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How a weight controller cleans up weight signals

2009-06-25 15:52   Article Source:hardyinst.com   View Times:

Let's take a look at how a weight controller can clean up the weight signal from a load cell. Consider the example of a signal coming from a typical weigh hopper, as shown in Figure 2a. Theoretically, the weight signal should move smoothly upward on the Figure 2a plot as material enters the hopper. But in reality, the signal may roll slowly, caused by the hopper's swinging and swaying or by material entering the hopper in pulses, such as from an improperly installed auger. Mechanical vibration, such as from a hopper agitator or nearby process equipment, or electrical noise, such as from large power lines nearby, can also cause fast jitter in the signal.

If the signal enters a weight controller equipped with an analog low-pass filter (typically rated from 5 to 20 hertz), the filter will strip off random jitter -- thus providing analog averaging -- and yield a signal similar to that in Figure 2b.

A weight controller equipped with a dual-slope, analog-to-digital converter can also help digitally average other random signal fluctuations. Once the controller digitizes the signal, it can average the readings to smooth out the slow rolling and yield a representative signal like that in Figure 2c. Such digital averaging is especially useful for averaging from 1 to 250 readings per weighing cycle when the weighing system is set up to take weight readings at single-unit increments (for example, at 1-pound increments rather than 5-pound increments on a system with a 200-pound range). In some applications, you may have to use a weight controller that also provides built-in proprietary algorithms that automatically eliminate the effects of signal fluctuations down to 0.25 hertz.

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