set point control

Thread Starter

bexy

Joined Apr 2, 2020
15
can a op amp be used for simple direct set point control for 0 to 10vdc signals. basically i am thinking + input is 0 to 10vdc signal and - input is a 10K pot. now the signals would need adjusting so the output is 0 to 10vdc but based on the 10K pot. so if the pot is at say 5k or half way and the + input is at 3 volts the output would go to maybe 7vdc conversly if the 0 to 20vdc input was 7 VDC the output would go down to 3 VDC. so as the input goes higher the output goes lower. have i got this right? is there a bettrer way? all thoughts welcome.
 

Reloadron

Joined Jan 15, 2015
5,997
I am not quite sure what you are asking but to quote:
"Operational Amplifiers, also known as Op-amps, are basically a voltage amplifying device designed to be used with components like capacitors and resistors, between its in/out terminals. They are essentially a core part of analog devices".
Introduction to Operational Amplifiers (Op-amps) is a good read on the subject. The idea being how the op-amp is configured in a circuit and what we want it to do. Basically a voltage amplifier with an inverting and non-inverting inputs. Just as an example a Differential Amplifier amplifies the voltage difference present on its inputs both inverting and non-inverting which is what I think you are getting at.

My choice for a basic set point control would be a circuit commonly called a comparator where a variable is compared to a set point. While an op-amp can be configured as a comparator there are also comparator chips better suited for the application.

Ron
 

crutschow

Joined Mar 14, 2008
27,457
Op amps can add and subtract signals using a different gain for each signal, but that cannot multiply signals, if that's what you are asking.
For that you need a multiplier IC.
 

Wolframore

Joined Jan 21, 2019
2,318
A transimpedance amplifier can multiply and theres a quad opamp multiplier circuit which I’m sure you can find. I’m confused after reading the requirement.
 
1st part: OK
generally you would use a reference IC (e.g. 10V), buffer it and drive the end of the potentiometer. Now, the potentiometer sees a precise 10V source.

You then have to buffer (x1 gain) to create a signal that you can use with a low impeadance, An easy way to current limit the output is to put a series resistor in the output and take the feedback at the source point of your 0-10V signal.

You have to deal with offsets, offset drifts, the need for a slightly negative power supply etc.


2nd part Totally lost
so if the pot is at say 5k or half way and the + input is at 3 volts the output would go to maybe 7vdc conversly if the 0 to 20vdc input was 7 VDC the output would go down to 3 VDC. so as the input goes higher the output goes lower. have i got this right? is there a bettrer way? all thoughts welcome.
 

Thread Starter

bexy

Joined Apr 2, 2020
15
probably did not explain myself well. i get a 0 to 10vdc signal from a pressure sensor and i am controlling an EC motor which accepts 0 to 10vdc as a speed control signal. between the 2 i need some sort of basic set point control if i was going to use PID i would go for a micro but as i only need direct control i was looking for something simpler. basically if the pressure rises in the controled space the EC motor has to slow down to drop the pressure. the pressure does not need to be held at a high tolerance value basically because air pressure varies A LOT any way and providing it is not to high or drops into the negative it is fine.
 

crutschow

Joined Mar 14, 2008
27,457
A simple inverting op amp with an added DC offset could be made to give a 10V to 0V output for a 0V to 10V input.
Is that what you want?
Example circuit below:

1610554669267.png
 
@bexy

I'm wondering if an OP-amp in a simple error amplifier mode with an additional window comparitor and sample/hold would wor?

Basically instead of a "setpoint", you set high and low limits on pressure and do say do a sample and hold on the output of the error amplifier, The "hold" woud occur when your outside of the window comparitor's range.
 

Thread Starter

bexy

Joined Apr 2, 2020
15
@bexy

I'm wondering if an OP-amp in a simple error amplifier mode with an additional window comparitor and sample/hold would wor?

