There are few areas of industry that feature as vast a range of products as mass measurement. This in itself creates a problem for the end-user in the choice of what equipment to purchase for the job at hand. To further complicate the customer's choice the difference in price between apparently similar systems can be huge. There are however simple guidelines that, if followed, will assure satisfaction when purchasing mass measurement equipment.
The first and most important step is to investigate what exactly it is that is required from the weighing system. Often the application is not engineered with the actual job at hand in mind. After all, any loadcell and transmitter will produce a 'readout'.
What should be looked for when engineering a system?
The weighing transmitter has a simple function to perform. It must supply the loadcells with an excitation voltage, receive the output signal back from the loadcell, convert it to an output and, in some cases, display the converted value. It can be seen that this is not a complicated process.
With this in mind, considering the following questions will result in the engineering of a good weighing system.
How accurate does the system need to be?
This will depend entirely on the quality and stability of the loadcell power supply, and the quality and stability of the analog to digital converter.
How fast does the system need to be?
If the loadcell signal is required to be transmitted to a secondary control unit, like a PLC, PC or DCS system, then a third component must be looked at: The speed and stability of the re-transmission output of the unit. This relies on the A/D conversion rate as well as the quality of the output card.
How friendly does the system need to be?
This does not necessarily mean that the unit must be easy to use; a good user-friendly menu does however make life simple and most importantly cuts down on downtime should adjustments or calibration changes be necessary. A friendly system should also hold its calibration and have the features that make the job simpler.
How reliable does the system need to be?
Reliability and quality go hand in hand. It is immediately apparent by looking at the technology used in the electronics as to the quality of the instrument. A reliable unit means better production and less downtime.
With the above in mind it becomes easier to choose the correct transmitter for an application. Weighing a product in motion will obviously require more accuracy and speed from the unit that you select.
The selection of a mass measurement system is often done without proper investigation and understanding of the application and the customer's requirements. The assumption that mass measurement equipment can be purchased over the counter is the biggest reason for a system's nonperformance. It is an exact science that requires good engineering of every application.
Power elect/tronics, (035) 753 2002
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