One PID connected to 2 actuators


I would like to maintain the tank level by using only one PID controller to control inlet control valve and outlet VSD pump.

When tank level goes up, open control valve less, slow down VSD
When tank level goes down, open control valve more , speed up VSD

In this case an the PID controller control the level well? If cannot, why?

Diagram attached.


you can consider using a common level measurement, but each controlled device will need different controller settings (gain, response time, and output limits, alarms...)
In this case an the PID controller control the level well? If cannot, why?

Although it is possible to loop the 4-20 ma signal through both devices (valve drive and vfd) , it would be impossible to troubleshoot.
When tank level goes up, open control valve less, slow down VSD
When tank level goes down, open control valve more , speed up VSD
This makes no sense.
In the first case why slow down the pump?
In the second case why speed up the pump?
It seems the pump is doing the opposite of what you want to do in both cases.
Why not leave the valve position constant and just control the pump.

If I wanted to get tricky I would start closing the valve and speed up the pump in the first case and
open up the valve and slow down the pump in the second case.

The big thing to remember is that to keep the level constant the inflow and the outflow must be equal
One thing missing from the drawing is level sensor..
That said the above points are also pertinent:
- close valve and increase pump speed to lower level; open valve and decrease pump speed to increase level.
- probably the easiest way to consider a PID with range 0-100% with look up tables to range the VFD and range valve opening, mindful the valve opening-to-flow ratio will unlikely be linear.
- the tables may give max. speed for the pump but would it ever stop ? Likewise the valve fully open but would it ever fully close ?
- Lastly, both pump and valve with contrived tables will give you a fairly complex set of parameters. Could you anticipate what PID action is required for all possible scenarios..

I managed with wet well level control with 3 pumps and 2 sets of pen stocks and 1 PID. Let us know how you get on !
I would use a PID controller for controlling the valve and a separate PID controller for controlling the pump, because it will be almost 100% sure that the parameters of those loops will be different. Of course you can set the limitation settings for the pump inside the frequency invertor, to prevent it from running too fast or too slow (heating up of motor windings!!!)

An important question you need to ask yourself: is the pump allowed to run dry? If not there must come an extra protection with a flow sensor to prevent it from running dry. A contact of the flow sensor must be wired in series to the start command going to the frequency invertor.

I assume there is a 4-20mA signal coming in from the level measurement. But of course: it can also be 0-10VDC......

For example:
4mA: tank is becoming too full, so valve must close and pump must run faster
20mA: tank is empty, so valve must open and pump must run slower.
This means that in -this case- the PID controller for the valve must be DIRECT acting and the PID controller for the pump must work INDIRECT acting.
If the level measurement is working the other way around (4mA=empty 20mA=full) you need an indirect acting PID controller for the valve and direct for the pump.

I would use a double PID controller, like for example our FZ series, that is a double PID controller.
With the 1st loop you control the valve and with the 2nd loop you control the pump.

The 4-20mA measurement you run in series trough both PID measurement inputs, so both PID controllers can use the same level measurement.

You can also let both PID controllers switch digital outputs on a specific deviation between setpoint/measured value for giving the start command to the pump frequency inverter.

I'm 100% sure this will work like a charm using one of our excellent controllers.

See the FZ series on this page:
(Assuming you got a bit mixed up and those who pointed at these did put things as it should...)
You gave no reason for this (process) setup -
E.G. - Maintaining the tank level while
- Receiving inflow (through the control valve)?
- Supplying outflow (through the VSD pump)?
Without answering these points the question remains theoretical.
Nevertheless, lets look at the problem the way you set it and the associated options.
The attached scheme (Fig. 1) gives an option to operate the way you set it:
- The controller output is set according to the control algorithm (PID or any other...!) and its parameters.
- One option (...not the only one...) is to send one Final Control Element (FCE) the controller output (OP) and to the other one 100%-OP this can be done in several ways and operating one of the FCE's in reverse mode is one of these.
Similar approach can be applied if we have more then just two FCE's and the attached Fig. 2 describes such an application we did for one of our customers.
You are (all) welcome to contact us to discuss any such or similar application:
[email protected]


This scheme has two degrees of freedom while only one is controlled. Control can be achieved by simply closing the feed valve and stopping the pump, right? The level will remain at setpoint, but there is no flow. Typically the control should cover both degrees of freedom, setting the flow through the system (one controller) and level control (another controller). While using the same controller for only level control, indirectly it will affect the flow, based on how much action is distributed over the feed control valve and pump.
You could drive the valve directly off the level, with some clever spanning and then run the pump off the PID, being careful of relative timing so the system would not "chase it's tail".

It would be some messing around, lack robustness and be confusing to anyone else that came along if their weren't copious notes.

So why would you do this, to save one PID controller, even if it was to be hardwired.