale
        self.forward = True
        self.pin_x = pin_x
        self.pin_y = pin_y
        self._pos = 0
        try:
            self.x_interrupt = pin_x.irq(trigger=Pin.IRQ_RISING | Pin.IRQ_FALLING, handler=self.x_callback, hard=True)
            self.y_interrupt = pin_y.irq(trigger=Pin.IRQ_RISING | Pin.IRQ_FALLING, handler=self.y_callback, hard=True)
        except TypeError:
            self.x_interrupt = pin_x.irq(trigger=Pin.IRQ_RISING | Pin.IRQ_FALLING, handler=self.x_callback)
            self.y_interrupt = pin_y.irq(trigger=Pin.IRQ_RISING | Pin.IRQ_FALLING, handler=self.y_callback)

    def x_callback(self, pin):
        self.forward = pin() ^ self.pin_y()
        self._pos += 1 if self.forward else -1

    def y_callback(self, pin):
        self.forward = self.pin_x() ^ pin() ^ 1
        self._pos += 1 if self.forward else -1

    def position(self, value=None):
        if value is not None:
            self._pos = round(value / self.scale)
        return self._pos * self.scale

If the direction is incorrect, transpose the X and Y pins in the constructor call.

Problem 1: Interrupt latency

By default, pin interrupts defined using the machine module are soft. This introduces latency if a line changes state when a garbage collection is in progress. The above script attempts to use hard IRQ's, but not all platforms support them (notably ESP8266 and ESP32).

Hard IRQ's present their own issues documented here but the above script conforms with these rules.

Problem 2: Jitter

The picture above is idealised. In practice it is possible to receive a succession of edges on one input line, with no transitions on the other. On mechanical encoders this may be caused by contact bounce. On any type it can result from vibration, where the encoder happens to stop at an angle exactly matching an edge. Code must be designed to accommodate this. The above sample does this. It is possible that the above latency issue may cause pulses to be missed, notably on platforms which don't support hard IRQ's. In such cases hardware may need to be adapted to limit the rate at which signals can change, possibly with a CR low pass filter and a schmitt trigger. This clearly won't work if the pulse rate from actual shaft rotation exceeds this limit.

In a careful test on a Pyboard 1.1 with an optical encoder pulses were occasionally missed. My guess is that, on rare occasions, pulses can arrive too fast for even hard IRQ's to keep track. For machine tool applications, the conclusion would seem to be that hardware decoding or possibly a rate limiting circuit is required.

Problem 3: Concurrency

The presented code samples use interrupts in order to handle the potentially high rate at which transitions can occur. The above script maintains a position value ._pos which can be queried at any time. This does not present concurrency issues. However some applications, notably in user interface designs, may require an encoder action to trigger complex behaviour. The obvious solution would be to adapt the script to do this by having the two ISR methods call a function. However the function would run in an interrupt context which (even with soft IRQ's) presents concurrency issues where an application's data can change at any point in the application's execution. Further, a complex function would cause the ISR to block for a long period with the potential for data loss.

A solution to this is an interface between the ISR's and uasyncio whereby the ISR's set a ThreadSafeFlag. This is awaited by a uasyncio Task which runs a user supplied callback. The latter runs in a uasyncio context: the state of any Task can only change at times when it has yielded to the scheduler in accordance with uasyncio rules. This is implemented in this asynchronous driver. This also handles the case where a mechanical encoder has a large number of states per revolution. The driver has the option to divide these down, reducing the rate at which callbacks occur.

Code samples

1. encoder_portable.py Suitable for most purposes.
2. encoder_timed.py Provides rate information by timing successive edges. In practice this is likely to need filtering to reduce jitter caused by imperfections in the encoder geometry. With a mechanical knob turned by an anthropoid ape it's debatable whether it produces anything useful :)
3. encoder_rp2.py Version specific to Raspberry Pico RP2 chip. This uses the PIO and Viper code to achieve fast response - upto ~10K transitions/s.
4. encoder.py An old Pyboard-specific version.

These were written for encoders producing logic outputs. For switches, adapt the pull definition to provide a pull up or pull down as required, or provide physical resistors. This is my preferred solution as the internal resistors on most platforms have a rather high value.