2 switch control circuit

[fade2blue]

Hi,
I have a hydraulic system I am working on that uses 2 pressure switches to safeguard/lockout the units power switch. This is a 12v DC system where both pressure switches must be tripped to allow the unit to recieve power. The sensed pressure may fluctuate while operating the unit so the switch may open cutting power. To get around this I am considering wiring a SPST relay as a latching relay on each switch to hold the connection. These relays are followed by 2 more SPST relays in series to ensure both pressure switches are tripped. The units are operated for several hours at a time. Im conserned with the relays failing due to heat from energizing the coils continuously. Is there another way to do this?

 

[itsmoked]

I'd just use some "off-delay" relays. Set them for something like 1/2 second. They would see your pressure switches close and they would close. If your pressure switches opened briefly for less than a 1/2s then the off-delays would remain closed. If you are running a PLC it can do this directly.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[fade2blue]

Thanks for the help. It turns out where the pressure is sensed it will peak and then drain off. So i will need some sort or latching/holding method. Alternatively I am looking at displacement sensors in the cylinders. Is there a type of relay with adjustable latching voltage that would match the output from the displacement sensor at a certain distance?

 

[roydm]

Yes, can you tell us what is the output from the displacement sensor,
4-20, DC voltage, AC>?
Roy

 

[fade2blue]

The output would likely be 0-10V DC.

 

[xJAyx]

What kind of safgaurd/lockout are we talking about?

 

[fade2blue]

This is for outriggers on an aerial lift truck. Where the pressure is measured it will spike when the cylinders contact the ground but will drain off due to the valve characteristics.

 

[xJAyx]

So your high pressure switch will trigger on the spike indicating that the outriggers have hit the ground? What if the outriger jams? wouldn't the same pressure spike be achieved? If you use a latching relay circuit it won't be able to tell the difference.

 

[fade2blue]

That right. Its not foolproof. The operators manually extend the outriggers using the control valve and use visual feedback to determine sufficient contact. The trucks are rated to be stable on slight inclines. So I could use a displacement sensor to trip the relay at a voltage corresponding to the required travel. But that still will not ensure the ground is contacted (if there is a low spot/hole on the ground). For that reason I like the pressure sensor as it relates to reaction force. Im using 4 SPST relays configured as stated in the original post but I am curious if there is a more integrated method.

 

[itsmoked]

Yes you could use a smart relay. If the trucks are 24V it gets easier. Look at one of these. There are many brands out there that are all very similar.

Hey they have added 12VDC!(never mind about 24V)

http://www.factorymation.com/s.nl/sc.2/category.1296/.f

Keith Cress
kcress -

http://www.flaminsystems.com

 

[itsmoked]

With one of the aforementioned smart relays you can add all sorts of logic. Spikes can be ignored entirely or used to your advantage. For a production situation you can get a programming key so that your production people can easily load the algorithm into each unit as needed.

Download the free software and see if you can get what you want working on the simulator.

Keith Cress
kcress -

http://www.flaminsystems.com