Jumping Analogue Signal

Hi, was wondering if you could help. I am using a Micrologix 1200 PLC analogue card that measures back analogue input signals between 0-30mV. The card is a IT4 Type used on weigh load cells etc.

Whats happening is that I'am measuring the weight of a substance (salt) that is being dispensed onto a conveyor, however, the analogue signal seams to be constantly jumping around before eventually decreasing as would be expected. The signal can even jump upwards which is causing problems as when I do my calculation to the amount dispensed I can get Zero etc and also it sometimes give a rapid decrease downwards before eventaully decraesing to that actual figure (I have this in a PID loop so control is poor)

I have written the code only to accept lower values each time so to avoid accepting a a current value greater than the previous value. This eliminates negitaive readings. I have also changed the card frequency down to 10Hz which is the lowest available on the drop down menu in configuration. Can it be set lower ?? i.e typing in a value not on drop down menu??. My PID has been set with limits to avoid rapid control to 0 or 100% but to be honest nothing sorts the problem.

Was wondering has anyone come accross or can offer a solution or could this be a problem with my weigh load cells?? Appreciate anyones help on this. Thanks - Rpax

The IT4 card is primarily a Thermocouple input card but it has a +/- 50 and 100mV range. There are a couple of things that I would suggest checking right away.

1)The stability of the load cell excitation power supply. This should be a clean ripple free DC signal. If you are having difficulty determining this then temporarily replace the power supply with a battery and compare the results for stability.

2)Vibration. Is there any vibration at all being transmitted from the hopper feed device, conveyor, or mounting structure to the hopper? Any vibration at all is going to cause problems with your reading.

Last, I have some comments about resolution. In any A/D conversion process there is some degree of increasing uncertianty about the least significant bits, the more insignificnat the bit, the more the uncertainty. And the resolution will also affect how small of a measurement you can take. For example, your input card is a 15 bit card. That means that you can resolve 1 part in 32767. If your load cell is rated for 30mV at 10kg, and you have the input range set for 50mV, then the best you are going to resolve is 509milligrams.

10kg/30mV * 50mV/32767counts = 509mg/count

Thats the theoretical resolution. However, its likely that there is a degree of uncertainty in at least the two least significant bits, that means that you probably have a couple of grams of uncertainty. So even in a completely vibration free system successive readings of the input could vary by 2 or more grams.

You haven't provided any information about the size of your load cell and the amount of salt being dispensed onto the conveyor, so I have no idea if this is helpful, but its something to keep in mind. Certainly if you have a 100kg load cell but your hopper and salt load are only 10kg or less then you are in real trouble with that setup.

If it is the problem then the solution starts with getting the right kind of instrumenation. You are using a 30mV signal on a 50mV input, squandering nearly half of your resolution right there, even more if you are using the 100mV input. I suggest you consider using a load cell amplifier and a 16 bit 4-20mA or 10V analog input. Even that may not provide you with the required resolution. You may need to use a exterenal weighing system with higher resolutions that interfaces with the PLC. AB has several encompass partners that make these kinds of devices.

possible troubleshooting

if I understand it correctly, you're trying to slow down the data fed to the PID due to the jumping values. Why not use a timer, every second or so send the weight to the PID ? That'll slow it down.

But how about fixing the real problem itself-why the jumping ? Is the load cell bad? How many load cell(s) being used ? Use two and average them out ? Any interference from electromagnetic or whatever ? I once saw a nextel phone causing an analog needle in a meter jumped when the two-way radio button was pressed.

Lads, just want to say thanks for taking the time to reply. Alaric\aotomaITion, your right, I actually have a 50kg load cell on the 15 bit card but i'd say the fundamental problem is with vibration were the load cell is located. Before having to revamp the process, might try your suggestions of delaying the reading on a cyclic timer. Let ye know how I go, Again thanks for help.

You'll NEVER get repeatable accurate readings until you deal with the vibration. If you delay the reading, you just delay the inaccuracies.

Oh dear OP ... vibration ? How could you missed that "little" issue ?