Shaft speed sensors for machines and industrial IoT
A shaft speed sensor measures the revolutions per minute of rotating components such as shafts, motors and gearboxes. It scans teeth, cams or markers without contact, using inductive, magnetic or optical principles, and outputs pulses or a speed value. In the IoT stack it provides the basis for standstill, slip and load monitoring.
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Speed sensors in practice
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What to look for
How speed sensors measure
Speed sensors detect a recurring marker on a rotating part and derive the rotational speed from it. Inductive sensors scan the teeth of a gear, bolt heads or cams on a shaft. Magnetic sensors based on the Hall effect or magnetoresistive elements respond to teeth made of ferromagnetic material or to magnets. Optical sensors work with a light barrier and slotted disc or with a reflective marker on the shaft. The pulse frequency and the number of markers per revolution give the speed in rpm.
There are passive and active sensors. Passive sensors, typically variable reluctance pickups with a permanent magnet and coil, generate an AC voltage themselves whose amplitude rises with speed, so they need a minimum speed. Active sensors are powered, deliver a clean square-wave signal and also detect very slow motion down to near standstill.
A simple example: if a sensor scans a gear with 60 teeth and counts 1,500 pulses per second, the shaft rotates at 1,500 rpm. More teeth give finer resolution at low speeds, but also raise the pulse frequency that the sensor and evaluation must handle.
How they differ from encoders and speed monitors
Encoders deliver the absolute or incremental angular position in addition to speed and are used for positioning tasks. If the only question is whether and how fast something rotates, a speed sensor is simpler and more economical. Speed monitors go one step further: they compare the measured speed with a set limit and switch when it falls below or exceeds that value, for example when a conveyor belt slips.
What to look for when selecting a sensor
Key criteria are the frequency range, the distance to the target, the tooth material and the output: pulse signal, analog value or IO-Link. IO-Link sensors transmit speed directly as a measured value and allow limits to be set in the sensor. If direction of rotation is required, two phase-shifted signals are needed. Certified speed monitors are available for safety functions. Typical solution examples include slip monitoring on conveyor systems, standstill detection on pumps and fans, and load analysis on gearboxes.
Frequently asked questions about speed sensors
How does an inductive speed sensor work?
An inductive speed sensor detects metal teeth, cams or bolt heads passing its sensing face. Each tooth generates a pulse. The evaluation calculates the speed from the pulse frequency and the number of teeth per revolution. Measurement is contactless and wear-free, so the sensor works reliably even in the presence of oil, dust and vibration.
How does a Hall effect speed sensor work?
A Hall effect speed sensor contains a semiconductor element that responds to changes in a magnetic field. A ferromagnetic gear tooth or a magnet passing the sensor alters the field and generates a pulse. Because the sensor is powered, it delivers a clean square-wave signal whose amplitude does not depend on speed. It therefore also detects very slow rotation down to near standstill.
What is an rpm sensor?
An rpm sensor is another term for a speed sensor that measures revolutions per minute. In industry it is mounted on shafts, motors, gearboxes or conveyor drums and scans teeth, cams or markers without contact. It outputs pulses or, with IO-Link, a direct speed value. The signal is used for speed control, slip detection and standstill monitoring.
What is the difference between a speed sensor and a rotary encoder?
A speed sensor reports how fast something rotates, while a rotary encoder also delivers the angular position of the shaft. Encoders output many pulses per revolution or an absolute angle and are used for positioning on axes and drives. For pure speed, standstill or slip monitoring, a speed sensor is usually simpler, more robust and more economical.
How do you measure shaft speed on industrial machines?
Shaft speed is usually measured with a contactless sensor that scans teeth, cams, bolt heads or a reflective marker on the shaft. Inductive sensors suit metal targets, Hall effect sensors detect ferromagnetic teeth or magnets, and optical sensors work at larger distances. The pulse frequency divided by the number of markers per revolution gives the speed.
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