Motion System Design

Measuring rpm

Tachometers directly report angular speed. Some are simple processors that merely digest output from an external sensor or encoder; others are integrated units that combine an encoder or sensor and a processor in one neat package. Both types, however, track mechanical motion (usually of a motor's output shaft) and convert it to an output signal directly proportional to rotational speed. This information may be fed to display panels, or sent downstream to a controller that adjusts motor speed accordingly.

In contrast to tachometers, absolute encoders produce only a proportional binary representation of angular shaft position; incremental encoders, on the other hand, generate a single voltage pulse per specified change in shaft angle, producing a pulse frequency proportional to speed.

This month's handy tips provided by Kris Paxton, design engineer with Electro-Sensors Corp., Minnetonka, Minn. For more information, visit

Q & A


Common tachometers can process frequencies from 0.1 to over 10,000 Hz. So conceptually, when using an incremental or other pulse-output sensor, incoming pulses are counted, timed, and scaled by a known pulses/rev value to calculate shaft speed. But there are several additional processes to measure high and low speed, deceleration, zero speed, and pulse-source averages.

For example, in slow applications the number of sensor pulses/rev can be a limiting factor. Measuring 0.1 rpm with a sensor that produces eight pulses/rev means time between pulses is 75 sec — for well over a minute to detect if a shaft stops — and generally not acceptable. That's why slow-speed applications often necessitate higher pulses/rev pulse sources, to reduce the time between pulses. Maintenance cycles in power generation is one example; massive shafts are never allowed to stop rotating, and are thus protected from damage.


On the input side, tachometers fall into three basic categories: Direct-coupling and noncontact (both with internal sensors) and processor-only sensorless types. Direct-coupling tachometers attach to shafts directly, just as rotary shaft encoders do. Noncontact tachometers detect shaft-mounted targets across an airgap; tachometers with internal Hall-effect sensors, for example, detect shaft-mounted magnetic discs or wraps. Sensorless tachometers have sensor input terminals that typically connect to pulse-output devices such as proximity switches, photo-eyes, Hall-effect sensors, and incremental encoders.

On the output side, tachometers can also be grouped into three categories: Analog, digital/network, and display. Display-output tachometers are simplest: Panel-mount numeric display devices. Analog-output tachometers typically produce voltage or current analog signals (0 to 10 V, 4 to 20 mA) with a calibrated linear scale factor (mV/rpm or mA/rpm) for connection to devices with analog inputs: PLCs, data acquisition devices, and displays. Lastly, digital/network-output tachometers have network interfaces such as DeviceNet or Modbus for communicating with enabled PLCs, PCs, and other controllers.


Many models include programmable speed alarms for underspeed and overspeed indication where shaft speed must be controlled autonomously. With these tachometers, a supervisory process is notified when speed crosses one or more user-set thresholds. Other tachometers measure bidirectional motion (forward and reverse) with a quadrature output encoder. This is useful where rotation speed and direction must be measured — on elevators, gantries, and especially cranes. Finally: Some tachometer models provide electrically isolated 24-Vdc power, eliminating the need for separate sensor power supply.

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