Saturday, July 14, 2018

Raspberry PI, PWM, servos, and PCA9685

The code mentioned below can be found in this git repo.
Pulse Width Modulation (PWM) is the technique used from a digital source to simulate an analog output.
For example, imagine that you want to dim an led from a digital device, to make it look like it is glowing. The digital device only has pins that can take 2 values: 0 or 3V3.
0 means that the led will be off, 3V3 means it will be on, at 100% of its brightness.
In short, it is on or off, and there is nothing in between.
But here is an idea to work around that issue:
To show it at 50% of its brightness, the idea is to turn it off 50% of the time, and on 50% of the time.
To show it at 25% of its brightness, it will be on 25% of the time, and off 75% of the time.
If the on-off cycles are short and fast enough, a human eye will no be able to see them, it will only have the illusion of the resulting brightness.
A human eye cannot make the distinction between images separated by less than one 10th of a second. That is why the movies are shot at 24 images per second, so you cannot tell the difference between the frames.
This technique is call Persistence of Vision (POV).
The #1 parameter of PoV is the human retina. To have an idea of how much it is important, just put your cat in front of a TV, and see how much he/she reacts. To a cat, it might just be a fuzzy screen...
The early movies - like Charlie Chaplin's silent ones - were shot at 16 images per second, fast enough to induce POV. They were later projected by faster projectors - 24 frames per second. That is why the characters seem to move faster. They were originally moving normally.

Here are examples of PWM applied to POV:
At work, for real:
The PCA9685 is a servo driver PCB.
The Raspberry PI does not have analog pins, we need to use Pulse Width Modulation to simulate analog values, a servo is an analog device.
We use for that the method setPWM(channel, 0, pulse), that will eventually write to the registers of the device.
An instruction like setPWM(channel, 0, pulse) means:
  • On channel channel (0 to 15 on the PCA9685)
  • in each cycle, turn the power on between 0 and pulse.
pulse has a value between 0 and 4095, that is 4096 distinct values, 4096 is 212, the PCA9685 is a 12 bit device.

The frequency

The frequency is provided in Hertz (Hz). A frequency of 60 means 60 cycles per second.
At 60 Hz, a cycle will be 1 / 60 second, which is 0.01666666 second, or 16.66666 milli-second (ms).

The pulse

For each of the cycles set above by setting the frequency, we need to determine the int value, between 0 and 4095, corresponding to the pulse in milliseconds we want to simulate with PWM.
In the class, this is done in this method:
public static int getServoValueFromPulse(int freq, float targetPulse) {
  double pulseLength = 1_000_000; // 1s = 1,000,000 us per pulse. "us" is to be read "micro (mu) sec".
  pulseLength /= freq;  // 40..1000 Hz
  pulseLength /= 4_096; // 12 bits of resolution. 4096 = 2^12
  int pulse = (int) Math.round((targetPulse * 1_000) / pulseLength); // in millisec
  if (verbose) {
    System.out.println(String.format("%.04f \u00b5s per bit, pulse: %d", pulseLength, pulse));
  return pulse;
The cycle length (in ms) obviously depends on the frequency.
The pulse required for the servo to work is emitted once per cycle.


As an example, let us calculate for a 60 Hz frequency the pulse value to send to setPWM(channel, 0, pulse) for a 1.5 millisecond PWM:
  • 1 cycle has a duration of 1 / 60 second, or 16.66666 milliseconds.
  • each cycle is divided in 4096 slots, we can say that 4096 bits = 16.6666 ms.
  • the solution is provided by a rule of three: value = 4096 * (pulse / 16.66666), which is 368.64, rounded to 369.

A comment about servos' compliance and reliability

Theoretically, servos follow those rules:
1.5 ms0 °Stop
2.0 ms90 °FullSpeed forward
1.0 ms-90 °FullSpeed backward
That happens not to be always true, some servos (like or have values going between 0.5 ms and 2.5 ms.
Before using them, servos should be calibrated. You can use the class can be used for that, you set the pulse values interactively, and you see what the servo is doing.
$> ./inter.servo
Connected to bus. OK.
Connected to device. OK.
freq (40-1000)  ? > 60
Setting PWM frequency to 60 Hz
Estimated pre-scale: 100.72526
Final pre-scale: 101.0
Servo Channel (0-15) : 1
Entry method: T for Ticks (0..4095), P for Pulse (in ms) > p
Enter 'quit' to exit.
Pulse in ms > 1.5
setServoPulse(1, 1.5)
4.0690 μs per bit, pulse:369
Pulse in ms > 0.5
setServoPulse(1, 0.5)
4.0690 μs per bit, pulse:122
Pulse in ms > 0.6
setServoPulse(1, 0.6)
4.0690 μs per bit, pulse:147
Pulse in ms > 2.4
setServoPulse(1, 2.4)
4.0690 μs per bit, pulse:589
Pulse in ms > 2.5
setServoPulse(1, 2.5)
4.0690 μs per bit, pulse:614
... etc.

Once you have determined the appropriate min and max values, you also have the int values to feed the setPWM with.

Some links: