Constant Force Circuit
Constant Force Circuit
(OP)
I have a project that I'm working on that requires a constant force (adjustable) to be applied by a double acting cylinder. Force only need be monitored in the extend direction (0-2500 lb f).
The setup looks very similar to a log splitter, but will have a load cell placed between the cylinder and its anchor point to monitor the applied force. This load cell will output a 0-10v signal that will be sent to a process controller.
I intend to place some sort of relief valve on the outlet side of the cylinder to regulate pressure and maintain constant force. This valve would need to accept a 0-10v or 4-20ma signal from the process controller so that the setpoint force will be maintained. It also needs to be fairly fast acting.
My question, does a valve of this type exist?
The setup looks very similar to a log splitter, but will have a load cell placed between the cylinder and its anchor point to monitor the applied force. This load cell will output a 0-10v signal that will be sent to a process controller.
I intend to place some sort of relief valve on the outlet side of the cylinder to regulate pressure and maintain constant force. This valve would need to accept a 0-10v or 4-20ma signal from the process controller so that the setpoint force will be maintained. It also needs to be fairly fast acting.
My question, does a valve of this type exist?





RE: Constant Force Circuit
The key equations are:
P(t)=P(0)+∫(β* Q(t))/V dt
Q(t)=K*x(t)*sqrt(Ps-P(t))
P is pressure
βis the bulk modulus of oil
Q(t) is the flow into a volume of compressed oil
V is the volume of compressed oil
K is the valve flow constant
x(t) is the valve spool position
Ps is the supply pressure
P(t) is the current pressure of the compressed volume of oil
P(t)=P(0)+∫(β* Q(t))/V dt
Q(t)=K*x(t)*sqrt(Ps-P(t))
The net force is Pa*Aa-Pb*Ab-friction
I have some graphical data for sinusoidal testing of welds in the same force range that I can post when back in the office Monday.
Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com
RE: Constant Force Circuit
http://www.moog.com/products/servovalves-servo-pro...
I plan to use a 2" cylinder just as you suspected. But, I'm also thinking that moving to a slightly larger cylinder may help accomplish tighter control. The larger cylinder will be less sensitive to slight changes in flow, no?
Thank you for your help.
RE: Constant Force Circuit
A bigger diameter cylinder is good if you are doing position control moving heavier masses.
When doing force control
dP/dt=β* Q(t)/V
Multiply both sides by the area.
dF/dt=β* Q(t)/x
where x is the distance from the end of the cylinder assuming no dead volume.
You can see that the rate in change in force doesn't care much about the diameter so from a purely force control point of view it is a wash.
For laboratory or testing applications I care more about the seal frictions. If the seal friction is the same between the 2 inch and 2.5 or 3.25 inch cylinders then the seal friction will be a smaller percentage of the force when using larger diameter cylinders. I would stick to the 2 inch diameter cylinder and use the savings to buy a better quality cylinder with lower friction.
I doubt a process controller can respond quickly enough. The analog inputs and outputs must be fast too.
What exactly are you doing?
Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com
RE: Constant Force Circuit
If not a process controller, then what type of controller could accomplish this?
RE: Constant Force Circuit
A small simulator system would need to respond very quickly in the millisecond range. A real system will be slower, much slower but much trickier than just trying to maintain a constant force.
I will look into this Monday. I am pretty sure we have customers doing force and torque limiting in drilling applications.
Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com
RE: Constant Force Circuit
RE: Constant Force Circuit
RE: Constant Force Circuit
Since this is a small model the controller will need to be fast.A PLC may be able to do the job but a hydraulic motion controller would be better especially if you want to collect data and connect to data analysis software like LabView.
Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com
RE: Constant Force Circuit
RE: Constant Force Circuit
There are no specific specifications, but naturally the closer to setpoint, the better. I'm hoping to see results in the +/- 3-4% range.
RE: Constant Force Circuit
However! Your designed of a solenoid moving a bellows looks good to me since there will be no seal friction and the solenoid can retract to reduce pressure to 0.
You should be able to get MUCH closer than 3-4% if you use a load cell for force feed back. You should be able to get within 1% if the force is to be constant. We do so all the time.
It is import that the servo valve have 0 overlap or be slightly underlapped ( v-spool ) and have low hysteresis.
Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com
RE: Constant Force Circuit
RE: Constant Force Circuit
However! Your designed of a solenoid moving a bellows looks good to me since there will be no seal friction and the solenoid can retract to reduce pressure to 0.
You should be able to get MUCH closer than 3-4% if you use a load cell for force feed back. You should be able to get within 1% if the force is to be constant. We do so all the time.
It is import that the servo valve have 0 overlap or be slightly underlapped ( v-spool ) and have low hysteresis"
Thanks again Peter. I will take your advice and find the nearest Parker dealer and go sit down with them. The Moog Valve I'm looking at is supposed to be available with zero overlap and very low hysteresis.
On another note, I noticed that Moog recommends placing an adjustable metering orifice across the head and rod end of the cylinder (after the Moog valve). The say it acts as a dampner and will increase accuracy of system. Ever seen something like that in action?
RE: Constant Force Circuit
BTW, the 1% assumes there are no outside disturbances.
Yes, in a real system the natural frequency will drop as the drill string becomes longer and heavier. Controlling a constant force is really impossible because it is like trying to control the force applied by a stretched out slinky. The average force can be controlled but it won't be constant because the the disturbances. I don't see how one can physically model as drilling rig with a small lab type model and get everything right.
Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com
RE: Constant Force Circuit
For the adjustable metering orifice, are we talking about something like this?
http://www.surpluscenter.com/Hydraulics/Hydraulic-...
RE: Constant Force Circuit
So how are you going to measure applied force? There is the load cell option and the differential force method. The load cell option is the most accurate but it doesn't work if the load cell can be damaged easily. The differential force methods requires a pressure sensor on both ends of the cylinder. NetForce=Pa*Aa-Pb*Ab. This method ignores seal friction.
Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com
RE: Constant Force Circuit
http://www.interfaceforce.com/index.php?5600-Axial...
I will likely be contacting you soon to get a quote on one of your single axis controllers.