Tachometers

Tachometers measure the angular speed of a rotating shaft. The speed of a shaft is measured in revolutions per minute (rpm). A tachometer may be as simple as a dc or ac generator that can determine the speed of shaft rotation by the amount of voltage the generator produces or the frequency of the output signal. The magnitude of the generator voltage and the frequency of the generated voltage will increase proportionally with speed. Frequency can also be measured by a photocell tachometer. The number of pulses produced by the photocell will increase as the speed of the shaft rotation increases.

Above: To monitor tach, one needs componets such as these: (left to right) hall-effect sensor, magnetic disk and tachometer.

Tachometers are used to determine the speed of a motor shaft for motor drives on conveyors, as well as to determine the speed of rotation of the screw shaft on a plastic injection molding machine. The speed of rotation of the screw on a plastic injection molding machine is important to control because the screw shaft is used to meter the amount of plastic that is drawn into the barrel of the machine for the next injection shot. If the speed isn't controlled, the screw will turn at different speeds and more or less plastic will be drawn into the barrel and the amount of plastic being used for each part will be inconsistent.

The speed of the rollers in large rolling mills is also important to measure. Here, the rpm of each motor is measured and compared to setpoints. A servo system will increase or decrease the motor speed to keep the rollers at the correct rpm. In packaging applications several motors must be synchronized as the production line speed changes -- this ensures that the equipment will function correctly. Tachometers are used to measure the speed of each motor; a controller adjusts each motor speed to match the speed of the production line.

The two basic types of tachometers (which use the change in voltage to determine speed) are: the dc generator tachometer and the drag-cup tachometer. These tachometers operate like an unregulated generator. The faster they turn, the more voltage they produce. The dc generator produces a dc voltage, and the drag cup tachometer produces an ac voltage. These tachometers can provide the direction of rotation information and speed, which is useful since most of these types of sensors are used to provide feedback signals. Because these signals are basically a voltage, a simple voltmeter may be used as an indicator.

The frequency-type tachometer counts pulses produced by a rotating field tachometer, toothed rotor tachometer, or the photocell tachometer. These tachometers produce sine waves or pulses that can be counted, and need more sophisticated digital circuits to complete the process of count, store, calculate, display, and reset to get a value displayed that represents rpm. The rotating field and the toothed rotor tachometers produce a waveform and the photocell uses a rotating disk that has a number of windows in it. A light source is positioned so that it will shine light through each window in the disk to a photocell detector as the disk spins. The disk is connected to the tachometer shaft, so when it turns the windows line up with the photocell and the photocell produces a pulse when it's struck by light. In each of these types of tachometers a pulse stream is generated which is proportional to the speed of the tachometer shaft.