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Variable displacement pump, accumulator and Rotary actuator

Variable displacement pump, accumulator and Rotary actuator

Variable displacement pump, accumulator and Rotary actuator

Hello everyone,

I am using Matlab's simhydraulics to design a closed loop hydraulic circuit using a variable displacement pump and a gas charged accumulator to drive a rotary actuator. I need some help with the calculations. Can anyone recommend me a source where I can find some similar circuits for example. I also want the theory (equations used for sizing and other things) for the three components.

RE: Variable displacement pump, accumulator and Rotary actuator

Are you trying to model existing components or are you designing each component from scratch?

Is your goal to develop controls or just qualify the ability to complete a task?

What is the application?

Ed Danzer

RE: Variable displacement pump, accumulator and Rotary actuator

Thanks EdDanzer. I am trying to model existing components and want to check the feasibility of an idea. The idea is to drive a variable displacement pump to charge an accumulator during some period of time and then use the energy stored in the accumulator to drive a rotary actuator. I want to match the torque generated by this actuator with a curve I already have. I have used a closed loop circuit with actuator directly connected to the pump output and got some favourable results but I want the accumulator to drive the actuator. That is why I want to see some circuits of how to use these three components  

RE: Variable displacement pump, accumulator and Rotary actuator

You are designing a system to do what?  If you are trying to control the rotary actuator then you have left out an important item.

The system described will not be very controllable.  The accumulator will make controlling the actuator difficult because the natural frequency will be drastically lowered by the accumulator.

There should be a valve between the accumulator and the rotary actuator. The valve will do the actual control.  The pressure compensated pump will just maintain pressure in the accumulator.

If the accumulator is there just to absorb shock there are better ways then how much shock?

Do you know the inertia of the load and the actuator?  If not then the formulas will not do you much good.


Peter Nachtwey
Delta Computer Systems


RE: Variable displacement pump, accumulator and Rotary actuator

Are you planning on having the pump supplementing the accumulator during actuation or will there be valve control?

Torque output will be pressure dependant. The accumulator volume, pre charge pressure and operating temperature will affect the pressure and volume available from the accumulator. The cycling of the accumulator can affect the gas temperature which will affect the pressure to the actuator.

You may not be able to use the SimHydraulics accumulator model because of variations in operating temperatures or the cycling constraints.

Ed Danzer

RE: Variable displacement pump, accumulator and Rotary actuator

Hello everyone!

@ hydtools.  Thanks for the link. The standby power circuit is somewhat related. I will have to understand this circuit in order to make my own.

@PNachtwey. Thanks! You are right about the valve. The problem is, that I do not know which type of valve should I use. The accumulator will be the driving source for the actuator solely. I know the inertia values for both the actuator and load.
@ EdDanzer. Thank you for the reply! The pump is used only for charging the accumulator for a short period. It will not supplement during actuation. I agree with you about the increasing temperature of the gas due to repetetive loading and unloading, but I just want to make an ideal circuit. If Simhydraulics won't do this, then can you give me an idea of which software is capable to model it?

RE: Variable displacement pump, accumulator and Rotary actuator

There is free software available which can be used to size/simulate the accumulator function.  It will also deal with the changes in temperature due to cycling.
It is available to download at www.Hydac.com.
If you need help with this, please call me at 1-905-714-9322 x2244.


RE: Variable displacement pump, accumulator and Rotary actuator

Simhydraulics can simulate your system.  The question is do you know how to use the tools at hand. The system can be simulated in Matlab alone. It can be done is C or Java from scratch or even Excel.  It takes time.  Buying a good set of golf clubs will not make me a better golfer.  It takes practice practice practice.

Software like Simhydraulics simulate systems using a system of differential equations all run in parallel using an algorithm similar to Runge Kutta.

Are you familiar with differential equations?  You don't need to use Runge Kutta most of the time.  You can get by with a simple first order approximation

pos(n+1)=pos(n)+vel(n)*∆t  The smaller ∆t is the more accurate.  Start at 1 millisecond or faster.

How I practice. I started working on a hydraulic motor simulation week back in my spare time
I am assuming I have a fixed displacement pump.
I am modeling a 3 gal accumulator.
A 100lpm valve with the spool response and leakage.
A 5 cuin displacement motor and leakage.
2000 lbm-in^2 inertia motor and load.
I am not done yet.  I need to add
The motion profile generator
the PID
Anti cavitation check valve.
Pressure relief valve.
Flow as a function of pressure for the pressure compensated pump.
Other things too add later
pipe and tubing lengths and the pressure propagation through them.
Capacitance of the piping and tubing.  Jack Johnson has so info on this topic.
A good static friction simulator. These are hard to do.  People write articles and whole chapters in books about just simulating static friction and how to model the transition to dynamic friction.

All this takes a lot of research, planning and programming. You will not learn this in a post or thread.


Peter Nachtwey
Delta Computer Systems


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