Monday, May 18, 2015

Load Sensor Technology Behind Truck Weighbridge Scales

Whether for mining sites, high-volume farms, or free-ways, heavy-duty weighbridge truck scales are built to handle an enormous amount of weight. For instance, truck weighing stations are usually fitted with these scales to ensure that the weight of the truck (and its load) is captured for variety of reasons (i.e. tax purposes, daily business operations, and ensuring that the weight falls within the safety guidelines.)

This weight is usually calculated through two measurements; the axle weight which constitutes the amount of weight carried by each axle and the Gross weight which is the combined weight of all the axles. Truck scales are usually built out of a combination of concrete and steel.

It's All in the Load Cell

To accurately measure the weight of a truck, these truck weighbridge scales usually use load cells to capture the weight. A load cell is simply a transducer or sensor which converts a force or load into an electric signal. This signal can either be a frequency change, a current change or a voltage change; all this does depend on the circuitry and the type of load cell that’s been used.

These load cells can be made using inductive, resistive or capacitive techniques. The most common load cell technology for truck weighbridge scales tend to be resistive; working on the on the principle of change of resistance in response to an applied load. In other areas they are termed as piezoresistive, which is something that changes in response to an applied squeeze or pressure.

Truck Load Sensor Componentry

The load sensors on most truck weighbridge scales are built using resistive bonded strain gauges. A strain gauge basically consists of wires which transmit a very mild electric current. The strain gauges can either be tension or compression based. A tension strain gauge is based on the slight change in shape of the cell that’s caused by the weight while a compression strain gauge is based on how much the cell compresses when some pressure is applied on it.

Usually 4 strain gauges are configured in a Wheatstone bridge configuration with 4 separate resistors connected. An excitation voltage of 10V is usually applied to one set of corners and the difference in voltage between the two corners is measured between the other 2 corners; at equilibrium, with no applied load, the voltage output is usually zero or very close to zero.

When the metallic member to which these strain gauges are attached is stressed by any application of force, the strain experienced does lead to a change in resistance in either one or more of the resistors. This small change in resistance does result in a change in the output voltage. This change, however small can be measured and then digitized after accurate amplification of the rather small milli-voltage signals to a slightly higher amplitude of 0-10V or 0-5V.

Weighbridge Scale Software

These readings can then be captured by appropriate weighbridge software and utilized accordingly. It is worth mentioning that most of these truck weighbridge scale load sensors come fitted with high level digital output formats such Wi-Fi, Bluetooth, USB and Xbee Wireless capabilities.

Software-integrated truck weighbridge load sensors allow users of the system to monitor the output on a PC with the help of appropriate weighbridge software and generate the requisite reports and undertake a wide range of analysis. This also allows for the storage of this data for future use.