Electronic – Are there double sided servos

When using the Servo library on an Arduino, a common source of servo buzz is that the interrupt-driven servo routines don't actually give a very stable output pulse. Because the AVR takes interrupts for servicing the millis() clock and other things in the Arduino runtime, the jitter in the Servo library is on the order of several microseconds, which translates to a lot of movement in the servo.

The fix for this is to write your own pulse. Something like this:

cli();
long start = micros();
digitalWrite(PIN, HIGH);
while (micros() - start < duration)
  ;
digitalWrite(PIN, LOW);
sei();

This will turn off other interrupts, and generate a much cleaner PWM pulse. However, it will make the "millis() timer miss some clock ticks. (The "micros()" function may be called something else -- I forget exactly what.)

In general, for timing critical code, you want to get rid of the Arduino runtime entirely, and write your own using the avr-gcc compiler and avr-libc library that powers the Arduino environment. Then you can set up a timer to tick 4 times per microsecond, or even 16 times per microsecond, and get a much better resolution in your PWM.

Another cause of buzz in servos is cheap servos with cheap sensors, where the sensors are noisy, or when the exact position requested with the pulse can't actually be encoded by the sensor. The servo will see "move to position 1822" and try to do it, but ends up with the sensor reading 1823. The servo will then say "move back a little bit" and it ends up with the sensor reading 1821. Repeat! The fix for this is to use high-quality servos. Ideally, not hobby servos at all, but real servos with optical or magnetic absolute encoders.

Finally, if the servos don't get enough power, or if you try to drive their power from the 5V rail on the Arduino, this will generate voltage-sag-induced buzz in the servos, as suggested above. You may be able to fix it with large electrolytic capacitors (which are a good idea for general filtering anyway) but you more likely want to make sure your servo power source can actually deliver several amps of current at the servo voltage.

First, I'll assume by "servo" you are really referring to hobby servo motors that are controlled by 1-2 ms pulses, not the general meaning of "servo" in electronics and control systems. You really should properly define this term in your question.

These hobby servos only use the 1-2 ms pulse as a way of communicating a analog level. The old all-analog hobby servos would integrate the pulse, hold it, and then use that as the control signal to compare the position feedback signal to. Newer digital types measure the pulse width digitally, then use that value to compare the position feedback to.

Either way, the driving of the motor is not synchronized to the pulse. The motor's drive signal is constantly updated internally to the saved control value derived from the last pulse. A new pulse only changes this control input.

That said, a sudden step in the control signal will likely cause a short term error, which the control mechanism will react to by driving the motor harder until it settles to the new position. Therefore, while the motor is always driven, it will usually be driven harder after a step change in the input, which can only happen immediately after a pulse.

Overall, I'd say it is good to stagger the pulses to multiple hobby servos if it's not much burden to do so. Note that this comes automatically if using a off the shelf radio link. These will send the pulses for each of the hobby servos it controls sequentially anyway. If you are controlling multiple hobby servos from a single microcontroller, then multiplexing might dictate sequential pulses too.

If it works out more simply to generate all the pulses at the same time, then go ahead and not worry about it. 20 ms is a short time, and the extra current from a sudden control input change will take longer than that in most cases. In the case of slow changes, the steps are small anyway, so the current should be relatively even. Put a bigger cap on the power supply if you're worried about it.