Damper Dilemma...


Thread Starter


I have 2 wood boiler dampers that, based on 4-20mA signal from a pressure controller, operate in unison to control the air moving through the boiler firebox. The dampers are positioned as the 4-20 signal dictates, and their position is fedback from separate 100 ohm "slidewire" resistance devices. Each damper has a separate controller strictly for positioning.

I'm convinced there is no device in the world that will replace "only the damper controller" so I have to come up with a plan "B". I have a new controller with slidewire feedback that is using the steam pressure signal. It will control one damper nicely, while monitoring its slidewire feedback... but if I add another controller in the steam pressure loop to control the second damper, they will not operate in unison. I could connect all this up to a PLC and add a couple of displays and switches for position control... ut that's expensive.

Wondering if anyone has any suggestions on how to make these units operate in unison. Assume there are 2 new separate PID controllers on the steam pressure signal, with a slidewire input from each damper to its respective controller. Thanks!

Bruce Axtell

If by "in unison" you mean that damper B is slaved to follow damper A, then just add a 2nd slidewire to damper A and let damper B follow it. This was a common practice years ago with Honeywell Modutrol motors, L&N and Barber-Colman motors. All those companies made devices to follow a slidewire signal. Today you could use a resistance-to-current convertor and use your 4-20ma motor for the slave. Barber-Colman had such convertors and they may still be available, though I haven't used them in many years. Action Instruments and Api both make resistance to current convertors also. An easier way is to feed your controller 4-20ma signal to both motors (in series). Just make sure your controller can drive into the total load. If each motor input has a 250 ohm input impedance, then your controller must be able to push 20ma into a 500 ohm load. Many controllers are weak in that department. The old controllers used to push 20ma into 1000 ohm loads, or even more.

Bruce A.
If you want "unison" operation, use only one controller to drive both dampers. Two controllers gives independent operation, not unison.

Use a single loop controller's 4-20mA output to drive both damper motors. You do that by wiring the motor inputs in series. The same signal to each motor will position each motor identically.

To wire the motors in series:
controller output (+) to motor 1 (+)
motor 1 (-) to motor 2 (+)
motor 2 (-) to controller (-)

Be more specific, is the problem that you have a simple loop controller that wants a single feedback signal?

What is too much $$$?

I would propose that you put a small PLC (MicroLogix?) on it, string the 4-20's in series so the one output controls both dampers, and monitor the feedbacks solely for the purpose of determining that you have a damper (or both) failing to mimic the CV. However, if everything is moving as expected (not hung up, etc.), the feedback is of no concern - you don't really care about the *exact* position of your dampers - just the value of the PV.
Send the 4-20 ma output from the first damper controller to a second controller via a series loop for use as a setpoint 0-100% for the second dampers opening. The 2nd Dampers slidwire will be the PV. Second controllers CV goes to the second damper. Properly tuned the 2nd should move simulataniously with the first as these are usually slow moving dampers.The second damper controller needs to be able to use an external setpoint scaled 0-100%. There are many cheap controllers like this on the market.

Stevan Simmons

In relation to this question and answer. I am considering a similar modutrol motor in an application. It has a 4-20 ma input. Is this actuator the equivalent of an actuator/positioner?

By this I mean will the actuator give a min/center/maximum position for a 4/12/20ma input? If so why would you need a slidewire feedback signal?

Bruce Axtell

Yes, a Modutrol motor that accepts a 4-20 ma signal will proportion the motor position based on the input current. If, for example, 4ma = full CCW, and 20 ma = full CW, then 12 ma will be mid point. That being said, there are several "gotchas" to watch out for.

First, rotary blade dampers are non-linear (logarithmic) in nature. Over 80% of your controllability will be in the last 20% of the stroke.

Also, the zero and span on the actuator may not correspond to full stroke. In addition, the linkage plays a huge role. Linkages can be configured to give little or no control in one part of the stroke and the majority of the control in the last few degrees of stroke. Motors also have different strokes...i.e., some are 90 degrees, others are 120 or 160 degrees. Depending upon the construction of the linkage, there may be significant backlash, which will result in sloppy control. It is possible for linkages to come disengaged. The motor still works but the damper doesn't. Dampers, depending on how they are implemented are also known to bind and jam up. That's why the feed back on the damper shaft. Just because the motor turns, doesn't mean the damper moves. This is even more true if you are trying to move two dampers in tandem. One
could be stuck while the other continues to work. The feedback will tell you this.

Bruce Axtell