Add a Door Guard Safety Relay to Output of Valve Amp

[mjohnsonc]

Guys,

I need your advice regarding connecting a door guard safety relay to the output of a Sauer-Danfoss Hydraulic Valve Amplifier and the proportional valve. I am concerned that switching the output while the valve is energized may cause inductive kick-back and damage the transistor output of the amp. Would adding surge suppression affect the proportional action of the amp? Should the door guard disconnect the 24 vdc input power to the amplfier instead of the output side?

Thanks in advance!

 

[TheStarr]

I think interrupting the output is the safest design for the machine. Unfortunately, I do think you will get a spike.

I would mount a TVS (Transient Voltage Suppressor between 4 and 6 on your drawing.

A TVS is like a zener diode, but deisgned to clamp high voltage spikes.

Search your supplier for a TVS with Axial leads. The size will be based on your working voltage and the size of the coil in the solenoid.

This should give you an idea: http://datasheet.digchip.com/343/343-4-001352-DL150.pdf

 

[monkeyhead]

From you schematic, it looks like the e-stop already kills the incoming power. Why wouldn't you just put your door guard relay in series with this e-stop circuit? Then you wouldn't have to worry about the inductive spike.

 

[panic mode]

i would think that something designed to drive inductive load is already protected but i'm with monkeyhead, why not use CR2 to kill the power?

 

[ydtech]

I'd agree that the output is probably already protected against spikes, but still it might be an idea to have some extra protection.

One possibility would be to put a capacitor across CR2, it will absorb the voltage spike while letting the current die off gracefully.

A lot depends on the current and inductance of SOL2, use something like 0.1 to 1 uF, 400V.

Put a resistor across the capacitor to discharge it after the spike. Not very critical, a time constant of a second or two is fine. Something like 22 or 47k. Just make sure it doesn't pass enough current to hold SOL2 in.

 

[ndzied1]

What I see a lot is a set of normally closed blocking valves sandwich mounted under the proportional valve. They are powered open to allow the system to move. When the Safety Relay drops out, the power to the blocking valves is dropped and they close off the oil to the prop valve.

 

[JesperMP]

What does the proportional valve do ?
If the valve is "killed", does that start a motion ?
If so, then you dont want to do like that, in stead you should remove the energy in system otherwise.
Just thought I would mention it.

edit: I think that Norm is on to the same.

 

[geoplc]

My guess would be there is internal protection on the amp itself, since it was designed to drive the indutive load it would stand to reason it would be protected if properly designed. Also NEVER EVER install any components (capacitor, etc) across the contacts of a relay or contactor, if shorted the relay contacts are essentially bypassed.

 

[mjohnsonc]

Thank you for all the great replies!

Opening the Door Guard removes power to the valve causing the machine to stop. I feel like I shouldn't add extra components to the output of the amplifier. I am going to wire the door guard CR2 relay to the power circuit as mentioned in an earlier post. I thought I would disconnect the amplifier output power as the Door Guard Safety relay drawings show. But then thought of the inductive kickback.

Any other comments about moving the Door Guard Saftey relay to the input side of the Amp?

 

[bernie_carlton]

For those who wonder, like Monkeyhead, why the guard door isn't simply wired into the Emergency Stop circuit, many machines have an load/unload point for product. This area is guarded such that nothing in the reachable area can move while the guard is opened. In addition, safe circuitry exists which monitors all motion in the load/unload area and when, after a processing cycle, all motion is stopped solenoids release a holding circuit on the guard door. Thus the door can be opened, the product unloaded and reloaded, the guard door closed, the guard door is locked and the processes continue without emergency stopping the entire machine. I suspect that the original poster's application is similar to this.

 

[mjohnsonc]

Bernie is correct about the configuration of the machine. The door guard stops motion while loading the machine so I can't drop the E-Stop each time. Sorry for not being more descriptive.

 

[mjohnsonc]

Bernie do think wiring the Door Guard safety relay to the input side of the amp is acceptable?

 

[bernie_carlton]

I don't have enough experience with this type of application - multiple safety shutoffs. Our only use of hydraulic driver cards had a full emergency shutdown of power to the card as well as all other motion items. I would contact the maker of the driver card for further advice.

Edit - In our particular operation the hydraulic unit was moving a vertical load. Having a possible single point of failure (the hydraulic hoses and fittings) we also installed a powered check valve in lower hydraulic hose connection such that, with power off, all flow was stopped.

 

[mjohnsonc]

Thank you all for the replies. I was told that Sauer Danfoss offices will be be closed this week so not sure I will have an answer from them regarding wiring to the output of the amp.