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Alvin Stewart
08-01-2014 10:33 AM

Need some advice on a slightly different loop control system

I've got a project coming up whe a proportional valve controld a hydraulic cylinder. The cylinder controld the control the window size of a slidind gate that allows varying amounts of material to discharge. The Output is a proportional valve with 12ma. closed, 12+m.a. increases flow through the valve in one direction and 12-ma. does the same in the opposite direction. The PV is a level sensor sensing the level of material in the hopper containing the material. The SP is the desired level to maintain in that hopper. The hydraulic cylinder will have no position feedback of the stroke. In the perfect world, the cylinder would open to create acertain size window for the discharge and then close at 12ma. The PV will vary based on the amount of material entering the hopper via a conveyor and being stratified to varying densities via vibration or pulsing air over the material in a water bath. Since the valve must output to a certain position, close, and settle for the next calculation and make the appropriate adjustment, the output will be going to a closed state 12ma. until it needs to make an output adjustment based on the PV. Can anyone offer some advice here. Any help or direction is greatly appreciated. A similar machine uses a rubber paddle drived at varying speeds controls the dischagre. The output is always at some level being sent to the VFD. That one is easy and easy to tune. Thaks again for any help here.
08-01-2014 01:23 PM
Top #2
Nnamdi Izuchukwu Nnona
08-01-2014 01:23 PM
Hi..Alvin...Will look for a way to introduce a path transducer in the Hydraulic cylinder If I were you(..Will give you greater control options..Unless I don't understand your problem clearly...Question...Do you have a way of monitoring the quantity that is discharged each time? feedback from the quantity of material discharged to set your next MV(manipulated Variable)?
08-01-2014 03:53 PM
Top #3
Alvin Stewart
08-01-2014 03:53 PM
The PV I have is in the level of the container via a float. No way to measure the quantityof discharge. I just have the level of the tank. It'snot just liquid but some solids mixed in with it. I hope this helps.
08-01-2014 06:03 PM
Top #4
Nnamdi Izuchukwu Nnona
08-01-2014 06:03 PM
Hi Alvin...I can only think of using the Controller Block in Siemens PLC....Considering the cost,If you already have it in the plant .. Using the Analogue output,you can vary the current sent to the proportional valve that controls the cylinder..It might solve your problem..I still think you need to find a way to receive a feedback from the quantity of material discharged back to the controller(Close Loop)...in this way you can have a proper tuning..Must the hopper be open through a hydraulic cylinder only?Issues with Speed of actuation will also affect the system(Saving Cost??)..Hope this helps...Cheers
08-01-2014 08:37 PM
Top #5
08-01-2014 08:37 PM
Dear Alvin,
Flow Control Set-up:
1. Level sensor detects the level of the material in the hopper
2. Each level detected by the sensor has a specific signal interpretation sent to the controller which sends feedback to the linear Hydraulic Cylinder to move to a specific position.
3. Every position of the cylinder dictates the flow of the material going into the hopper

Feedback control:
1. Set your controller output of 4-20mA proportionally to 0% to 100% Cylinder opening.
Meaning if signal is 0mA then the cylinder will be in the intial position (0% opening of the material gates) while the 20mA signal the full position (100% opening of the material gates).

2. Whatever the output of your Controller (4-20mA, 0-10V) just make it a ratio of 0% to 100% opening.

3. After that, perform the fine tuning of the opening and closing of valves using the PID setting of the controller

As far as I know the control valve can move the cylinder at different positions, back and forth, as per command from the controller. Though it may be limited to 4-positions.
If you want to have multiple seamless position of the cylinder stroke then you may use hydraulic stepper motor for positioning.

Please let me know if this clarifies or helps solve your issue.
08-01-2014 11:22 PM
Top #6
Olof Dahlberg
08-01-2014 11:22 PM
This is not a PID problem. You need to keep the output at 12 mA except for when the level leaves an allowed interval, in which case an adjustment to the hydraulic actuator is made by activating in the appropriate direction for a short time. The control tuning parameters will be the size of the allowed level window, amplitude and length of valve signal offset (from 12 mA), and control impulse time interval.
08-02-2014 01:59 AM
Top #7
Richard Dambaugh
08-02-2014 01:59 AM
Positioning a hydraulic cylinder is a motion control problem. Typically these are configured to use a bipolar output (such as -10 to +10 VDC) which takes into account if the output fails, usually to zero, the machine won't haul off and start moving unexpectedly. Using 4 to 20 ma obviously does not have that luxury, so if the output fails, one needs to account for the axis moving to the extreme. Using 4 to 20 ma is the cheap way to avoid using a proportional valve controller.

As far as motion control, typically the output is ramped, controlling both the velocity and acceleration to form S curve velocity profiles. One needs to compute the accel & Decel break points based on the movement requirements. The S curve profile helps the equipment to last longer using ramped acceleration instead of step changes in acceleration.

Some PLC's like Rockwell's ControLogix5000 have motion control logic built in. Others can be easily written or try surfing the web for prewritten stuff.
08-02-2014 04:39 AM
Top #8
Ivo Dimitrov
08-02-2014 04:39 AM
Depends on the how precise your process must be, but I'll try to have 3 positions only - closed, half-open (half-closed) and closed. If you can control the level with 1/2 open/closed when is 10% more/less and after that open/closed completely when the level gets to the set point, you may be able to have some smooth process. In addition, you can take a look on the loading conveyor speed - if you can to slow down or speed up that may make the process even better... if all of this is not good enough, you can add 2 more positions as 25% and 75% and already have 50%, with 0% and 100%, that should be very flexible...
Sometimes 2 or 3 position automation control makes simple and better result on the field then PID, less complicated and match more understanding for operators/personnel, keep in mind you are not part of this machine/line/process and you are not available 24/7...
Good Luck
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