INSTRUMENTATION

Load cell instrumentation is the electronic equipment used to excite, measure, condition, display or transmit the electrical signal produced by a load cell.

A conventional strain gauge load cell typically produces a very small differential signal in millivolts per volt (mV/V). Instrumentation provides the bridge excitation and accurately measures this signal before converting it into useful engineering units or another electrical format.

Examples include load cell amplifiers, signal conditioners, digital indicators, data acquisition systems, transmitters and wireless telemetry.

Most strain gauge load cells produce only a small mV/V signal and therefore cannot be connected directly to conventional voltage or current inputs without suitable electronics.

Load cell instrumentation supplies stable bridge excitation, amplifies the differential output, filters electrical noise and converts the measurement into a usable analogue or digital signal.

The instrumentation is therefore an integral part of the measurement chain and can have a significant influence on accuracy, resolution, stability and measurement uncertainty.

mV/V means millivolts of output per volt of excitation and is the standard way of expressing the sensitivity of many strain gauge load cells.

For example, a 2 mV/V load cell excited with 10 V produces approximately 20 mV at its rated load: 2 mV/V x 10 V = 20 mV.

At zero load the bridge output will be close to zero, and the signal changes approximately proportionally with applied force.

A load cell indicator normally conditions the load cell signal and provides a local numerical display in engineering units such as N, kN, kg or tonnes.

A signal conditioner primarily converts the raw load cell output into another electrical signal for use by a PLC, DAQ or control system and may not include a display.

Some instruments perform both functions and may additionally provide relays, alarms, data logging or digital communications.

Load cell instrumentation can provide analogue and digital outputs to suit different measurement and control systems.

Common analogue outputs include 0-5 V, 0-10 V, +/-10 V, 0.5-4.5 V and 4-20 mA. Digital options can include USB, serial communications, CAN and industrial fieldbus interfaces depending on the instrument.

The optimum output depends on the receiving equipment, cable distance, required update rate, resolution and electrical environment.

Both allow a conditioned load cell signal to be connected to industrial instrumentation or PLCs, but they have different characteristics.

A 0-10 V output represents load as a voltage and is straightforward to interface over relatively short cable runs. A 4-20 mA current loop output is generally more robust over longer cable distances and in electrically noisy industrial environments.

The 4 mA live zero also makes it possible for suitable systems to distinguish a zero measurement from certain wiring or power failures.

A raw mV/V strain gauge load cell can only be connected directly to a PLC if the PLC has a dedicated strain gauge or load cell input capable of supplying bridge excitation and measuring the differential mV signal.

Otherwise, a signal conditioner or transmitter is required to convert the load cell output into a signal accepted by the PLC, commonly 4-20 mA or 0-10 V.

PCM can supply the load cell and instrumentation as a matched system and configure the output for the customer's control system.

A digital signal conditioner converts the analogue output from a strain gauge load cell into digital measurement data.

It normally provides bridge excitation, amplification, analogue-to-digital conversion, filtering and calibration, and may provide functions such as tare, peak capture and scaling.

Digital conditioning can be particularly useful where measurements must be integrated directly into software, data acquisition or networked control systems.

Wireless telemetry allows load cell or strain gauge measurements to be transmitted without a conventional wired connection between the sensor and receiving instrumentation.

It is particularly useful on rotating, moving or difficult-to-access components where cables would restrict movement or be impractical.

Telemetry systems can be used for applications including rotating shafts, wheels, drivetrains, machinery and temporary test installations. Depending on the system, power may be supplied by batteries or transferred inductively.

A load cell junction box combines the signals from multiple load cells into a single measurement output. They are commonly used in weighing platforms, tanks, hoppers and other systems where several load cells support a common structure.

Many junction boxes also provide adjustment or corner balancing facilities to compensate for differences between individual sensors and mechanical load distribution.

The junction box must be compatible with the load cell bridge resistance, wiring configuration and required environmental protection.

Yes. A load cell and its associated indicator, amplifier or signal conditioner can be calibrated together as a complete measurement chain. This allows the calibration to include the behaviour of both the sensor and instrumentation rather than calibrating each component independently.

The system can then be configured to provide a required engineering unit display or analogue output at known applied forces.

PCM can calibrate compatible load cells together with their instrumentation and provide calibration documentation for the complete measurement system.

The correct instrumentation depends on the load cell output, bridge resistance, excitation requirements, required accuracy and resolution, measurement speed and the system receiving the measurement.

Consider whether a local display is required, the required analogue or digital output, cable length, environmental conditions, data logging requirements and whether the application is static or dynamic.

For complete measurement systems, selecting the load cell and instrumentation together usually produces a better result than treating them as independent components.