How can I make smaller steps with my servo?

Question

I've got a Raspberry Pi with which I control a servo. The servo controlls the rudder in an autonomous boat I'm building. I'm using Golang and this gpio library to control the PWM.

The input I get from the program ranges from -100 (full left) to 100 (full right). I've got some code (proof-of-concept) which works great (paste here).

I use:

• a frequency of 100
• a cycle length of 100
• I vary the duty cycle between 3 (full left) and 14 (full right).

This works great, but since the library only accepts ints as input for the duty cycle, I only have 11 discrete steps to which the servo can be set. I would like to be able to have more fine grained control though. I would like to use a minimum of about 25 discrete steps.

So I played around with the numbers of the frequency, the cycle length and the duty cycle minimum and maximum, but I really can't get more steps.

Could anybody give me some hints as to how I can create more discrete steps? All tips are welcome!

So far so good.

Answer 1

Your pin frequency is far too low. This establishes the basic clock used to produce the PWM.

You normally want the PWM signal to have a period close to 20 ms, which corresponds to an update rate of 50 Hz. The pulse width needs to vary between 1 and 2 ms. If you want 200 steps to cover this range (-100 to +100), the pin frequency (PIN_FREQ) needs to be

$$\frac{200}{2\text{ ms} - 1\text{ ms}} = 200\text{ kHz}$$

If you want to update at 50 Hz, that means that the cycle length (CYCLE_LENGTH) should be

$$\frac{200\text{ kHz}}{50\text{ Hz}} = 4000$$

and to vary the pulse width between 1 ms and 2 ms, you use duty cycle (dutyCycle) values between

$$200\text{ kHz} \times 1\text{ ms} = 200$$

and

$$200\text{ kHz} \times 2\text{ ms} = 400$$