Feedforward component to power controller from measured power output

[motoz]

Seems as the PI controller parameters quite often causues trouble with rapidly shifting loads, eg. when switching to/from hot water production. Rather than switching PI parameter values to a hardcoded set when producing hot water, wouldn't it make sense to use a feedforward component from the load as measured by the flow meter and out-return temp diff? Or maybe it does that already, I don't have a flow meter so I'm just speculating. And even without flow metering, I think a settable feedforward component to add/remove when switching on/off hot water production could help greatly. Especially since the controller knows when the load is about to change so the feed forward component could be applied in advance to eliminate the burner dead time from the loop.

[MHES]

No, there are no feedforward component based on the flow and return temperature, and no matter what you do, you can't make the controller to guess what will happen after HW-production.

Some installations runs with a 3-way valve which switch the hot water from the boiler to either the HW-heater or to the house-heating, some uses a 2-way valve which just adds the HW-heater to the general load of the heating system.
Some runs with a low HW-temperature compared with the general boiler temperature, others runs with a high HW-temperature.

In order to do a propper regulation, you need to know what will happen after the HW-production, and you don't.

Best regards
Michael

Seems as the PI controller parameters quite often causues trouble with rapidly shifting loads, eg. when switching to/from hot water production. Rather than switching PI parameter values to a hardcoded set when producing hot water, wouldn't it make sense to use a feedforward component from the load as measured by the flow meter and out-return temp diff? Or maybe it does that already, I don't have a flow meter so I'm just speculating. And even without flow metering, I think a settable feedforward component to add/remove when switching on/off hot water production could help greatly. Especially since the controller knows when the load is about to change so the feed forward component could be applied in advance to eliminate the burner dead time from the loop.

[motoz]

I agree that guessing would often not work. But when there is a flow-meter installed there is no guessing involved. The controller knows of every process disturbance exactly when it happens so the natural thing would be to feed that info forward to the controller and eliminate the disturbance before it affects the process. A PID controller is really never the best algorithm for any process, it's just a simple one that is often good enough. And when something is known about the process it is always possible to do much better than a PID, especially when the disturbance can be measured or even just semi-accurately estimated before it affects the process. Just food for thought, since it seems as PID problems are not that rare. In this case I think that a PI combined with feed forward from the flow meter would give radically improved results.

[MHES]

Well, if you have the flow value you can of course use it as a feed forward to the regulator.
But you need to know the ratio between change in flow and needed extra power.

Additionally with the flow meters used by NBE, you get one puls pr. 10 l, meaning with a normal flowrate of 4-600 l/h, it will take 1-1,5 minut before the change in flow will be registered.
The change in temperature will me measureable long before, and the same will happend when the HW-production ends.
If feed forward based on flow should be useable, the signal has to be detected much fatser.
And you need to know the ratio of extra power needed on base of the flow change.
However the change of flow can be caused by several different parts, with very different impact on the needed power.

When feed forward is used with succes in regulations, you have a fixed ratio between change in indirect value (ex. flow) and the following offset of PV (process value).

I fully understand your point of view, but i doubt it will make regulation easier for the users.

Best regards
Michael

I agree that guessing would often not work. But when there is a flow-meter installed there is no guessing involved. The controller knows of every process disturbance exactly when it happens so the natural thing would be to feed that info forward to the controller and eliminate the disturbance before it affects the process. A PID controller is really never the best algorithm for any process, it's just a simple one that is often good enough. And when something is known about the process it is always possible to do much better than a PID, especially when the disturbance can be measured or even just semi-accurately estimated before it affects the process. Just food for thought, since it seems as PID problems are not that rare. In this case I think that a PI combined with feed forward from the flow meter would give radically improved results.

[motoz]

But you need to know the ratio between change in flow and needed extra power.

Combined with boiler temperature and return temperature, that's already known. But...

it will take 1-1,5 minut before the change in flow will be registered.

That buries the idea completely. I think it would need to be at least 10 times more responsive to be useful for feedforward.

[MHES]

Sorry, but the needed extra power when HW-heater is switched on is not known from a combination of flow, return temperature and boiler temperature 
...it can only be known from experience or hard theoretical calculations, and then what when switching in/out huge room-heaters (radiators) og floor-heating systems ?
No, it won't work 

Agree, you need a much fast responce if you use feed forward.

Best regards
Michael

But you need to know the ratio between change in flow and needed extra power.

Combined with boiler temperature and return temperature, that's already known. But...

it will take 1-1,5 minut before the change in flow will be registered.

That buries the idea completely. I think it would need to be at least 10 times more responsive to be useful for feedforward.

[motoz]

I think I must respectfully disagree here. When the water flow in kg/s and it's temperature when leaving and when returning is known, you know exactly (sans measurement errors of course) how many kWh/h is leaving the boiler. The exact same amount + some extra for boiler efficiency need to be fed to the burner as pellet kg/h. There are no hard calculations here, but that is of course a moot point since the flow isn't measured in realtime. If it were measured in realtime, if switching on the HW heater would increase output power by eg. 10kW, then the input need to also be increased by 10kW. Now we must wait for the PI controller to increase input power as a response to falling boiler temp, with all the problems that gives, instead of just increasing the input by 10kW right away and let the PI take care of any smallish adjustments that still need to be done because of incorrectly guessed boiler efficiency, pellet energy content, measurement errors and so on.

