of several different types including the optical type the laser type and the induction type a speed signal 36 is fed back to a microcontroller 38 either through a signal wire or through a wireless system the microcontroller 38 compares the actual speed with the desired speed and adjusts the valve opening using the control signal 40 this is done at a very high sampling rate using control algorithms control algorithms may include pid algorithms feed forward algorithms or fuzzy logic algorithms used independently or combined for greater performance the desired speed of the tool is selected through a user interface 42 which may include a number of input buttons on the tool a liquid crystal display lcd 46 buzzers and or light emitting diodes led 48 or other interactive devices the buzzers and leds are used to inform the operator of a important situation for example if the air available to the tool is insufficient to allow the controller to maintain the desired rotational speed in this case the microprocessor may stop the rotation of the tool sound the buzzer and illuminate some leds to warn the operator both the display and the buttons may be combined through a touch screen interface the lcd displays information relevant to the.