Servo control and Arduino PWM outputs

tl;dr Arduino PWM (ie analogWrite) doesn't work for servos. Use the built-in Servo library or one of the many variations and deal with the loss of some analog output pins or having to call a function at least every 50ms.

I had problems with the servos being a bit jerky, or twitchy when not being moved. I'm using the built-in Servo library, but was annoyed that it disabled PWM outputs on some pins - I used these pins for the motor control, and this particular issue wasted several hours debugging. After I discovered this, I was annoyed because I had found a convenient place to plug in motor control and was too lazy to move it.

I tried the SoftwareServo library and found it no better, in fact possibly more twitchy, sometimes not responding at all - but this may have been due to power problems. But I put a LED on the output pin and it seemed to flicker inconsistently, due to it not being a hardware output and dependent on processing loop time, whereas it should have been a smooth or at least consistent flicker.

I thought all this was silly, why were there different libraries for servo control, and both had annoying issues? I could just control the servos directly with the hardware PWM, right?

Well I tried just that, and the servos did very weird and completely wrong things - a single change it degree made it move a little, or maybe 90 degrees.

Turns out that servo motors are not controlled by PWM, or Pulse Width Modulation, at least that which is output by the Arduino (the timing is probably wrong). They are controlled by Pulse Position Modulation (PPM). The wikipedia page on servos is a little misleading on this matter, although it is possible there are servos that are controlled by different kinds of PWM.

All this behavior finally became clear when I found this post on the arduino forums: 

Help with PWM for controlling servos and speed con

So after moving my motor PWM pins, making sure the arduino power was isolated from the servo power, replacing the oversized pan servo with a smaller, faster, lower-power one, and replacing the tilt servo with one that hadn't been put through the first batch of testing where it held up the camera itself, I have a stable camera :)