VSD Speed control with Pressure

[Jethro Baidoo]

The smart-a$$ answer is to use a twisted, shielded pair and connect it to the correct terminals on both the PLCs analog output module and the the drive.
There will also be a parameter in your VFD that you will need to set to configure it to accept the 4 -20 mA signal as its speed command.

I know...but how do I get the 4-20mA output in the 1st place??

 

[parky]

You need to place the speed integer into a scaling block the reverse of the analogue input scaling however, to control it you need to use some king of PID block presumably, the scaled analogue input will be the process variable (PV) and the required set point (SV) as well as the control values i.e. PID. the output from the PID block then re-scaled to the raw required for the analogue output
for example at the moment you may have
Scale analogue input
Raw input : Raw analogue input
In Min : Min for raw input
In Max: Max for raw input
Out Min: Min for scaled output i.e. 0
Out Max: Max for scaled output i.e. 10.0
Scaled Out: Actual scaled analogue i.e. 0.0-10.0

PID Block
Process variable: Actual scaled analogue input
Process set point: Required setpoint i.e. 5.0
P: Proportional component
I: Integral component
D: Derivative component
PID Output: for example 0-100%

Scale analogue output block
Scaled required value: 0-100%
Min Out: 0
Max Out: 2047
Scaled out : 0-2047
I believe there are some standard blocks for scale & unscale
but here is a link of how to build your own.
https://cruysen.nl/plc site/siemens files/1scaling.pdf
And here is a link to the PID function
https://cache.industry.siemens.com/dl/files/084/1137084/att_27126/v1/Stdpid_e.pdf

 

[drbitboy]

I know...but how do I get the 4-20mA output in the 1st place??

What type of analog outputs does the S7-300 have now?


Is there an expansion card for the S7-300 that outputs 4-20ma?


Would this help:


https://www.hicomponent.com/isolated-4-20ma-0-10v-5v-current-voltage-converter-din-rail.html


? I got that from sending the search terms [transducer 0-10v to 4-20ma] to The Google.


Also see this thread: http://www.plctalk.net/qanda/showthread.php?p=256426#post256426

 

[drbitboy]

Building on @parky's earlier post.

Below is an example image of what I suspect the final system will look like; there may be but one pump, but the approach is the same. Every intersection in that image where a line touches or crosses a box's or circle's boundary is an interface that needs to be understood, specified, and connected logically or physically, in a way that will make the system work. I count at least two dozen such interfaces assuming one VSD/Motor/Pump, and there will be additional implcit interfaces (scaling, transducers, etc.). Also, the PID box is but a single, simple box in that diagram, but it comprises perhaps 30% of the total complexity.

TL;DR

Each interface needs to be carefully designed, so the first question to ask oneself is if one can, or can learn to, understand every interface, and how they work together.

This thread is asking a lot of this forum. It is not reasonable to ask its members, many of whom get paid for consulting on projects comprising tens or hundreds of tasks of similar complexity, to design your system from scratch, and that is impossible with as little information as has been provided.

So here are some suggestions.

1) Ask a specific question e.g.

• How do I connect an S7-3xx Model XYZ card's analog output 0-10V signal to the 4-20mA speed reference on VSD Model ABC?

OR

2) Treat this like a homework problem.

• Show your work
  • A diagram of how you think this will work.
  • What has been designed and/or connected so far.
  • Show any PLC programming that has been done, even for testing e.g. to bring the pressure signal into the PLC
  • Etc.
• Give complete details of all components
  • Which S7-300?
  • What inputs, outputs, and expansion cards does the S7 have? What are the part numbers? What are the inputs types (voltage, mAmp, etc.)?
  • Will there be an HMI or physical controls? How will the pressure setpoint be provided to the system?
  • What are the part numbers of the VSD, the pressure sensor, the power supplies, HMI, etc.
  • What software is being used to program the S7?
  • What inputs and outputs does each component have? 2-wire? 3-wire? Voltage? mAmps?
  • Etc.
• State your experience level.
  • What pieces of this have you done before?
  • What pieces are a mystery to you?
  • Is PID a new concept to you?
  • Have you programmed S7-300 PLCs?
  • Are you facile with numbers and math?
  • Etc.
• If this level of detail turns into a long document, post it as an attachment; PDFs are handy for this. Or post it on some publicly-available website such as Github or Google Drive.

