Turn a weight reading into a reliable process decision

A weighing controller processes information from a weighing system and, depending on the model, provides weight indication, fill cutoff, tolerance checking, dosing or data transfer. Displaying kilograms and controlling a production process are different tasks. Reliable weight measurement requires the mechanical arrangement, load cell, measurement electronics and control logic to work together.

The roles of the load cell, indicator and controller

A load cell converts applied force into an electrical signal. In common strain-gauge load cells, minute deformation of the metal body changes the resistance of the strain gauges. A weighing indicator processes and scales the signal. A controller, when equipped with the necessary outputs and software, commands a valve, feeder or reject mechanism.

Distinguish how the load cell works from the displayed unit: the sensor responds to force, while calibration allows the weighing system to indicate kilograms or tonnes. The difference between mass and weight matters here. A load cell alone is not a complete scale or filling-control system.

Input signals and connection compatibility

Raw bridge-type load cell sensitivity is commonly specified in mV/V. For example, a cell rated at 2 mV/V with a suitable 5 V excitation produces a nominal signal change of approximately 10 mV from zero to rated load. This small signal needs dedicated weighing electronics. A standard analogue input accepting 4–20 mA or 0–10 V requires a suitable transmitter; modules with dedicated load cell inputs are different.

Four-wire and six-wire connections require different arrangements. In a six-wire system, sense leads help monitor excitation voltage at the load cell. Use the manufacturer's wiring diagram for conductor colours, sense connections and shielding. For multiple cells, check the combined bridge load and the terminal's excitation capacity. Digital load cells also require compatible power, interfaces and protocols.

Capacity, tare and mechanical installation

Selecting a weighing sensor requires more than the intended product quantity. Account for the vessel or platform's own mass, maximum product load, uneven load distribution and impact forces. Taring does not reduce the mechanical load on the sensor or increase its capacity. Gross is the weighing result before tare subtraction; net equals gross minus tare.

Load cell types must suit the mechanics: single-point cells can serve platforms, suitable weigh modules can support vessel legs, and tension cells can serve suspended arrangements. Selection depends on the manufacturer's loading direction and mounting limits. Side loading, a platform rubbing against its surroundings or rigid pipework supporting a vessel can distort the reading. Software filtering cannot replace correct mechanical installation.

How are filling and dosing controlled?

In production-integrated weighing systems, coarse feed provides rapid filling, while fine feed controls the flow near the target. Material already falling or still flowing after a valve closes can continue to reach the scale. The cutoff point must account for this trailing material. The result is compared with the tolerance after the scale has settled.

For example, with a 25 kg target and an average 0.15 kg arriving after cutoff, fine feed might stop at approximately 24.85 kg. These values are illustrative: actual settings depend on product flow, valve delay and measurement response. Continuous gravimetric feeding can instead calculate feed rate from the vessel's mass loss over time; its controlled variable differs from the weight of an individual package.

Static weighing differs from checking moving products

Static weighing allows the load to stop and the reading to settle. In dynamic conveyor checkweighing, the product moves, so belt speed, product spacing, vibration and the available weighing time affect the result. Automatic weight checking also requires an out-of-tolerance product to be rejected at the correct moment. A fast indicator alone does not make a complete checkweigher.

Measurement update rate and display refresh rate are different specifications. Strong filtering can reduce fluctuations while delaying the response. Set filtering, stability criteria and cycle time together. A delayed reading can cause overfilling or an unsuitable evaluation window when checking moving products.

PLC integration, communication and outputs

When integrating a PLC, consider gross/net selection, stability, overload and instrument fault status as well as the weight value. With Modbus communication, data type, decimal scaling, register layout and update interval must match. An RS-485 port alone does not establish Modbus support. Check the load rating of relay or transistor outputs and select suitable drivers or interposing control devices according to the wiring diagram.

Calibration, checks and intended use

Calibration establishes the relationship between reference values and instrument indications, including associated uncertainties; zero or span adjustment changes the indication. Calibration and instrument adjustment are different operations. Include checks with suitable reference loads, repeated loading and different platform load positions in the application plan. Reassess performance after mechanical work or sensor replacement.

Intended use matters when considering weighing standards: OIML R 60 concerns load cells, R 76 non-automatic weighing instruments, R 51 automatic catchweighers, and R 61 automatic gravimetric filling instruments. A load cell certificate alone does not establish compliance of the complete system. Requirements for trade use, conformity and legal verification must be assessed separately for the location and instrument category.

When comparing products, specify capacity, target tolerance, cycle time, cell count, input signal, outputs and communication together. This helps identify a system that performs the actual weighing and control task, rather than one that merely has a suitable-looking display.