A belt weigher (also called a weightometer, belt scale or conveyor belt scale) works out tonnes from two measurements: how much material is on the belt, and how fast the belt moves. The speed sensor supplies the second. Six types are common across Australian mine and quarry sites: spiral cage (tach wheel), magnetic pickup, rotary encoder, trailing arm, Doppler radar and Hall effect. This guide covers how each works, which integrators they suit, the NPN versus PNP trap on brand swaps, and how to find a fault when the speed reads wrong.
Why does belt speed matter so much on a weightometer?
Tonnage is load multiplied by speed. The integrator uses a simple relationship:
tonnes per hour = load on the belt (kg/m) x belt speed (m/s) x 3.6
Load comes from the load cells, speed from the sensor; both feed the same multiplication, so a speed error is a tonnage error the same size: 2% out on speed is 2% out on tonnes. On a belt moving 1,000 tonnes per hour (an illustrative figure), a sensor reading 2% slow makes the integrator report 980, and the missing 20 is invisible unless someone checks the speed against reality. The speed sensor is half the measurement. For the parts around it, see how a belt weigher works.
What are the six types of belt weigher speed sensor?
Spiral cage (tach wheel): the bicycle speedometer
Like a bike speedo where a magnet trips a sensor each turn, a spiral wound pulley rides against the belt and turns with it, while a proximity sensor counts targets on the end of the pulley shaft, so a faster belt gives more pulses. The spiral shape sheds material and resists slipping, and as few as one target will do the job. It pushes the carry belt up by at least 10 mm where it sits, which can upset idler alignment, so it belongs well clear of the weigh frame. It reads true belt speed, is cheap and easy to re-bearing, and suits almost any integrator, but the surface wears in time, and a worn or dirty pulley can slip on a wet or oily belt. Bearing seizure is the number one failure mode: keep the bearings greased, spin the pulley by hand to confirm it still turns freely and runs true, and keep the target gap small and consistent. Keep the pulley clean too, because only 3 mm of material built up on a 300 mm pulley reads about 2% low. Spring tension is a check for the spring loaded arm and wheel types, not for a spiral cage.
Magnetic pickup (reluctance): the guitar pickup
Like a guitar pickup sensing a string near a magnet and coil, a mag pickup faces a toothed gear on the shaft, where each tooth induces a small AC voltage, so it self-powers and touches no belt. It is rugged and long-lived, but its amplitude falls with speed and can drop out below about 1 Hz, and it reads drive speed. Keep the air gap about 0.5 to 1.5 mm; for no signal, measure coil resistance (100 to 500 ohm, where open circuit means a broken coil).
Rotary encoder: the barcode scanner on a spinning drum
Like a laser reading a barcode inside a spinning drum, an encoder turns a slotted disc past a detector, and thousands of slots per turn give very high resolution, a steady signal at any speed, and direction sensing. Against it: cost, fragile glass discs, sensitivity to dust and vibration, and a need for external power and careful coupling. Shaft-mounted it reads drive speed unless driven off a measuring wheel. Keep the housing clean and watch for voltage drop on long runs.
Trailing arm (measuring wheel): the surveyor's wheel
Like a surveyor's clicking measuring wheel, a precision wheel on a hinged, spring-loaded arm rides the belt and drives an encoder or proximity sensor, giving direct belt speed at a higher resolution than a spiral cage, which suits accuracy-critical work. Where a weigher is used for trade, the approval document for that weigher is what sets the requirements, and it governs things like the load cells and the sealing of the calibration settings rather than dictating the speed sensor type. Check the approval rather than assuming a trailing arm is needed. It is more complex and can bounce over belt splices, so check the pivot moves freely and the spring holds the wheel down.
Doppler radar: the speed camera
Like a speed camera reading the frequency shift of radar off a car, a Doppler sensor fires a microwave beam at the belt and reads the shift, giving true surface speed with no contact: no belt wear, no moving parts, and reliable running in dust and water. It is the dearest option, needs calibrating to the belt surface, and its output may need conditioning for some integrators. Keep the sensor face clean and hold the mounting angle.