Please note with care the "Etc." entries above. Given the questions asked it will be hard to know what is important, so in this case more is indeed more.

 

[Jethro Baidoo]

You need to place the speed integer into a scaling block the reverse of the analogue input scaling however, to control it you need to use some king of PID block presumably, the scaled analogue input will be the process variable (PV) and the required set point (SV) as well as the control values i.e. PID. the output from the PID block then re-scaled to the raw required for the analogue output
for example at the moment you may have
Scale analogue input
Raw input : Raw analogue input
In Min : Min for raw input
In Max: Max for raw input
Out Min: Min for scaled output i.e. 0
Out Max: Max for scaled output i.e. 10.0
Scaled Out: Actual scaled analogue i.e. 0.0-10.0

PID Block
Process variable: Actual scaled analogue input
Process set point: Required setpoint i.e. 5.0
P: Proportional component
I: Integral component
D: Derivative component
PID Output: for example 0-100%

Scale analogue output block
Scaled required value: 0-100%
Min Out: 0
Max Out: 2047
Scaled out : 0-2047
I believe there are some standard blocks for scale & unscale
but here is a link of how to build your own.
https://cruysen.nl/plc site/siemens files/1scaling.pdf
And here is a link to the PID function
https://cache.industry.siemens.com/dl/files/084/1137084/att_27126/v1/Stdpid_e.pdf

Thanks parky...I think the trick is with the unscaling to get the PQW as speed reference.

 

[Jethro Baidoo]

Building on @parky's earlier post.

Below is an example image of what I suspect the final system will look like; there may be but one pump, but the approach is the same. Every intersection in that image where a line touches or crosses a box's or circle's boundary is an interface that needs to be understood, specified, and connected logically or physically, in a way that will make the system work. I count at least two dozen such interfaces assuming one VSD/Motor/Pump, and there will be additional implcit interfaces (scaling, transducers, etc.). Also, the PID box is but a single, simple box in that diagram, but it comprises perhaps 30% of the total complexity.

TL;DR

Each interface needs to be carefully designed, so the first question to ask oneself is if one can, or can learn to, understand every interface, and how they work together.

This thread is asking a lot of this forum. It is not reasonable to ask its members, many of whom get paid for consulting on projects comprising tens or hundreds of tasks of similar complexity, to design your system from scratch, and that is impossible with as little information as has been provided.

So here are some suggestions.

1) Ask a specific question e.g.

• How do I connect an S7-3xx Model XYZ card's analog output 0-10V signal to the 4-20mA speed reference on VSD Model ABC?

OR

2) Treat this like a homework problem.

• Show your work
  • A diagram of how you think this will work.
  • What has been designed and/or connected so far.
  • Show any PLC programming that has been done, even for testing e.g. to bring the pressure signal into the PLC
  • Etc.
• Give complete details of all components
  • Which S7-300?
  • What inputs, outputs, and expansion cards does the S7 have? What are the part numbers? What are the inputs types (voltage, mAmp, etc.)?
  • Will there be an HMI or physical controls? How will the pressure setpoint be provided to the system?
  • What are the part numbers of the VSD, the pressure sensor, the power supplies, HMI, etc.
  • What software is being used to program the S7?
  • What inputs and outputs does each component have? 2-wire? 3-wire? Voltage? mAmps?
  • Etc.
• State your experience level.
  • What pieces of this have you done before?
  • What pieces are a mystery to you?
  • Is PID a new concept to you?
  • Have you programmed S7-300 PLCs?
  • Are you facile with numbers and math?
  • Etc.
• If this level of detail turns into a long document, post it as an attachment; PDFs are handy for this. Or post it on some publicly-available website such as Github or Google Drive.

Please note with care the "Etc." entries above. Given the questions asked it will be hard to know what is important, so in this case more is indeed more.

thanks drbitboy...I will let you know the outcome