Speed sensors (industrial rotational speed)

01

Match the target, air gap and speed range

A magnetic pickup is a permanent magnet, a pole-piece and a sensing coil in a cylindrical case; a passing iron or steel target distorts the flux and generates a voltage whose size depends on the target size, its speed and how close it passes. Red Lion rates each pickup by a 1-Volt Threshold Speed — the surface speed of a reference gear that gives a 1 V peak open-circuit output at a 0.005 in air gap — and by a minimum gear pitch at which output drops to 40–60% of the reference-gear output. Its MP-62TA (5/8-18 UNF thread) reaches 1 V at 10 in/sec, or 50 RPM on a 60-tooth 16 D.P. reference gear, with a 24 D.P. minimum gear; the smaller MP-25TA needs 135 in/sec (1000 RPM on a 60-tooth 24 D.P. gear) with a 48 D.P. minimum. Output falls as the gap widens, and Red Lion does not recommend magnetic pickups for counting because counts are lost at low speeds. [1]

Murphy's magnetic pickup bulletin gives the output frequency as engine RPM × number of flywheel teeth ÷ 60, and says the gap should be set to clear the gear teeth while giving adequate voltage at the lowest measurement speed with the control equipment connected. Its stated outputs show how test conditions change the figure: 200 V p-p for one pickup on a 20-pitch gear at 100 inches per second, 0.005 in gap and 100 kOhm load, against 20 V p-p for another on a 60-tooth, 6 in mild-steel wheel at 1000 rpm, 0.01 in gap and 10 kOhm load. [2]

Powered sensors work to zero speed at larger gaps. Electro-Sensors' HE950 Hall-effect gear-tooth sensor gives full output down to zero Hz up to 12 kHz at a recommended 2 mm to 3 mm gap, provided tooth width, valley width and valley depth each exceed 2 mm, the gear is thicker than 3 mm and made of low carbon steel. [3] Micro-Epsilon's DZ140 eddy-current system reads aluminium or titanium turbocharger blades from 200 to 400,000 rpm at distances up to 2.2 mm with a ø3 mm sensor, counting 1 to 16 blades per revolution. [5]

Editorial calculation: using Murphy's formula, a 60-tooth gear at 3,000 rpm gives 3,000 × 60 ÷ 60 = 3,000 Hz, and the HE950's 12 kHz limit on the same gear corresponds to 12,000 × 60 ÷ 60 = 12,000 rpm. Diametral pitch converts to metric module as 25.4 ÷ DP, so Red Lion's 16, 24 and 48 D.P. gears are about module 1.59, 1.06 and 0.53, and a 0.005 in gap is 0.127 mm. [2] [3] [1]

Give the supplier the target type (gear, keyway, bolt heads, magnet wheel or blades), tooth count, module or diametral pitch, tooth and valley dimensions, target material, minimum and maximum speed, the gap you can hold including runout, whether zero-speed or direction sensing is needed, the receiving device and its input, supply voltage, cable length, thread and housing, temperature, ingress and hazardous-area requirements, and quantity.

02

Match the output, supply and interface

Passive magnetic pickups need no external power and give an AC voltage that rises with speed; Red Lion lists coil output impedances such as 1200 Ω ±20% and 400 mH for the MP-62TA, and its ASTC accessory detects pickup outputs as low as 20 mV and converts them to an NPN open-collector output for devices not designed for a pickup input and for longer transmission. Red Lion recommends 2-wire shielded cable with the shield on the instrument's ground terminal, run apart from power and control wiring, and offers M12-connector models. [1] Murphy also specifies two-core shielded cable with the shield earthed at one end only. [2]

Powered sensors give a conditioned output. The HE950 runs on 5-24 Vdc (26 Vdc absolute maximum) at 15 mA maximum, switches an NPN open collector rated 20 mA, can be mounted up to 1,500 feet from the control unit on 22AWG 3-conductor shielded cable, and needs a 2.2 kOhm pull-up resistor if the receiving device has none. [3] Electro-Sensors' selector guide lists speed sensors with 2-wire 4-20 mA analog, DeviceNet, single or quadrature PNP or NPN frequency, and passive-coil outputs, in M18x1 mm or 3/4-16 housings of stainless steel or aluminium, with intrinsically safe or explosion-proof versions. [4]

The DZ140 eddy-current controller takes 9 ... 30 VDC (short-term up to 36 VDC) at 50 mA and gives a TTL pulse output — one pulse per blade or one per rotation — plus a 0 … 5 V analog output with 10-bit resolution and linearity < ±0.2 % FSO. [5]

Editorial note: compare a pulse sensor's output type (sinking NPN, sourcing PNP, TTL or AC voltage) and a passive pickup's voltage at your minimum speed against the input threshold of the tachometer, speed switch or PLC card before ordering.

03

Shaft, wireless and belt-slip sensors in the listings

Listings range from Hall-effect, magnetic and photoelectric speed sensors and speed transmitters to gear speed sensors, wireless optical machine-speed sensors and conveyor belt-slip speed sensors.

04

Target, sensing element, pole-piece, electronics, housing and cable

The target heads this table because Red Lion and Electro-Sensors state output and zero-speed performance only for a defined gear, so its teeth, pitch and material belong in the order. [1] [3]

05

Speed range, output, supply and gap against your receiving input

Accuracy and frequency range each rest on a single datasheet here (the DZ140 and the HE950), so ask every other supplier for its own figures.

06

Send the target, speed and receiving device together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Target: [gear teeth; pitch/module; material]
Speed: [min rpm; max rpm; zero speed?]
Gap: [mm; runout]
Output: [AC; NPN/PNP; 4-20 mA; TTL]
Supply: [V DC]
Install: [thread; cable length; connector]
Environment: [°C; IP; hazardous area]
Quantity: [units]

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