Arhi tipped us to a PID library for Arduino. If you require precision control of electro-mechanical devices like heaters, servos, water pumps, etc. PID is one way to implement that control.
It is an algorithm that controls a device by constantly adjusting its control or power signals to keep the device at preset state. Using sensors to detect differences between the preset state and the actual state the device is in, the controller calculates which signals it should send to the device to eliminate the difference.
Via the forum.