Pressure Transducers vs. Load Cells

Hoping Peter N. will help out on this one...

In a hydraulic positon/force control loop what are the advantages/disadvantages to using a load cell to measure force instead of deriving the force indirectly from 2 pressure transducers? In what type of applications would a load cell would be preferred?

Thanks!

In a force / position system, they are dependent on each other. you cannot correctly measure force applied using pressure transducers with this type of system. be prepared to
spend some time to debug the system.

A lot depends on the application.

Load cells give you the force applied, you can convert that signal to pounds, newtons, ft pounds, and so on.

in regards to the pressure transducers, thats a bit tougher.
you have a hydraulic pump feeding valves, lines, and eventually a cylinder. Even though the pressure transducer can be placed at the cylinder, you still have a resistive force from the cylinder rod seals, and it varies along the cylinder. Past experience has proved it.

its been years since i worked on one of these, and i'm not sure i have done a good job explaining it.

regards,
james

I prefer load cells for precise test equipment and differential force for big industrial things.

Load cells are usually more accurate and have faster response. They are easier to set up because there is just one analog input. Load cells measure the true applied force. The down side is that they are usually mounted out by the tooling or where the work is being done and often get damaged. Load cells are usually pretty safe in a lab environment. I sometime joke about load cells having a half life as short as their time constants when in nasty environments.

Pressure sensors should be mounted at either end of the cylinder and that is usually far enough away where they don't get damage. The pressure sensors on either side of the piston should have the same response rate. The main problem with differential force is that the applied force can't be measured down below the seal and piston friction. The force that is reported is
NetForce=PressureBE*AreaBE-PressureRE*AreaRE. Where RE is the rod end and BE is the blank end. The problem with that formula is that there should also be a seal and piston friction term. When dealing with a press the differential force works just fine because the rod and piston seal friction force is very small relative to the many tons that are being applied.

Another advantage of differential force is that it can be used for damping and a second derivative for the PID but that is rarely a requirement press control because presses are usually very stiff. A load cell doesn't provide any information about accelerating, just applied force when contact is made. If acceleration info is required then an accelerometer is required and that makes two inputs again.

In both cases the response should be fast. Motion controllers can close the loop at 1 millisecond or faster so the force feedback should be at least that fast. I have customers that didn't listen and used pressure sensors or load cell amplifiers that had response times in the 100 millisecond range. They wasted a lot of their time and our time trying to make it work. The time constant for a force feed back devise should be about 200 microsecond. In 5 time constants or 1 millisecond the analog value will be with 1% of the true value. 200 microseconds is 5 KHz. If the pressure sensors are mounted remotely in a manifold some place because the hydraulic guys are lazy then there are additional delays. The speed of sound, the pressure or shock wave, travels about 4 ft/millisecond or 1.2 m/s. Where the pressure sensors are mounted makes a big difference.

James said:

you cannot correctly measure force applied using pressure transducers with this type of system.

Click to expand...

I was long winded so you beat me. However you are wrong about being able to measure force using two pressure transducers but you are right about the piston and rod seal friction. I am pretty sure agarb is talking about presses judging by past post and history so seal friction is insignificant to the total pressures. agarb doesn't need to worry about friction changing as a function of position because presses do the forming at the same position each time.

Also, the debug tools are pretty good now.

Thanks for the info. The potential application is a 75 ton press (8 inch cylinder). Customer also want to go down as low as 1.25 tons controlled to within +/-0.05 tons. They plan to validate operation with a load cell. Do I need to worry about seal friction at this level?

agarb said:

Thanks for the info. The potential application is a 75 ton press (8 inch cylinder). Customer also want to go down as low as 1.25 tons controlled to within +/-0.05 tons. They plan to validate operation with a load cell. Do I need to worry about seal friction at this level?

Click to expand...

Yes, I have found that seal friction can easily be 2000lbs*0.05=100 lbs and maybe more depending on the pressures. Can you use a load cell for feed back on this machine? You may check with your hydraulic supplier to see if they have low friction cylinders but I bet they will not give you a guarantee.

Peter,

I said you could not do that because of a past system i did for the auto industry. it was based strictly on load cells and LVDT's.

agarb,
not sure how you are going to measure the 1.5 tons.
i know you said load cell, but what size? this much power will crush a load cell very quickly. think about over loads to protect it. With an 8" cylinder and 1.5 tons, i would say you would need to worry about friction, i have a 5" cylinder and when it is empty, it takes 50 pounds force to start it.

regards,
james

agarb, is this press using a ram or cylinder?

My calculations (pi/4)*(8in)^2*3000 psi=150796 lbf. That is cutting it awfully close considering there may be opposing pressure on the other side of the piston.

Controlling force ramps will be tricky over that range because the oil will flow out as function of the pressure drop either into or out of the cylinder/ram. This means the gains will change as function of the current force.

User programs will allow the output scale to be changed on-the-fly

James a good controller should have 15 bit resolution. With a little over sampling the resolution is effectively 17.3 bits. Even with 15 bits the resolution on the force will be about 5 lbf. Load cell accuracy should be with 1% but that doesn't leave much wiggle room either because 1.25/75 tons is 1.666%.

An application like this may require two sensing methods. I haven't heard of using two different ranges load cells in parallel. I think the one with the lower range would be damaged if it takes the full force. If there was a way to remove or protect the load cell then having a load cell for 0-5 tons and another for 0-75 tons would be a good way to go. The 1% of 5 tons is 0.05 tons but even that is cutting it too close.

What are these guys going to be making where that much precision is required?

Sometimes I think it would be cheaper to build two presses instead of going to extremes to make one press do everything.

Most of the load cells you buy will be 200tons or less. On our presses we use load cells as much as we can. Many times it depends on how you mount them. From a control stand point both load cells and transducers work well. I have used both up to 800tons. Get a quality cylinder with low friction seals and use a good valve. On an 80 ton press I can hold 1 ton easy with either one. What will your cycle be like? Will you be moving and stopping at a force? Will you be ramping force?

I don't know much about the potential application, it was mainly a pre-sales application question that my boss was asking about.

However, I currently have a 50 ton press on the floor (7" bore, 3.5" rod). For giggles, I gave the controller a 1 ton setpoint. In a steady state, with a 1 ton setpoint, I see the actual force staying within plus or minus 0.05 tons of setpoint. Of course, I have no way to verify the actual force other than using the pressure transducers in a differential method. The transducers are only 1/4% accurate of 3000 PSI full scale. The more I think about this I'm not really sure it proved anything...