Worms in hydraulic systems

Since I am given a chance to show off

I have attached a pdf file with the same motor but the simulation is much more complex. There are 9 differential equations solved in parallel. I have graphed what all the differential equations are calculating. The pump is only a 10 gpm pump and cannot keep up with peak flow on page 24/26. The flow to the A port is greater than the flow from the pump. The extra flow comes from the accumulator. Velocity can only be maintained using an accumulator. The pump takes too long to get to full flow and couldn't keep up the the motor's demand on its own. The accumulator is a 3 gal which isn't very big. On page 25/26 I calculate the sum of square errors between the target and actual angle. If I increase the size of the accumulator the error will drop. It shows how important sizing the accumulator is.

I can change the load, sizes and anything I want. It is cheaper than buying all the different machinery.
http://www.deltamotion.com/peter/Mathcad/Mathcad - Hydraulic Motor 8 20-26.pdf

Tom, I am going to rub it it in so you don't step in it again.

http://www.deltamotion.com/peter/Mathcad/Mathcad - Hydraulic Motor 8. 20-25 with added load.pdf

I added a 200 in-lbf load at 2.5 seconds to the previous simulation. The original load was a constant 2000 in-lbf so at 2.5 seconds the load is instantly 2200 in-lbf. The same flow is available but the motor slows down. So much for V=Q/A. Velocity depends on the force/torque and in this case the motor slows down until the sum of torques is zero. The motor only speeds up again because the integrator in the PID winds up to open the valve to reduce the pressure drop across the valve even more and increase the pressure drop across the motor so it has more torque and can accelerate the extra load.

The extra load needs extra torque to keep moving. To get extra torque there must be a bigger pressure drop across the motor. This means the pressure drop across the valve is lower and the flow is therefore lower. As the integrator winds up the valve opens up so there pressure across the motor increases ( 2.8-3.5 seconds ) until the ramp down starts. If this was an open loop system then the valve wouldn't open up to reduce the pressure drop across it and the motor would continue to move at the slower speed where the sum of torques is zero again.

Now remember to wipe your shoes off.

I'm tied up today and tomorrow teaching a class. Nore to come soon!

Ill be sure to pick up some popcorn! lol.

Tom's timing was good. I was working on that simulator when he started this thread. I am going to make many graphs showing the motion under different conditions. Our marketing guys requested that so they have a better idea of how hydraulic systems work and being better able to interpret the graphs generated by our controller.

Peter Nachtwey said:

http://www.deltamotion.com/peter/Mathcad/Mathcad - Hydraulic Motor 8. 20-25 with added load.pdf

I added a 200 in-lbf load at 2.5 seconds to the previous simulation. The original load was a constant 2000 in-lbf so at 2.5 seconds the load is instantly 2200 in-lbf. The same flow is available but the motor slows down.

Click to expand...

Will your simulator let you reduce the FLOW while all other variables remain equal?

What happens THEN?

Here is a simulator for sale, real 4D. Costs 4K + shipping.

Kalle

KalleOlsen said:

Here is a simulator for sale, real 4D. Costs 4K + shipping.

Kalle

Click to expand...

Wow.... that is WEIRD!!!! I have actually been scouring EBAY for a used tractor (honest, no kidding) and I recognized THIS one as one of the ones listed... I honestly thought my laptop was acting up because it kept taking me to this TRACTOR i recognized.... I recognized it from the strange stiffening vanes in the bucket....

Oh, and..... "ha,ha".

Does it come with a manure bucket?

Try to lift the corner of that red barn in the background...

Force still makes it go

Stationmaster said:

Will your simulator let you reduce the FLOW while all other variables remain equal?

What happens THEN?

Click to expand...

http://www.deltamotion.com/peter/Mathcad/Mathcad - Hydraulic Motor 8 out of oil.pdf

I reduced the pump flow to 0.1 gpm from 3 gpm. I reduced the accumulator from 3 gal to 0.1 gal. The system quickly runs out of oil and can't keep up with the motion profile after about 1 second. The control signal quickly increases to full open and is fully open at about 1.4 seconds. To make a point I kept the bump in the load at 2.45 to 2.5 seconds. The angular velocity drops rapidly if though the pump flow isn't changing. This shows the velocity is dependent on the force or torque balance and oil flow just equalizes pressure/energy. Eventually the pump can start to supply oil fast enough to maintain a speed at about 20 radians per second but soon the target generator starts ramping down.

This is an extreme case that should never be seen but we have inducted many hydraulic designers into the Delta Hall of Shame for lessor mistakes.

No flow, no go.

Yes... the force causes movement for the first gazillionth of an inch, but without replacing that gazillionth of an inch worh of oil with adequate flow, the pressure drops dramatically and the movement cannot continue. I'm gonna have to lean towards "both" being required to maintain velocity.

Your wording ("flow just equalizes pressure/energy") sounds as if you seek to minimize the contribution of flow. The bottom line is.... the system won't work, the cylinder or motor won't move, and the process won't get completed without BOTH.

I am not seeking to minimize the role played by Pressure. Force can't be created without it. The load can't be moved without it. And the cylinder creating greater force than the load is what makes the load move that first little bit. I get that. But I still contend that Flow is JUST as important in maintaining the velocity.

You are interpreting what I am saying wrong.

