what's the difference between Force Control and Pressure Co

what’s the difference between Force Control and Pressure Control?

In the RMC, pressure and force are exactly the same as far as the control.

Compared with proportional pressure reducing valve, What are the advantages of controlling the pressure with servo value。

I pressure reducing valve is basically controlled by pressure pushing on a spring to let the oil flow. How accurate can that be? All it means is that oil will be by passed to keep the pressure down. Pressure relief valve react only to over pressure, they can’t anticipate.

By itself a servo valve is pretty useless but in a control system with a fast pressure sensor and controller the controller can see a rapid rate of pressure change and start relieving oil before the pressure gets to the set point. This helps minimize overshoot. Relief valves can’t do this so by the time a relief valve reacts it is too late.

A servo valve can control the pressure/force precisely using pressure feed back. The servo valve can increase or decrease the pressure/force.

Over half of the pressure/force applications are actually force control applications where a force is being applied to an object. This means the differential force across the piston must be controlled. A relief can’t do that because it has no idea what the pressure is on the other side of the piston let alone knowing the surface areas on either side of the piston so a true force can be calculated.

In short, pressure relief valves are safety devices not control devices. Use servo valves for control.

thank you for your reply
Another question about press control

In press control mode,What reference data of value。

Can give me a example?
Is Pressure-signal characteristic curve important for press control?

Actually, Peter was mostly referring to basic relief valves only, not proportionally controlled ones. A proportional pressure control valve is certainly useful in some pressure control situations and the RMC has successfully controlled them. If you also need position control, you need a servo valve to do both the position and pressure control.

A proportional pressure control valve cannot move a system backwards, but if you are just controlling pressure, it can work fine. Make sure the step response time is small.

A linear characteristic curve is the best, since that will allow the Pressure Feed Forward in the RMC to work the best.

A valve that has a lower value of ΔP/in would make the pressure or force easier to control. However, there are other things like the volume of oil that is being compressed that make a big difference too. Larger volumes require more oil to be added to increase the pressure.
What makes a press tricky to control is when the valve is too big. Then opening the valve just a little bit will cause the pressure to change too rapidly. If you want the closing speed to be fast use jack or kicker cylinders to move the main ram with a smaller valve and turn the fill valve off to the main ram just as the pressure starts to increase. Now the smaller valve is controlling the pressure on the main ram. The RMC75 and RMC150 allow changing gains on-the-fly so doing this is easy.