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 10
th 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 2
12, 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
i2c.servo.pwm.PCA9685.java, 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.
Example
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:
| Pulse | Standard | Continuous |
|---|
| 1.5 ms | 0 ° | Stop |
| 2.0 ms | 90 ° | FullSpeed forward |
| 1.0 ms | -90 ° | FullSpeed backward |
That happens not to be always true, some servos (like
https://www.adafruit.com/product/169 or
https://www.adafruit.com/product/155)
have values going between
0.5 ms and
2.5 ms.
Before using them, servos should be calibrated. You can use the class
i2c.samples.IntercativeServo.java 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:
