Dual 4-20mA signals for a motor

Thread Starter

KC30803

Joined Jan 30, 2024
9
Hi all,
I have the circuit attached as a way to ramp voltage up and down based on a customer's input of dual 4-20mA signals. This will then be turned into a PWM signal to control our motor. Is there a graceful way to handle the 4-20mA signals, as there can't be an instantaneous change. The ramp must go to zero before ramping up again (forward -> idle -> reverse). Thank you for your help.
Screenshot 2024-01-30 102536.png
 

Irving

Joined Jan 30, 2016
3,868
Welcome to AAC.

The source of the current loop is presumably driven by the user who is at liberty to change the current instantaneously. What you need is an integrator that generates a ramp from the given input.

eg
1706640252041.png

Not sure why you have 2 current loops, unless one controls CW and the other CCW speed, in which case that's just an extension of the above.
 

Thread Starter

KC30803

Joined Jan 30, 2024
9
Welcome to AAC.

The source of the current loop is presumably driven by the user who is at liberty to change the current instantaneously. What you need is an integrator that generates a ramp from the given input.

eg
View attachment 313959

Not sure why you have 2 current loops, unless one controls CW and the other CCW speed, in which case that's just an extension of the above.
Hi Irving,
Thank you for the reply. How would I go about making sure that if the user sends both signals for CW and CCW the higher one is taken/ they're subtracted from each other. Thank you.
 

Irving

Joined Jan 30, 2016
3,868
How is the user inputting these commands? It would be unusual to provide CW and CCW commands separately. More common would be speed and direction through eg a joystick or rotary control.
 

Thread Starter

KC30803

Joined Jan 30, 2024
9
How is the user inputting these commands? It would be unusual to provide CW and CCW commands separately. More common would be speed and direction through eg a joystick or rotary control.
It is dual 4-20mA signals from a PLC, and they seem to be using the second signal for braking. Thank you for your help.
 
Last edited:

ericgibbs

Joined Jan 29, 2010
18,835
Hi KC,
I assume that 4mA will correspond to the PWM Full OFF of the 20mA the Full ON Speed of the motor?

When you say braking for the 2nd 4mA to 20mA, that will mean the two PWM Ramping signals will have to be synchronised in some way?
Also, are the two 4mA to20mA inputs interlocked in sync?

E
 

Thread Starter

KC30803

Joined Jan 30, 2024
9
Hi KC,
I assume that 4mA will correspond to the PWM Full OFF of the 20mA the Full ON Speed of the motor?

When you say braking for the 2nd 4mA to 20mA, that will mean the two PWM Ramping signals will have to be synchronised in some way?
Also, are the two 4mA to20mA inputs interlocked in sync?

E
Thank you for the reply. Yes, the 4mA signal will be off and 20mA will be the full on signal. The second signal will be for braking/reverse functionality. I believe they will have to be synchronized but I don't have a good method of doing so. I'm unfamiliar with them being "interlocked in sync." I thought about using an H-Bridge to accomplish what I need but upon further research that method is for powering the motor and being able to flip polarity on the power provided. Thank you for your help.
 

ericgibbs

Joined Jan 29, 2010
18,835
I need but upon further research that method is for powering the motor and being able to flip polarity on the power provided
Hi,
If you flip the motor power, when say running at full PWM speed that will mean that full power braking will be applied, do you really want that?
Describe how the two 4-20mA sources are generated.
E
 

Thread Starter

KC30803

Joined Jan 30, 2024
9
I do not want that, I have an integrator designed for ramping down. They're being generated by a PLC, likely driven by a yokogawa or other temp controller.
 

Irving

Joined Jan 30, 2016
3,868
It is dual 4-20mA signals from a PLC, and they seem to be using the second signal for braking. Thank you for your help.
So how do they select direction? Or is it always one way?

Can you explain more how the overall system works? What does the motor drive? And the 'user' is a PLC?
 

Thread Starter

KC30803

Joined Jan 30, 2024
9
The direction will be selected based on which signal is greater and braked based on the opposing signal. The motor is driving a fan that can be used as a push or a pull for a furnace. The PLC will control the signal based on the temperature in the furnace.
 

Irving

Joined Jan 30, 2016
3,868
If you're using a 4-20mA current loop you should simulate it correctly as a floating current source with a burden resistor. You cannot assume one or other side is grounded.

Notwithstanding that, I don't see how that circuit satisfies:
5V for full push, 0V for off and 5V for full pull
as its subtracting the two signals and you only have one output, so you'll have +X for full push and -X for full pull. Most motor controllers need 0 - 5v or 0 - 10v analog for speed (not PWM) and a logic direction. Plus you still don't have any slew rate control so direction changes will still be stressing the motor.

Without more info about how these two competing inputs are related temporally AND knowledge of the motor controller's input requirement you cannot say what is needed. And that's without any consideration of undershoot or overshoot issues for which you'll need to implement a PID feedback loop or similar if you want to avoid any wild swings in furnace temperature.

BTW why have you chosen that opamp?
 
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