A load cell delivers a signal of a few millivolts. No control system can do anything with that, and no person can read it. The evaluation electronics sit in between – and the choice follows four questions rather than a list of types.
1. Who reads the value?
That is the basic decision.
- Weighing transmitters amplify and pass on – analogue or digital, with no display and no keys. They sit on a DIN rail in the control cabinet, where a PLC processes the value further.
- Weighing indicators have a display and keys for zeroing, tare and limit values. They are used where a person operates the scale – in their own housing or for mounting into a machine front. Many also have a signal output and can do both.
- Large displays only show what another device delivers – for reading from a distance.
- OEM boards are amplifiers without a housing, built into your own device; for series machines the most compact solution.
2. How many cells, and individually or summed?
A tank scale with three or four cells can be connected in two ways. Through a summing box the cells are wired in parallel, the electronics see one sum and need only one channel – the simplest and least expensive option. Multi-channel transmitters read each cell separately: that shows the load distribution, detects a faulty cell and allows corner trimming in software. Versions from one channel up to eight are available.
3. Which signal does the control system need?
Analogue is enough as long as one measured value per channel is sufficient and a free analogue input is available. 4…20 mA is immune to interference, needs no software and works over long cables.
Digital pays off as soon as several values per device, diagnostics or remote configuration are needed. IO-Link is the simplest option – unshielded three-wire cable, M12 connector, parameters carried over when a device is replaced. PROFINET, EtherCAT and Modbus TCP tie the scale directly into an existing plant network.
Where a cable cannot reach – rotating parts, suspended loads, mobile setups – there is wireless telemetry in the 2.4 GHz band, or Bluetooth for the short link to a tablet.
4. How fast, and how safe?
A tank scale needs only a few measurements per second. Checkweighing in a cycle, dosing with fast cut-off or recording force curves on a press demand high conversion rates – there the electronics are the limit, not the cell.
If the measurement is to take on a protective function, an ordinary amplifier is not enough. Under measuring electronics SIL / PL there are amplifiers with two galvanically isolated outputs for SIL3 and PL e, as well as devices with a built-in protective function for SIL1 and PL c that work without a safety controller. The background and the figures for plant certification are on the page Functional Safety SIL | PL.
The connection itself
Almost all devices work with a classic strain gauge full bridge. Three things need checking: the bridge excitation (5 to 12 V is common), the rated output of the cell in mV/V and the connection method. With long cables the 6-wire configuration is preferable, because the sense leads compensate the voltage drop in the cable – otherwise the reading drifts with cable temperature.
What we need for a recommendation
- number and rated output of the load cells
- where the value has to go: PLC, PC, display on site
- the signal or fieldbus you want
- mounting location and protection rating
- how often the measurement is needed
- whether a safety function is involved
To office@octogon.org or +43 676 362 8453.