Troubleshooting load cell applications

May 12th, 2016, Published in Articles: EE Publishers, Articles: EngineerIT, Articles: Vector

 

This article explains how a remote connectivity solution helps load cell application service technicians ease their troubleshooting task.

Before the introduction of the electronic load cell, mechanical lever scales were widely used in many industrial and commercial applications. The accuracy of these mechanical scales depends on the scale calibration and maintenance.

Today, load cell-based weighing systems dominate the entire weighing industry and are the industry standard. Though load cells are reliable, they are subject to damage due to overloading, lightning strikes, chemical or moisture ingress and mishandling (such as damaging the cable, or dropping the cell). Troubleshooting a scale fitted with more than one load cell to detect defective cells is always a tedious procedure. Multiple load cells also make troubleshooting harder. The larger the scale, the harder this can become.

Fig. 1: Weighing platform.

Fig. 1: Weighing platform.

Basic components of load cell-based weighing systems

Weighing platform

This is a weighing platform to hold whatever is to be weighed, allowing the force of gravity on the entire item to be transmitted to the load cells.

Load cell

A load cell is an electronic device (transducer) used to convert a force into an electrical signal. Load cells are wired to a weight indicator via a junction box.

Junction box

The junction box is where the load cell cables are terminated and summed. The weight indicator is also terminated here. The summing board is a passive circuit that joins the signals of the cells to the indicator as one signal. These boards normally have a trim potentiometer for each cell so that the signal of each cell can be adjusted so that no matter where a weight is placed on the scale, it will weigh the same. There are two types of summing method. Adjusting the excitation voltage to the individual cells (excitation trim), or shunting signal of individual cells (signal trim). On larger scales such as vehicle scales, the cells can also be trimmed in groups of two in sections.

Weight indicator

The weight indicator digitises the low-level signal voltage from the load cells and converts the voltage to a displayed weight. Normally, the weight indicator is mounted away from the scale in an area where the weight can be viewed easily on the indicator’s display. The indicator also supplies a regulated voltage known as “excitation” to power the load cells. This voltage is normally 10 or 5 V DC. The signal from the load cells will not exceed 30 mV with the maximum capacity applied to the cell.

Fig. 2: Structure of weight scale using a load cell.

Fig. 2: Structure of weight scale using a load cell.

How load cells work

Each load cell contains four wires. Two wires are used to provide power to the load cell (excitation), and two return a voltage reading proportional to the weight on the cells (signal). There are many varieties of load cell on the market but strain gauge-based load cells are the most commonly used. A strain gauge is a thin strip of metal designed to measure mechanical load by changing resistance when stretched or compressed within its elastic limits.

There are four strain gauges bonded in the inner surface of load cell that form a Wheatstone bridge configuration (refer to Fig. 3). When stress is applied to the load cell, two strain gauges will be stretched, and the other two will be compressed. The stretched gauges will increase their resistance and the compressed gauges will decrease their resistance. If no stress is applied, there will be no voltage difference between the +SIG and -SIG output. If force is applied to the load cell, the strain gauge bridge will become slightly unbalanced and a voltage will appear between +SIG and -SIG output. The magnitude of this voltage is based on the load cell design and is rated in mV/V (millivolts of signal per volt of excitation) at full load. For a 10 V excitation to a load cell rated at 2 mV/V, the maximum output with a force of full capacity would be only 20 mV.

Fig. 3: Wheatstone bridge configuration.

Fig. 3: Wheatstone bridge configuration.

Troubleshooting challenges when servicing strain gauge type load cell based scale

A scale not displaying weight correctly, drifting, not providing repeatable measurement, or one that is unstable, are indications that the scale needs service. Regular maintenance on the scale can improve the over-all reliability of the load cells and provide an opportunity to discover water leaks and spills running under the scale, allowing them be corrected before the cell goes bad.

To troubleshoot a load cell-based scale, technicians must perform a cornering test to determine if the scale weighs the same no matter where the weight is placed. Secondly, they must perform a gross calibration test to determine if the scale is consistently weighing correctly with each application of weight. Bad load cells can be identified by measuring output from each load cell’s output with a handheld multimeter.

When performing corner testing, technicians must apply weight on the four corners of the platform. The signal to the weight indicator should read the same for each cell. On larger scales, such as a truck or a rail scale, one challenge technicians face is determining the multimeter reading without straining their eyes. Eye strain tends to be less of an issue when the technician is troubleshooting a small scale. On many scales, it is possible to identify a cell using a meter and measuring the signal while standing above that cell. This can speed up the process of finding a bad cell.

If resources allow, the technicians may also have to ask someone to read the multimeter reading. If not, they have to move the multimeter each time they move. Sometimes, the scale troubleshooting task is not limited to just the workshop and requires technicians to perform scale trouble- shooting outdoors, even if it is raining or snowing. This situation is worse if technicians need to view more than one meter reading at the same time.

If the weight indicator is mounted in a control cabinet, viewing the weight indicator from the scale platform is completely impossible. Such conditions make troubleshooting tedious.

Wireless remote connectivity

Wireless remote connectivity can be achieved by means of the new U1177A IR-to-Bluetooth adapter – an accessory that enables wireless remote connectivity via a Bluetooth connection (see Fig. 4).

Fig. 4: Attaching IR-to-Bluetooth adapter to the handheld multimter.

Fig. 4: Attaching IR-to-Bluetooth adapter to the handheld multimeter.

By attaching the Bluetooth adapter to the IR port located at the back of a digital multimeter, users can establish Bluetooth communication with a hand-held multimeter via an Android-based phone or tablet PC (see Fig. 5). This phone or tablet PC must be preloaded with free Android-based application software called Keysight mobile meter or mobile logger.

Fig. 5: Mobile meter application on Android phone.

Fig. 5: Mobile meter application on Android phone.

The mobile meter allows users to perform up to three multimeter measurements simultaneously and in real time (see Fig. 5). With an Android device in hand, service technicians do not need to strain their eyes to view multimeter readings that are far away, or move the meter each time they move to identify bad load cell. This also makes it possible to monitor voltage readings within an industrial cabinet with the door closed.

The mobile meter also allows users to extend their reach to two or three places without the need to be physically present at various points. This allows technicians to make measurements from a safe distance, eliminates the need to walk back and forth between the measure target and control points, and monitors multiple measurements simultaneously.

For troubleshooting or identifying intermittent problems such as a power supply failure in the junction box, a mobile logger (a free Android application) allows users to log data over a long period of time and provides trending graphs from Keysight handheld digital multimeters.

An array of functions are available, such as the ability to send e-mail automatically or to short message service (SMS), and use pan and zoom functions via the Android device’s touch screen. Alternatively, data logging and monitoring activities can be performed at a PC using downloadable GUI data logger software.

Contact Steve Alves,  Concilium Technologies, Tel 011 678-92000, steve_alves@concilium.co.za

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