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4-20ma

mgoutier 4 months ago updated by Ed Schippers 4 months ago 7

I am looking to control a 4-20ma device from and MC43, and I was curious if anyone has done this with the current outputs using a max range of 20ma? It is just such a small part of the range I was curious if anyone has had issues, or have done it. 

you will struggle to control it if it works at all.

We got over it using a can to analogue device from third party supplier. (Seneca)

Hi,

I have a new project and am planning to use four 4-20mA inputs and well as one 4-20 mA output.

I'm not sure in regards to the issues AndyR indicted in the previous post.  I've worked with IQAN for 12 years now.  I've occasionally had issue with COUT's as you can not reliably control anything between the 0.0 mA to 100 mA. range.  I am just now getting ready to try the 4-20 mA interface on a MC43.

I get the feeling you are concerned about how well something can be controlled with only 0-20 mA of current.  You should not be.  This has been in use for at least 75 years, and is one of the most common interfaces for industrial controls due to the inherent noise immunity, device self powering from current loop, etc. .  See https://en.wikipedia.org/wiki/Current_loop


The A/D's that IQAN uses are 12 bit, which provides 4096 bits total.  20 mA/4096 = 0.00000488 Amps/bit, i.e.

4.88 microamps/bit which should be plenty to achieve accurate control

I'd like to hear more details from AndyR as to what issues were encountered, and why he had to switch to a Seneca device.

Thanks,

Using the current loop input for the MC43 should be straight forward.  However, trying to control a 4-20 mA device with an MC43 output will not be possible.  The proportional outputs of an MC43 are designed to be valve drivers and are rated at maximum of 2500 mA.  Referencing Appendix A of the Instruction Book, the Output range is 100 to 2500 mA for Current Outputs.  For PWM outputs, the minimum load is 60 mA.

My earlier comments reflect the issues we experienced some years ago (Iqan design3.0) on a marine project where we had to supply a twin MTU propulsion plant with a 4-20mA speed demand (MTU will not allow third parties to access their ECUs via can) after playing for a short time with the Iqan hardware available  at that time it was soon apparent the current drivers were neither accurate nor repeatable enough to use at such a low range of their potential range of 0-2000mA - as you know the outputs are designed for driving current driven proportional coils, not command loops. 
maybe things are better now with the latest hardware - I have not had the need to investigate. The solution we came up with then has stood the test of time and the vessels the system controls are still in service 10 years down the line.  the Seneca units we used are still available and are simple to set up and program. They provide three independent analogue outputs either voltage and/or current.

further to the above, standard industrial command signal outputs (voltage and/or current) have been requested by me and others on this forum previously to the Iqan design team but we understand to implement this requires considerable hardware modification and as such we cannot expect to get it for some time if at all.

We have used the IFM CR3008/CR3003 modules which work great with iQAN PWM outputs to control a device that wants an analog 4-20mA or 0-5V signal.

i'm using the Seneca modules also for years now. big advantage is that it uses CANbus, so it does not uses up Iqan outputs