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IO-LC3 specification clarification


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We are looking at using the LC3 to measure a pair of load cells at speeds where I am concerned about the response time of the Unitronics system. We got bit a while back by not paying enough attention to conversion times of certain modules and found out that some were quite long.

Anyhow, I want to make certain I understand clearly the response I can expect when measuring 2 load cells with the LC3.

The published spec is 12.5msec. So the first question is is that total or is it per channel? Will it actually do 12.5msec for both channels independantly, 12.5msec per each channel is use (total 25msec), or worst of all 12.5msec per each channel whether they are used or not (32.5 msec)?

I also found this disturbing spec in the help file.

Note

Minimum settling times for projects using multiple loadcells are


  • 12.5ms for one active loadcell

  • 675ms for two active loadcells

  • 1,012.5ms for three active loadcells

What does this mean exactly? Is this settling on the electronics end? The processing/filtering/program end? How can it go from 12.5ms to 675ms?!? What is the settling time indicating and how is it related to the conversion time? How does this correlate to using multiple LC3 modules. Does the above minimum settling times apply on a per module basis, or does it continue to get exponentially worse for every channel I use?

Could I use two LC1 modules instead, and get 12.5ms each to elimate the severe degradation in performance.

On a different but unrelated note, we had a customer considering these for their high speed check weighing machines.

It seems as though the IO-LC1 and IO-LC3 are not suitable for high speed check weighing applications based on how slow and unresponsive the electronics are.

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I also found these two seemingly conflicting statements in the specification sheets

The number of Loadcell inputs used per expansion module affects the sampling rate for each of them,

resulting in lower filter depth and effective resolution.

The filter settling time can be programmed separately for each Loadcell input without affecting the

other(s).

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Hi Damian,

My answer is no substitute for the wisdom of the creators, however I would suggest the specs on settling time are to be taken at face value.

If you want speed, then multiple LC1 modules sounds like the way to go.

I suspect that in the IO-LC3 the A/D convertor is multiplexed to the loadcell signals. When only one signal is active, it is fed directly to the convertor. When 2 or 3 are active, the multiplexer comes into play and there must be extra time between each sample for the old signal to be cleared and the new signal to settle (think about capacitance and high-impedance circuits).

(I have seen a similar spec like this with a small Allen-Bradley PLC. Our customer added a second analogue sensor to the PLC and configured a second input on the PLC to read it. Following that we got a call to say our sensor was too slow. It turned out the analogue input sample time of the PLC increased dramtically when the second input was configured. I think it also had something to do with one input being 0...10V and the other 4...20mA.)

As for filter and settling time.

1. the slower sample rate will mean much fewer values to work with. So if the base sample period is 675ms, you should expect strange results if you settling time is less than this value, and it probably should be 5-10 times the sample time.

2. I don't think the statement about independent setting of the filter parameters conflicts with the statement about the interaction between base sample time and filter depth. It just means that once you take account of the slower sample rate and how it will affect your filter times, you can still set different filters. For example one settline time can be 5 times the sample time, and the other can be 10 times the sample time.

I hope this helps.

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Hi Simon, Thanks for your input. It is always appreciated.

I did expect that they were multiplexing a single A/D converter, but the jump from 12.5ms to 675ms doesn't make sense to me at all. Why would adding one channel add 650ms to the cycle, but adding a third only add 337.5 ms? And why so long in either case? The price of the LC3 is very attractive if you compare it to purchasing multiple load cell amps, but I find myself leaning towards doing that and just mounting a standard analog input card. From my standpoint they should have three seprate A/D converters and multiplexing the data side instead. In this day and age what does an A/D converter cost?! Even the expensive ones are cheap.

1. the slower sample rate will mean much fewer values to work with. So if the base sample period is 675ms, you should expect strange results if you settling time is less than this value, and it probably should be 5-10 times the sample time.

I'm not certain how they are doing the filters, but It would seem the sample times alone almost render the filtering useless when using more than 1 channel. Unless your process is really slow.

2. I don't think the statement about independent setting of the filter parameters conflicts with the statement about the interaction between base sample time and filter depth.

You're right. The act of adjusting the values of the filters on each channel does not affect the filtering globally. The act of adding more channels does affect the filtering. I was reading more into it than what was really being said.

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Hi,

I just thought that I would put in my 2cents worth. We have put together several weighing

systems lately and what we have determined is that we use the LC3 for large bulk tanks

that don't need the speed and the LC1 for speed.

Speaking for speed and reliablilty, we have some systems that we fill 5gallon buckets of

stucco with a 3inch ball valve by gravity. With the head pressure always changing the system

always corrects itself for the next bucket. We average 1.25 to 2.5 second fill times. We love

it using the LC1. We have also done a couple of machines to do high speed filling that us

2 LC1's and we haven't noticed and loss in speed or reliability.

Good luck.

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Hi,

Can I participate to this interesting discussion?

OK - LC3 has one A/D converter. There is built in buffer for filtering.

In LC1 the buffer is filled in the first half secind after powering the system and thhen the result is given in FIFO. When you have 3 channels, each time you switch between them you need to reset the buffer and to wait untill it will fill again. That's why the min settling time for two and 3 channels is way greater than the one for one channel.

Small tip: In advanced functions of FB Loadcell you have option "Disable all other chanels" This way you make one channel only runnning in LC3 with the speed of LC1. This is not universal soluiton, but if you don't need more then one chanel at a time, you can use LC 3 and in each moment switch to the relevant chanel. With this easy lader "gimnastics" you will get 3 x LC1 in gthe place and price of one...

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Hi,

Can I participate to this interesting discussion?

OK - LC3 has one A/D converter. There is built in buffer for filtering.

In LC1 the buffer is filled in the first half secind after powering the system and thhen the result is given in FIFO. When you have 3 channels, each time you switch between them you need to reset the buffer and to wait untill it will fill again. That's why the min settling time for two and 3 channels is way greater than the one for one channel.

Small tip: In advanced functions of FB Loadcell you have option "Disable all other chanels" This way you make one channel only runnning in LC3 with the speed of LC1. This is not universal soluiton, but if you don't need more then one chanel at a time, you can use LC 3 and in each moment switch to the relevant chanel. With this easy lader "gimnastics" you will get 3 x LC1 in gthe place and price of one...

Emil, Thanks for the detailed response. It was definitely more along the lines of what I was looking for. Could you elaborate more on what this buffer is for and is doing?

Thanks,

Damian

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