Hall effect: the automatic door sensor
Like a supermarket door sensing a magnet pass by, a Hall effect sensor detects a magnet on a rotating shaft and gives a clean digital pulse each pass. Unlike a mag pickup it works at any speed, even a crawl, and is solid-state and cheap, but it needs power, the pulse count is low, the sensing distance is short (about 5 to 15 mm), and it reads shaft speed. Check the magnets are secure and confirm the gap.
What is the difference between NPN and PNP, and why does it bite on brand swaps?
Most proximity and Hall sensors are either NPN or PNP, and getting it wrong is a common reason a brand-new sensor reads nothing.
An NPN (sinking) sensor is the drain: it pulls the output down to 0V (active low), and the load wires between output and the positive supply. A PNP (sourcing) sensor is the tap: it pushes current out to the positive supply (active high), and the load wires between output and 0V. Same job, opposite direction.
It bites on brand swaps because families default differently. Some integrators favour one polarity and a few accept only one, so always check before a brand swap. CST is not one of them: its integrators take NPN or PNP, with a jumper on the board setting which, and a two-wire sensor sidesteps the question entirely. Feed an NPN sensor into a PNP input (or the reverse) without checking and the integrator sees no valid pulse. Many modern units are configurable by DIP switch or software, but check the setting before you connect.
Wire colours are the same for both (IEC 60947-5-2): brown (BN) is the positive supply (10 to 30 V DC), blue (BU) is 0V, and black (BK) is the switched output. Only the load's other end differs: to the positive supply for NPN, to 0V for PNP. Always verify with a multimeter before you energise, since not every maker follows the standard.
Which integrator works with which speed sensor?
Almost every integrator takes a proximity input; what varies is the polarity it expects and whether it also takes a mag pickup or encoder:
| Integrator family | Typical speed input | Note |
|---|---|---|
| Thermo Scientific / Ramsey Micro-Tech (MT 9101, 2101) | Proximity, NPN or PNP | Very common on Australian mine sites; flexible input. Set PPR to match the sensor. |
| CST (IPC-14 on the WIM2C board, and the later WIM3) | Proximity, either polarity | Takes a two-wire or three-wire proximity switch, NPN or PNP. A jumper on the board selects which, so set it to match the sensor before you connect. CST prefers a two-wire sensor, which avoids the question altogether. |
| Schenck Process (DISOMAT Tersus, INTECONT) | Proximity, PNP (sourcing) | Expects PNP by default, catching out swaps from NPN brands. Strong HTL encoder input. |
| Web Tech AutoWeigh (Masterweigh 5, Masterweigh 6, and the current Masterweigh Novus) | Proximity, magnetic pickup or pulse tacho | Australian made. The speed input reads a pulse voltage rather than a switch polarity, roughly 2.5 to 50 volts peak, square wave or sinusoidal, so a proximity switch, a magnetic pickup or a pulse tacho will all drive it. Match the pulse voltage and the sensor supply rather than NPN against PNP. Common on older QLD and NSW coal sites. |
| EMC Industrial (MP1, MP2) | MP1 NPN; MP2 NPN or PNP | MP1 is NPN only; MP2 is configurable. |
| Siemens Milltronics (BW100, BW500) | Proximity, PNP (or NAMUR) | PNP standard; NAMUR 2-wire on some models. HTL/TTL encoder for high accuracy. |
| Thayer Scale (EZ Series, Model C) | Proximity, NPN or PNP | Weigh feeders and feed-rate control. Model C defaults NPN; EZ Series configurable. |
How do you troubleshoot a belt weigher speed sensor?
First, safety. Fault-finding means opening junction boxes and taking live measurements, so treat it as electrical work. Isolate and lock out where the task allows, and only take live readings if you are trained and authorised. Use a properly rated multimeter, verify the wiring before you re-energise, and treat every circuit as live until proven dead. Conveyors run by variable speed drives (VSDs, sometimes called VFDs) carry both a shock risk and electrical noise. If you are not set up for this, stop and call us.