Stationmaster said:

Yes... the force causes movement for the first gazillionth of an inch, but without replacing that gazillionth of an inch worh of oil with adequate flow, the pressure drops dramatically and the movement cannot continue. I'm gonna have to lean towards "both" being required to maintain velocity.

Click to expand...

You can't calculate the velocity from the available flow.

Your wording ("flow just equalizes pressure/energy") sounds as if you seek to minimize the contribution of flow.

Click to expand...

I didn't say that but you can have all the available flow you want and if you don't have a net force the system will not move. There is no more to discuss.

The bottom line is.... the system won't work, the cylinder or motor won't move, and the process won't get completed without BOTH.

Click to expand...

I have said that oil is a means of transferring energy from one place to another.

I am not seeking to minimize the role played by Pressure. Force can't be created without it. The load can't be moved without it. And the cylinder creating greater force than the load is what makes the load move that first little bit. I get that. But I still contend that Flow is JUST as important in maintaining the velocity.

Click to expand...

I don't dispute that pressure is required to generate the force but pressure alone doesn't generate force. Those that don't realize this can't solve problems like Alaric had with his hydraulic system two years ago where the piston backed up against the port so that the pressure only had the area of port to push on the piston and not the whole piston so the piston didn't move.

Look at the equations in the first pdf that I posted. Pressure and flow are all part of the calculation but the acceleration and resulting angular speed is determined by the net torque divided by the inertia. There are no formulas where you can calculate the velocity without taking into account the force/torque and the mass/inertia.

Okay, I think I get it. The difference is in the predictions necessary for engineering. The systems I work with (irrigation with centrifugal water pumps) don't, for the purposes of engineering, take limitless Flow for granted when calculating/predicting system performance. I was trying to get a parallel between the two disciplines.

In the case of "hydraulics", even though both Pressure and Flow are required to do the Work, it would seem that you would do all your calculations with Pressure, and then just insure adequate peak Flow at some point in the design.

I see the difference.

Thank you,

Stationmaster

Wow, Kalle! That rig is way more sophisticated than the one I used. Our cylinder was a single acting, and there was no hydraulics on the bucket. The dump was gravity tipped when you released a pin, and to get it back horizontal you had to lower the bucket and reverse the tractor until the bucket pivoted back and re-latched.

I've attached a math model of the system, as near as I can recall the details from about 50 years ago. This is one of those brute force systems, where the interest in and effect of accelration are both negligible. I created the mathematical model to demonstrate this.

There are a few points I want to note.

1) This is an open loop system. The pump flow is constant at constant rpm (ignoring insignificant items like volumetric efficiency and leakage). When the valve is in center position the pump flow is dumped back to the sump and the cylinder is blocked.

2) When the cylinder is blocked the pressure is a function of weight and cylinder diameter. There is no motion (no go) because there is no flow out of the cylinder.

3) The final velocity when raising the bucket is determined strictly by the pump flow rate SO LONG AS THE CYLINDER PRESSURE EXCEEDS THE PRESSURE CREATED BY THE LOAD. If this is not the case, there is no upward movement and there is no flow. (See Peter, I know pressure matters - it's just not the only thing that matters!)

4) Cylinder and load acceleration when raising it is determined by the relief valve setting. Once the cylinder velocity and flow equals max pump flow the pressure drops. Until then the pressure equals relief setting.

5) The time to accelrate the load is inconsequential to the operator and negligible to the system designer. This is not a servo system or a motion control system - lifting the load is all that counts. In point of fact I'm not sure the relief valve on that old tractor could respond in 90 miliseconds, but hey - it's a theoretical analysis!

Of course, if the load increases during lifting the cylinder will decelerate. How much is a function of the bulk modulus of the hydraulic fluid and the effective "spring rate" of the cylinders and hoses. I'm sure some bright young PhD candidate could figure it out if anybody cared. However, for a system like this nobody cares! (The load increase during lifting isn't common, but I have visions of an encounter with a very large flying cow as one possibility.)

6) When lowering the bucket the system changes from constant flow to constant pressure. Now the downward velocity is determined by feathering the control valve until the speed of descent matches expectation.

7) The load will accelerate at constant gravity, with the velocity increasing until the pressure drop through the valve equals the pressure caused by the load. That pressure drop increases as the square of the flow. The flow that causes the equilibrium of pressures dictates cylinder speed.

OK - I'm going to quit now. I'll never change Peter's mind, and I don't have more time to devote to this. I think I've accomplished my objectives:

a) Demonstrating to the youngin's that not all hydraulic systems are about motion control and not all systems require precise operation.

b) Demonstrating that you can adjust your mathematical model of the system to suit the physical and commercial constraints of the system. Exercise of good engineering judgement and experience means doing an analysis that verifies you meet the functional requirements of the customer and the cost constraints of the supplier. There is no benefit in doing a sophisticated analysis that doesn't help one of these parties.

c) Annoying Peter a little. I must confess to a rather juvenile annoyance of my own after the last thread on this subject was blocked. This was mostly caused by the implication that I don't understand the fundamentals of either hydraulics or Newtonian physics. The annoyance was further aggravated by the implication that there is only one way to look at any hydraulic system and that any analysis that varies from the one true methodology is defective.

Thanks for all of your patience while I indulged myself. I hope at least a few of you benefited and got a couple of chuckles out of this.

And Peter, don't take it too seriously. I KNOW you are an expert in your field and I respect that!