The PWM Amplifier
The most basic PWM motorized amplification device is often referred to as the bridge. With a bridge, if the transistors labelled are set to on, electricity flows through the motor in a uni-directional way and the motorized cogs turn in that same direction. However, turning off these transistors and turning two other transistors will cause electricity to flow in the opposite direction and the motorized cogs turn in reverse mode. It is worth noting that if all the transmitters are turned on at once. a directional shortcut will cause the motor to fuse and the amplifier will no longer work. Therefor, for this motor to function all transistors must be switched off or just two at a time.
Sophisticated lab integrated mechanisms permit the designer of the motor to adapt to these conditions and prevent this from occurring. But beware, as this is not a simple process due to the rapid switching of the transistor. When all these transistors are switched off, the motor is consequently off. On top of this the designer must control amount of current passed to a motor to prevent overheating and other damage to the device. To achieve this the current must be continuously monitored via the bridge or even the motor by rapidly shutting the motor down. Under lab conditions this can be quite difficult, as the time frame for doing so is minute.
Here is a link to some more information on lab automation equipment