No signal at all
Start mechanical: is the sensor wheel or target actually turning? If not, look for a seized bearing (spin it by hand and listen for grinding), a spring or arm that has lost belt contact, a shifted bracket, or a belt that has tracked away; on shaft sensors, a sheared coupling. If it is turning, check for a signal at the junction box. Nothing there: on a proximity sensor, measure the supply at the terminals, but check what that integrator is meant to provide before you call it a fault. It varies by brand, from about 5 V on a Web Tech unit to 12 V on a Tecweigh and 24 V on a CST, so a reading well under 10 V can be perfectly normal. Never feed an external supply into an integrator's sensor supply terminals to make a low reading up, because some units are damaged by exactly that. If a long cable run has dropped the voltage, some integrators let you lift their own sensor rail from the board instead. Then watch the sensor LED switch as a target passes. On a mag pickup, measure coil resistance; on an encoder, confirm its supply. If the signal reaches the sensor but not the integrator, the fault is between them: damaged cable, wrong terminal, or NPN/PNP mismatch. On runs over about 50 m, check for voltage drop at the sensor end.
Intermittent or dropping out
Random dropouts usually mean a loose terminal (vibration works them free), moisture in the junction box, or interference from VSDs, HV cables or contactors. Use shielded cable and ground the shield at one end only, normally the integrator end, and check the proximity gap. If it drops out only when the belt is wet or loaded, suspect slippage: check a spiral cage is clean and still turning freely, check a trailing arm is not bouncing, and remember a shaft sensor reads fine while the belt slips on the pulley. If it drops out at a particular speed, a mag pickup may be losing amplitude (check the integrator's minimum frequency), or VSD interference may bite at certain motor speeds.
Wrong or inaccurate reading
Reading too high usually means a PPR (pulses per revolution) setting that does not match the sensor, double-counting (for example both encoder channels), a wheel diameter entered wrongly, or noise adding counts. Reading too low points to a PPR set too high, belt slippage on the wheel, a wheel diameter set too large, or missed pulses. The check that settles it: compare the calculated belt speed against the actual speed by timing a marked length of belt or a hand tachometer. More than about 2% discrepancy warrants investigation, and because tonnage tracks speed one for one, that 2% is 2% off your tonnes.
How do I choose the right speed sensor for my conveyor?
Match the sensor to the job. This is the selection we usually make:
| Application | We usually fit | Why |
|---|---|---|
| Standard belt weigher (not trade-approved) | Spiral cage (tach wheel) | Direct belt speed, low cost, universal. The AU mining default. |
| Trade-approved (legal-for-trade) install | Trailing arm, with encoder | Direct belt contact with encoder precision. The weigher's approval document sets the requirements, so check it rather than assuming this type is needed. |
| High accuracy or feed control (weigh feeders) | Rotary encoder | Highest resolution, steady at all speeds. |
| Harsh environment (heavy dust or water) | A sealed spiral cage, or non-contact sensing where the integrator supports it | Nothing riding the belt to wear or seize. Confirm the integrator will accept the signal before you specify non-contact. |
| Retrofit with no belt access | Magnetic pickup | Self-generating, shaft-mounted, no power needed. |
| Variable-speed conveyor | Rotary encoder | Steady at all speeds, no low-speed dropout. |
Frequently asked questions
Our tonnage looks wrong but the load cells check out. What next?
Check the speed. Time a marked length of belt, or use a hand tachometer, and compare with the displayed speed. More than about 2% off warrants investigation, usually slippage, a PPR mismatch, or a wrong wheel diameter.
Can I swap a Schenck speed sensor for a different brand?
Yes, but check NPN versus PNP first. Schenck expects PNP while several brands default NPN, so match the sensor's output to the integrator's input, or reconfigure before connecting.
Do you supply and install speed sensors across Australia?
Yes. We supply, install, service and calibrate belt weighers and their speed sensors on mine and quarry sites Australia-wide from our Brisbane base.