[MHES]

Arhh, you forget one important factor in your calculation 
The output temperature will fall as soon as the flow will increase, depending on the by-pass flow in the "hold the return temperature circuit".
Your calculation will show the actual used power, but limited by the capacity of the burner/boiler.
If you switch in additional load of ex. 3kW on a 30 kW system running actually 50% load (15 kW), the result in the power calculation will show almost same power, until the burner is capeable to produce more power 
This a classical example of "What came first, the hen or the egg" 

Best regards
Michael

I think I must respectfully disagree here. When the water flow in kg/s and it's temperature when leaving and when returning is known, you know exactly (sans measurement errors of course) how many kWh/h is leaving the boiler. The exact same amount + some extra for boiler efficiency need to be fed to the burner as pellet kg/h. There are no hard calculations here, but that is of course a moot point since the flow isn't measured in realtime. If it were measured in realtime, if switching on the HW heater would increase output power by eg. 10kW, then the input need to also be increased by 10kW. Now we must wait for the PI controller to increase input power as a response to falling boiler temp, with all the problems that gives, instead of just increasing the input by 10kW right away and let the PI take care of any smallish adjustments that still need to be done because of incorrectly guessed boiler efficiency, pellet energy content, measurement errors and so on.

[motoz]

That is a very good point, and that is also why it can't work with the slow flow meter that is used. To work, the flow must be correctly measured before the heat buffer in the boiler runs out, and the smallar the boiler the harder it will indeed be.

You say that the temperature will fall as soon as the flow increases, that is correct, but it does not tell the whole picture. It will *start falling* as soon as the flow increases. How quickly it falls depens on the size of the heat buffer in the boiler compared to the output power. Normally the controller uses this falling temperature as it's only reference for balancing input power with output power, ie. it knows absolutely nothing about the process.

If however the increased flow can be measured before the temperature has fallen substantially, then an almost perfect estimate of the required power input can be made without waiting for the feedback loop to react at all. If the temperature falls so fast (small boiler) that the power output becomes limited by the input power, the estimate for required increase of input power will be too low and the boiler temperature will still fall, leaving more work for the PI feedback loop. The worst case will be that the forward component is zero and all work is left to be done by the PI feedback loop, ie. it will work exactly as it does now.

My point is, whenever an estimate for forwad feed can be made that is better than nothing, it will improve control. The more that is known about the process, the better the estimate can be. For instance the feedforwad algorithm can be made to compensate for the fact that an increase in measured output power is soon followed by a decline if input power can't be increased fast enough but not the other way, since that is a known quality of the process. 

It is however possible that the forward feed estimate is worse than nothing, for example if it comes too late. And that is why it can't work with a slow flow meter, I won't try to guess how fast it would need to be with the small blackstar boiler, but I fail to see any theoretical reason why it couldn't work given fast enough reaction time. It's completely possible that a fast enough flow meter simply doesn't exist.

(this is obviously no longer a "Ønsker til funktioner i chip" subject, more of a theoretical discussion on process control in general, please regard as such)

[MHES]

Hi,

Well, changing the subject from an NBE-burner issue to general issue on regulation technics, changes the whole context 

I fully agree that feed-forward is a useable function in regulation in many cases, I use it a lot myselves in my proffesional life 
Often it is the most precise way instead of cascading PID-regulators 

But it is not useable on the NBE-burner control unit 

Best regards
Michael
 

That is a very good point, and that is also why it can't work with the slow flow meter that is used. To work, the flow must be correctly measured before the heat buffer in the boiler runs out, and the smallar the boiler the harder it will indeed be.

You say that the temperature will fall as soon as the flow increases, that is correct, but it does not tell the whole picture. It will *start falling* as soon as the flow increases. How quickly it falls depens on the size of the heat buffer in the boiler compared to the output power. Normally the controller uses this falling temperature as it's only reference for balancing input power with output power, ie. it knows absolutely nothing about the process.

If however the increased flow can be measured before the temperature has fallen substantially, then an almost perfect estimate of the required power input can be made without waiting for the feedback loop to react at all. If the temperature falls so fast (small boiler) that the power output becomes limited by the input power, the estimate for required increase of input power will be too low and the boiler temperature will still fall, leaving more work for the PI feedback loop. The worst case will be that the forward component is zero and all work is left to be done by the PI feedback loop, ie. it will work exactly as it does now.

My point is, whenever an estimate for forwad feed can be made that is better than nothing, it will improve control. The more that is known about the process, the better the estimate can be. For instance the feedforwad algorithm can be made to compensate for the fact that an increase in measured output power is soon followed by a decline if input power can't be increased fast enough but not the other way, since that is a known quality of the process. 

It is however possible that the forward feed estimate is worse than nothing, for example if it comes too late. And that is why it can't work with a slow flow meter, I won't try to guess how fast it would need to be with the small blackstar boiler, but I fail to see any theoretical reason why it couldn't work given fast enough reaction time. It's completely possible that a fast enough flow meter simply doesn't exist.

(this is obviously no longer a "Ønsker til funktioner i chip" subject, more of a theoretical discussion on process control in general, please regard as such)