PLC Controlling a Hydraulic Servo Valve

[Bindasbanda]

Hey guys,

I am new at this and trying to figure out a way to control the speed of a hydraulic rotary actuator such that it starts slowly , increases speed and then slows down to stop at the exact setpoint.

I am not willing to use an analog module as my budget is not allowing that.

What i am thinking to do this by using a Hydrolic Servo Valve . i have never used one before, this is my first time.

How can i vary the frequency from the plc (using pwm or pid etc).
I am using Schneider PLC.

Any other way i can achieve this? Help me out people!!

 

[Ken Roach]

Welcome to the Forum !

Generally the way this goes is PLC -> [Analog Signal] -> Solenoid Valve Driver -> [PWM] -> Valve Solenoid.

Your PLC might have an output circuit that has enough power to drive your solenoid directly. Typical PLC frequency outputs are meant to be signal-level only, not power-level.

Has the solenoid already been selected ? Describe its size, flow, speed... anything you can tell us.

"Schneider" covers dozens of different PLC models. Post some details.

I've done hydraulic positioning with multiple parallel discrete valves; we had "1", "2" and "creep". When we wanted to go fast, we turned on 1 and 2. To go slow, only 1. To "creep" into position... "creep".

Do you have any position feedback devices on the rotary positioner ?

 

[widelto]

Allen Bradley has a special module for hydraulics 1756-HYD02

 

[Peter Nachtwey]

Allen Bradley has a special module for hydraulics 1756-HYD02

That module is made for linear actuators. The M02AS uses SSI feedback. One can get absolute rotary encoders with up to 17 bits of resolution per turn.

Since the OP is using a Schneider PLC of some sort the best alternative to the M02AS is our RMC75E-MA1. Many Scheider PLCs support Ethernet/IP if not Modbus/TCP so there would be easy Ethernet communications. Unless the PLC is handling a lot of I/O the PLC may not even be necessary.

I don't see how this can be done without spending more money and what is sad is that money has been spent on things that probably will not work. Anything controlling a servo valve should have a analog output. I think it would be a waste of money to simply buy an analog output card. They are slow and have low resolution. Also, nothing has been said about the feed back device. I recommend an absolute SSI rotary encoder that at least provides 13 bits of resolution = 8192 counts per revolution.

 

[iant]

welcome to the site.
You need to price the control systems with servo / analogue.
even look at a cheeper AB plc (with analogue)
or stepper output.
to use PID - you need to supply an analogue output.

Alternatively have two or three hydraulic solenoid valves with two or three flow controllers
and do the control your system with digital signals.
as it is hydraulic this will be the cheapest and easiest method.
PROVIDED
you dont need accuracey greater than 1mm

 

[Bindasbanda]

Thanks for all replies. i am trying to achieve precision using the equipment i already have at the moment.

PLC Being used is Schneider model : TWDLCAA40DRF

40 I/O 24 c 24 V inputs 14 relay outputs and 2 transistor outputs.


Rotary encoder being used is
 ' Autonics ENCODER 8-M-39-40_ENC ' . 250mm diameter and 1mm pulse every 1mm is the resolution. Required accuracy is exactly 1mm.
you can google it to see there datasheet


the current setup up i did is as attached


Solenoid 2 flow is regulated through a flow controller. At present 2 opens only for slow start up then both 1 & 2 open at fast speed and then sol 1 closes for slow speed to activate. both shuts down to stop. I was thinking to replace 1 and 2 solenoid by a servo hydrolic valve. Is it possible controlling it with my PLC digitally.

so there are two speeds in my system and speed is not reducing gradually.

Problem is motor is not stopping exactly at the position required , it moves 2 to 3 mm front as i have no 'creep' speed to reach the exact point. Is this a programming fault or hardware fault. or may be i cannot estimate the % error i am getting from the rotary encoder or may be the resolution of the rotary encoder is too small!! ...mind boggling!!

 

[iant]

at 1mm per pulse your are already in trouble.
as the hydraulic motor speed is constant and the hydraulic motor speed is settable via your valving.
you normally stop the hydraulic motor by blocking both ports of the Hyd Motor.
this will stop it and hold it in Position.

As far as your accuracey
because you have 1 mm per pusle you will have +/- 1mm so 2 mm out is possable. to get better you need 2 or more pulses per mm.

transistor output
this will drive a stepper motor and will give you considerable accuracey.
not cheap though