Basically instead of a "setpoint", you set high and low limits on pressure and do say do a sample and hold on the output of the error amplifier, The "hold" woud occur when your outside of the window comparitor's range.
OK, sorry bit slow here can you please give me an example of howq that would work.
 

Reloadron

Joined Jan 15, 2015
5,997
What you want is a process control. You have a process variable which is the pressure and you will have a set point. As to op-amps you want a differential amplifier amplifying the difference between two inputs, your process variable and your set point. The ideal is when both inputs, set point and process variable are the same resulting in a zero output. Two analog inputs with a difference output driving your motor. Anyway you can look at differential amplifier circuits. The same process control can likely be done using a uC (micro-controller) or any number of discreet components. Regardless you will need to trim / scale for your specific ranges of pressure and motor speed.

Then there are added features like a display showing the SP and PV depending on what you want. Just for starters it's a process control.

Ron
 

Thread Starter

bexy

Joined Apr 2, 2020
15
How do you want to "adjust" the output?
That pot would affect both the gain and offset.
good point i need to consider gain as well, i have used op amps to give more or less gain to a 0 to 10vdc signal, now i have to figure out how to add set point contorl to that
 
@bexy

I don't really have the ability to draw.

https://www.researchgate.net/figure/Window-comparator-circuit_fig6_335372082

1610599912672.png

A "similar to" circuit above , you would have a pressure upper limit and a pressure lower limit.

The other block is the error amplifier. https://en.wikipedia.org/wiki/Error_amplifier_(electronics). That would be what you would like for your setpont,. Vul is SP+hysteresis, Vll is the setpoint - hysteresis.

When your in that dead zone, the feedback mechanism is not working. You run a sample and hold circut, so it holds the fedback value of the motor control voltage when it crosses Vll going toward the setpoint and when it crosses Vul heading toward the setpoint.

This is a sample and hold: https://en.wikipedia.org/wiki/Sample_and_hold

it's probably easier to do it with a computer.
 

Dodgydave

Joined Jun 22, 2012
10,041
That circuit is a Window Comparator.
You can change the "Window" span by altering the Upper and Lower Voltages with the center resistor.

opamp107.gif
 
Last edited:

Reloadron

Joined Jan 15, 2015
5,997
good point i need to consider gain as well, i have used op amps to give more or less gain to a 0 to 10vdc signal, now i have to figure out how to add set point control to that
You mentioned your sensor gave you an analog 0 to 10 VDC right? So all you need for SP is a pot. If you want an analog out to your motor start looking towards differential amplifier circuits. A window comparator as shown above will give you ON/Off control and not vary the motor speed so which do you want?

Ron
 
A window comparator as shown above will give you ON/Off control and not vary the motor speed so which do you want?
You missed my point. The window comparitor would contral a sample and hold with the result of the error amplifier.
Hopefully, if designed right, it wouldn't bounce. It might have to be more complicated than that.
 
Last edited:

Reloadron

Joined Jan 15, 2015
5,997
You missed my point. The window comparitor would contral a sample and hold with the result of the error amplifier.
Hopefully, if designed right, it wouldn't bounce. It might have to be more complicated than that.
I get your point and agree a window comparator with a sample and hold would likely do fine.

Here is what I don't quite get. The thread starter made it clear initially that he had a sensor with a 0 to 10 volt output. OK, that's a good start. He also had a motor with a 0 to 10 volt input for what I assume to be some RPM (speed). Now there is no mention of actually reading set point or pressure ranges, in fact we have no idea what he or she is actually doing. What I also missed in your post was sample and hold with the result of an error amplifier so yes, that would be an analog value. All he needs is his analog input from the pressure sensor, a set point pot for another analog input and the resultant output of an error amp or differential amp to his motor. Compare a set point to a process variable and drive the motor speed with the difference. Three operational amps like even an 0P7 or ancient 741 (dual +/- 15 volt supply) and that should work. When SP value and process variable value are the same the difference is zero.

Ron
 
Top