1756-HYD02 module for hydraulic pump displacement control?

Would a 1756-HYD02 module be able to control the swash plate angle on a hydraulic pump/motor equipped with LVDT feedback?

We have an application with dual direction swash plate motors that run in speed or torque control mode and I was curious if we could use the HYD02 module to control the swashplate angle and have an outer torque or speed loop in the PLC command the desired swashplate angle.

It should work but the HYD02 is an inner loop. The inner loop needs to follow the command from the outer loop using a PID. Usually the inner loop is a velocity loop but in this case it is a position loop. You will need to do some roll your own programming unless there have been blocks made specifically for this application. If not then the outer loop must output a position that may as well be -100% to +100%. The inner position loop must gear to this position generated by the outer loop.

Controlling speed should be easy. Controlling torque, pressure, is difficult to do the swash plate angle isn't proportional to the torque and the outer loop updates must be fast. This is where a RMC75 would be better because the outer loop can be done faster in the RMC75. The general technique would be the same as what I suggested for the HYD02.

My initial concept was to have the PLC run the speed/torque loop, and send a swash plate position command to the HYD02 module, which appears to align with what you said. How fast of a loop would be ideal for running torque control? I have a couple applications running speed control loops at 3ms using the Control logix high speed analog modules that are working well for us.

One aspect that will complicate things is that 3 motors run into 1 gearbox, and there are 3 gearboxes on the setup. So the speed/torque loop in the PLC for each gearbox would have to send a position command to the 3 swash plate control loops. The 3 gearboxes are connected together through an axle with a differential, so the system will need to switch from speed to torque control (one loop at at time) during testing.

The torque is proportional to the pressure difference between the A and B ports of the hydraulic motor. The rate of change in the pressure between the servo valve and the motor can be very high because the flows are high to achieve speed but the volume between the valve and the motor is small. The equation for the rate of change in pressure is

P(t)=P(0)+∫(β* Q(t))/V dt
Q(t)=K*x(t)*sqrt(Ps-P(t))
P is pressure
β is 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
or in terms of a sample time
ΔP=β*Q(t)*Δt/V

If the bulk modulus of oil is 200,000 psi and the flow is 0.01 cubic inches per second, Δt=0.003 seconds and the volume of oil between the valve and pump is 10 cubic inches then the ΔP=0.6 psi for the 3 milliseconds or still 200 psi/second. I would say your chances are pretty good if my assumptions are right but estimating the rate of change in pressure difference in torque will be critical to controlling torque if the loads change quickly.

I would need to look into the hydraulic specs to get the size/flow of the servo valves, but I do know that the motors are 500CC Rexroth units, so we have a potential for very high torques factoring in the gear reduction. The system is also regenerative in nature (the output port of the 3 "input" motors are plumbed to the inlet side of the output motors) so there will not be a static pressure supply at the inlet ports of 6 of the 9 motors.

Just to explore my options a bit, with 9 inner control loops and 3 (or 6 counting spd & tq) outer loops, would the RMC75 still be the ideal choice, or would the RMC150/151 be more suitable?

Paul351W said:

Just to explore my options a bit, with 9 inner control loops and 3 (or 6 counting spd & tq) outer loops, would the RMC75 still be the ideal choice, or would the RMC150/151 be more suitable?

Click to expand...

I would need to know more about your application and have access to info at work to come up with the best option. A phine call would be best.

In some cases the pumps are spread out a bit and a RMC75 for each pump or two pumps make sense but if they are all together then a RMC150 is cheaper. The RMC72 has two outputs so it can control two pumps. The RMC150 only has 8 outputs so 2 RMC1510s would be required or maybe a RMC151 and RMC75

BTW, I would prefer to use SSI feed back because it can provide more resolution. That means I would recommend our SSI cards and the Rockwell M02AS over the HYD02.

The regeneration feature is going go make this application non-standard no matter which path you take. Servo systems like to have a constant supply pressure. The gain of the system changes as the supply pressure changes. You may need another pressure sensor to monitor this supply pressure so the gains can be changed on-the-fly on each scan.