Position: Following the advice in the previous four sections will ensure that your weight cell signal arrives at the weight controller in the cleanest form possible. But chances are, the signal still won't be absolutely clean. Why not Remember that the weight cell transmits a signal that represents mechanical force, and vibration is a mechanical force. Similarly, the weight controller measures an electrical signal, and RFI and EMI are electrical signals. But even if you can't entirely eliminate mechanical and electrical noise sources, you can select a weight controller that helps clean up less-than-perfect weight signals and improves weighing accuracy .
Figure 2: Cleaning up weight cell Signals
How a weight controller cleans up weight signals. Let's take a look at how a weight controller can clean up the weight signal from a weight 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 with Weighing Transmitter .
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.
Weight controller requirements. The weight controller requires several other features to ensure weight accuracy. The controller should have an analog-to-digital converter that can be synchronized with a 60-hertz line frequency to avoid the problem of "60-hertz hum" caused by noise from 60-hertz power lines and equipment. The controller's internal components should provide proper analog signal shielding to isolate the signal from stray interference. The controller's analog circuitry should also have high-grade electrical components to accurately process the weight cells' low-voltage weight signals.
Finally, consider three key weight controller specifications to ensure that your weighing system is accurate:
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Nonlinearity: ±0.01 percent of span (that is, the weighing system's selected operating range).
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Temperature effect on zero: ±0.0027 percent of span per degree Fahrenheit.
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Temperature effect on output: ±0.0027 percent of span per degree Fahrenheit.
As with a weighing load cell, nonlinearity effects on the weight controller are negligible for small weight changes. You can also ignore the temperature effect on zero if the controller tares before starting the weighing cycle. However, you do need to consider how temperature effects on output can affect your weighing accuracy.
