Fruit growers often struggle with weighbridges not performing accurately. Errors of 5-10g (and even more) are common. But what causes this? Most factors are mechanical in nature, but the control system can also play a part.

Cup linkage design

Certain cup designs are simply bad. Inspect your cups closely. What you need is for the cup to somehow be pushed along the track, instead of dragged by the rod. This is typically done by having linkage arms on the side of the cup, connecting the rear of the cup to the rod in front, or by having the cup sit loosely inside a frame.

If the cup is simply hanging on the rod from its front end, friction on the rod causes it to carry a small percentage of the weight in an uncontrolled manner, causing significant errors.

Cup orientation on scale

It is important for the cup to be completely flat on the weighbridge as it passes by. Bad linkage design can often cause the cup to be tilted forward, so that some of the weigh points are hanging in the air, with predictably disastrous effects on results.

 

Cup weigh points

Figure 1: A good cup design will ensure accuracy in measurements.

A good cup is designed so that both the front and rear end of the cup move onto the scale simultaneously. This is typically achieved by having a 3-point system (one weigh point in the middle front and two in the back), or by having two sets of staggered weigh points whereby the front points are narrower than the rear. The advantage of this is that the entire cup is on the scale for a longer period, giving the system more time to weigh the cup.

Weighbridge alignment

The area where the cup transfers from the track onto the weighing area needs to be aligned so that the cup does not bang against the weighbridge, but gently transfers onto it. A good idea is to place your finger in the cup as it moves over the bridge. You want a relatively smooth ride. If it is bad, you will feel an ugly bump as it goes across.

Lubrication

A well-designed cup can give accurate results without much fuss. If the system is calibrated, you will always get a good result. A bad cup design can still give accurate results if you do some basic maintenance. The weighing area, especially where the cups are sliding, should be lubricated daily. The rods where the cups are hanging should also be lubricated from time to time using silicone spray. Regularly check the cup orientation by watching from the side to make sure the cups remain perfectly horizontal during their travel.

Mechanical integrity

Make sure your loadcell and weighbridge are properly fastened and that the weighbridge is completely isolated. It should not be touching any adjacent hardware and should not have any debris. Clean gently, as the loadcells are sensitive devices.

Filter design

The raw readings the machine receives from the loadcell are rather noisy (blue line in Figure 2). The machine needs to apply some clever filtering techniques to tease a good ‘Table Mountain’ signal out of the mess (red line in Figure 2). Good control systems have digital filters providing excellent characteristics. One way to know if you have a digital filter or not is to answer this question: Does your machine have any loadcell adjustments that require a screwdriver? If so, your machine likely has analogue (old) filters and will not work as accurately.

Figure 2: Displayed loadcell readings.

Conclusion

A good machine will give accurate results within 1-2g of the true value, all the time. A bad machine or cup design will always take a lot of care to give reasonable results, and, even then, accuracy will never be much better than 3-5g. Use this information to choose wisely when you buy a new machine, and if you already have a ‘bad’ machine, inquire over your retrofit and upgrade options. Perhaps your old machine does not have to remain as inaccurate.-Carel Hauptfleisch, Agrisort Technologies

For more information, phone Carel Hauptfleisch on 021 981 7062 or visit www.AgriSort.com.

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