Low Pass Filter info in RS 5000

[ghaulk]

I was wondering if anyone could tell me excatly how a LPF instruction works in RS Logix 5000?

 

[kamenges]

Can you limit the scope of your question a little bit. Do you want to know where to use it, how to connect it, how to configure it or what the specific equations are that the plc uses to determine the output?


Keith

 

[ghaulk]

Yes. It seems that the instruction has a buffer in it that apparently has many numbers that it's averaging. (I assume) I would like to know if you can change the buffer size, or even see the buffer numbers that are being used.

 

[kamenges]

Where to begin.

There are many others on this forum that can explain this better than me. But I'll try to give this a quick shot. This isn't an averaging filter. There isn't some large buffer of data being held. At the most there are three output values being maintained. The rest of the information is rolled into the latest filter output.

Think of this filter acting like up to three RC filter in series. The number of RC filters is determined but the .Order field. How aggressively these filters act is based on the .WLag field.

Back to the RC filter example. If you know what a capacitor charging curve looks like you know what a first order filter output curve looks like. It is generally held that a capacitor will reach its full charge value in five 'time constants'. One time constant is the time take for the output to reach 62.5% of the difference between the input and the current output. As it happens the time constant is also equal to 1 / WLag of WLag is in radians/sec.

So, to make the filter respond faster you can make .WLag larger. Also, as .Order increases you will add delay from input to output. But as .Order increases you also get a more discriminating filter (passes low frequencies but cut's off high frequencies).

I hope this generates more questions. Filters are generally an interesting subject.

Keith

 

[ghaulk]

We are using this filter prior to a PIDE. When we raise the .WLag (.5 is max), we do see an increase in response time but this in turn makes the PIDE untunable(not sure if that's even a word but you understand). If we lower the .WLag the PIDE comes right in and the process will run somewhat stable. This process is a loss in weight application. While we are running a stable rate of loss, everything is fine. When it comes time to refill the hopper the Low Pass Filter takes up to 45 seconds to update the weight. We need it to update much quicker. Does this make any sense?

 

[russrmartin]

Limitations

I think I see where you are coming from. Based on my limited experience, and from reading several of PeterNachtwey's rants on feed forwards, this sounds like a classic example of where one is needed, or would help improve control.

From what you are saying, you have the loop tuned pretty well to accept normal variances in weight scale output as the process is running. However, when you refill the hopper, this is a too much too fast change which drives system performance down.

My suggestion to you would be to try and obtain information which you can input into the system to indicate that this large signal change is going to take place, and design the loop to respond accordingly.

In short, I think you're simply seeing the limitations of the system at work.

Russ

 

[kamenges]

I guess I don't exactly understand what you are controlling. My guess is that your actuator is controlling the rate of loss on weight. You are using load cells to measure the weight, which means you are determining rate by taking the differential of this value. Am I about right?

IF this is the case, russmartin is right about the use of feed forward. Since you know what the target rate is you should be able to use that to set a baseline actuator output. This feed forward value should account for the majority of the output. Since it is based on the command and not feedback it is unconditionally stable. You can then run with much lower gains since the whole signal doesn't need to come from the PID loop.

Then again, this is all a guess on the system you have. A good system description would help.

Keith

 

[ghaulk]

Sorry about that. I guess I should have done that from the start. What we have is a Hopper sitting on top of a load cell with a vibratory feeder underneath. We receive the signal from the load cell through a Red Lion Signal Conditioner and then to the PLC. We then have to filter the signal in order to send it to the PIDE which in turn controls the vibratory feeder.

Having explained all that, I was thinking yesterday about using a signal conditioner with filtering built in. Then we could eliminate the need for it in the PLC. What do ya think?

Thanks,
Greg

 

[kamenges]

Most signal conditioners don't have particularly discriminating filters. They are designed to take out PWM artifacts, which are in the kiloHertz range and above. You are going to have a huge noise contribution due to the feeder.

One of the issues you most likely have with the filter in the plc program is update rate. To effectively filter a signal you need to be sampling faster than the signal. The plc scan and analog update time may be getting in the way of this. You might also want to try using the filters on the input module itself. These run much faster and can do a much better job of attenuating noise.

If that doesn't work take a look here. Go under the product name index and look at the GSAF-10. I've used these in tension control applications in the past to get rid of the vibration of a load cell roll. It is a 9-pole filter and you can specify the corner frequency. It produces some phase delay but for your application that shouldn't matter much. I ordered a 15 Hz corner frequency and could barely see a 60 Hz full voltage input on the output side. I recommend the Bessel filter as it won't ring given a step input.

If you really want the Cadillac solution look at this. Take the time to view the videos; they are rather impressive and very applicable to what you are doing. Specifically look here. there is also a version for the 1769 I/O format. It will perform the rate of change calculation for you. They are a bit pricey but so is your time.

Are you determining feed rate but looking at the change in weight?

I also still recommend you implement a feed forward term if you haven't already. This may end up being somewhat non-linear but anything you can add to the output that the PID doesn't need to generate will help, especially when you change feedrate and start up.

Keith