Proportional Valve issue 1

[Hakim Eng]

Hello guys,
i have a linear hydraulic cylinder controlled by a plc through 4-20 mA proportional valve.
The whole hydraulic system composed of:
- proportional valve.
- blocking valve.
- pressure switch " to ensure oil presence"
- a position transducer.
when trying to move the cylinder up/down it works perfectly, but when trying to position the cylinder at an intermediate position the load goes donw by the inertia effect even the blocking EV is not energized at all ?!
Also, when powering-off the machine,i left the cylinder at initial position " up ", and when coming next day, i found the cylinder at the bottom position even though no power was present.


What cause the cylinder to change it's position with no Electro-valve energized?

Any help please
Thanks in advance

 

[btrueblood]

Sounds like your "blocking" valve isn't. Look into adding load control valves or pilot-operated check valves.

 

[Hakim Eng]

But the problem happens with several cylinder with the same hydraulic system i mentioned.
Could you please recommend me a documentation that describes the principal of operation of these proportional valves.
Thank you so much for your help

 

[lukin1977]

What cause the cylinder to change it's position with no Electro-valve energized?

Looks like gravity. The oil return line is open and the cylinder falls down by gravity

 

[lukin1977]

https://www.youtube.com/watch?v=q1labA0dx0M