Make a sensitive weigh scale platform the easy way.
Estimated reading time: 6 minutes
It’s easy to design a platform scale if you use Mini-Mounts to support your platter. You are not limited to a particular size. As a result, you can make a large platform without reducing capacity at the corners. If you use a single-point load cell to make a platform scale, you have to reduce the capacity at the corners. Check your load cell specs. We match the load cells of the Mini-Mounts. We have “current matched” them for connection of the four cells in parallel. As a result, you can achieve accuracy within 0.05% across the whole platform. Our 1945 USB transmitter uses this load cell connection method and connects to your PC computer through a single cable. Consequently, all you have to do is plug them in.
Alternatively, you can use a regular weight indicator such as our TI-500E. In that case you need to cut off the connectors and wire all the load cell cables in parallel (like colors to like colors). If you need better accuracy, we can do this too, by selecting matched sets of Mini-Mounts.
- Capacity – 30 or 50 kg, giving total capacity of 120 kg (260 lb) using 30 kg mounts or 200 kg (440 lb) using 50 kg mounts.
- Corner capacity is equal to cell capacity – 30 or 50 kg (66 or 110 lb).
- Fully overload protected in all directions except tension.
- Mounting surface must be flat for 2″ diameter around top of Mini-Mount.
- Place Mini-Mounts as close to extreme corners as possible, for best accuracy and scale capacity.
- Use 1/4-20 screws to secure platter to Mini-Mounts, or weld studs if you need to lift platter off mounts.
- Secure mounts to base structure with 1/4-20 countersunk screws.
- If you need to move mounts to position them, use 1/4-20 or smaller screws with washers.
- For other scale components, see Scales.
Scale capacity and resolution
We rate the load cells at 30 kg or 50 kg capacity, but that has little to do with your platform scale capacity. Be sure not to exceed the capacity of the load cell, because if you do, the overload stop will come into action. The safe design limits the load, regardless of what you are trying to do with it. At the other extreme, if you try to use a 50 kg load cell to weigh a light object such as a banknote with accuracy, it is unlikely to be successful.
Today’s weight indicators and our 1945 transmitter use very stable electronics with high resolution. Consequently, this enables you to set up the scale with a capacity of 100 to 150 lb (50 to 75 kg) and a graduation size of 0.01 lb (0.005 kg) and still get good stability. We set up our self-checkout platform scales in this way and, as a result, they can detect a greeting card with envelope or a packet of razor blades.
If you use four 50 kg Mini-Mounts to make a 150 lb (70 kg) scale, you can place a 110 lb (50 kg) load right on the corner of the platter, or a full capacity load 2/3 of the distance between the center and the corner. If you use a very heavy platter or a small conveyor with internal roller drive, this will limit your maximum capacity, of course.
What is RMS jitter?
The ultimate resolution, or the smallest weight you can measure, is affected by electrical noise. This is seen as jumps in the digital reading, or jitter. The noise is random and we use the RMS (root mean square) value as the measure. Nobody wants to use a scale that jumps around and the ones in self-checkout applications depend on stable readings to report the weight of the items. They check the item weight against the stored database of weights for the product, to verify that the correct item is being bagged.
The conversion from analog voltage to a digital number has 24 bit accuracy, or a value between -8 million and +8 million. The practical resolution is, of course, much less and the 1945 transmitter gives -150,000 to +150,000 counts with an RMS error of 1 count. Most weight indicators, if designed with the latest ADC chips, should show a similar resolution.
The Scale App at right has many useful features to help you set up your scale to perfection. As well as a weight indication, the app has scale settings, full calibration, filter settings and a settling time indicator. Also included are auto zero tracking, under/over notification and re-zero buttons, file store and program update. A useful feature allows you to reset the altitude and latitude when you move the scale. This allows it to be adjusted without the need to recalibrate with test weights.
Filtering and response time (or speed)
Every weight indicator, and the 1945 transmitter too, has a stability filter. It removes reading jitter caused by vibration, wind, shocks and objects dropped onto the platform. When this filter is set too high, it increases the response time. Therein lies the problem: you can have a fast responding platform scale or you can have a very stable scale, but you can’t have both. It’s a compromise and the trick is to set the filter to get the best of both worlds.
Most transmitters and indicators use the simplest “low-pass” filter, usually 3 stages of a digital equivalent of the common R-C electronic circuit. The 1945 transmitter uses an advanced recursive digital filter that gives faster settling than the simple filter above, typically 0.3 to 0.6 sec. as well as high rejection of vibration and noise. Like all filters, it can be adjusted and will give the best results when the best compromise has been reached. Finally, make your platform structure stiff and you will have a platform scale that’s both fast and accurate.