[DonRotolo]

The PWM signal used for R/C cars (and in FIRST) is a coded signal, where a pule of 1.0 mSec width is considered "zero", 1.5 mSec is considered "127" and 2.0 mSec is considered "255". These pulses are sent every 20 mSec or so, but this timing can and does vary by application, and can be anything from about 4 mSec to 100 mSec, as reasonable values.

A piece of hardware (think "IC Chip") is then used to translate the 0 - 255 signal to a motor drive signal. Using a Victor as an example, a 0 is full reverse, and 255 is full forward. Between those two extremes is where it gets interesting.

In the case of 3/4 forward, what the victor actually feeds the motor is 75% of the time a full forward signal, and 25% of the time a full reverse signal - flipping back and forth very fast, one or 2 thousand times per second (but this can be from dozens to tens of thousands per second). Because of the mass and inertia of the motor, it reacts as if you just sent 75% of the regular voltage, instead of actually reversing.

This means, the victor has only two output states: Full forward and full reverse. By switching between them very fast, the motor sees the 'average' and reacts accordingly.

This drive signal is also a kind of PWM, but really we call it is "Square wave" with a certain "Duty cycle", which varies between 0% and 100%. You can have 2 kinds of drive: full reverse to full forward, or 0 to full but with a separate signal for direction.

An H Bridge circuit simply takes the duty cycle signal and amplifies it greatly, using the available supply voltage (such as 12 volts for a FIRST robot). You can make it go from 0% (stop) to 100% (Full) with just 2 transistors or FETs (one for + and one for -), but to reverse the current (like a Victor) you need four. If you don't need variable speed, you can use 4 relays to make an H bridge, but variable speed is where the fun is (transistrs and FETs can switch on and off far faster than relays, so that's what we use for variable speed).

Hope this gets you started, there's a ton of info out there on H bridges and R/C PWM signals.

Don.