Linear Positioning and Feedback

You say, "position 500 lbs," but you don't say where. If it's vertically moving, that's one thing, but if it's moving horizontally on rails, that's much easier.

To move a 500 lb inertial load 12" in 1 second is one of the problems, and hydraulics may be the only way to do it inexpensively, as I haven't calculated the required torque or power needed for a ballscrew, which would be my first choice if not too demanding.

1/4 turn of a 4-pitch screw gives 1/16" travel, but that requires accelerating and decelerating time of 0.00521 seconds. 1 HP is 550 foot-pounds per second, so to move 500 lbs 1/16" in 0.00521 seconds means that you need about 1 HP (0.91 HP). 1/4 turn of a ballscrew means that the angular velocity of a motor needs to reach 2880 RPM in 5 milliseconds -- otherwise known as impractical.

External linear measurement with hydraulics may be the only feasible way to do it. Look up LVDT as a linear feedback device. Others are inductosyn, linear potentiometer, etc.

Thanks Bill, The load is horizontal, moving on linear bearings. I do prefer using a basic hydraulic cylinder to move the load, I just want to find a way around the cost of a std LVDT setup. You can buy a 12" digital vernier caliper (china) for $15 that has a resolution of .0005". If I could find something "off the shelf" like that which could take an input for target position and output to "forward" or "reverse" relays.

OT

Some of those Chinese counters have a BCD output this could probably be fed into a stand alone counter with pre settable totalizer points also a good counter would have scalable settings for a given input so you could adjust your distances. some of these controllers are quite sophisticated. The out put signal could shutdown your hydraulic feed and with some hysteresis settings it might work. You might have to use two valves one a larger flow than the other and a very small flow for the final position.

Thank you Garth, we have been using two parallel valves each with independent flow controls to accurately find the target position and it has been acheiving .006"-.0100" results (which is better than needed).

Where would I source the equipment you described?

Thanks, OT

Before a suggestion a question:

in any given position will be the mass under a load trying to chnage achieved position?

NN, once the load is positioned and the closed-center valves are centered, the load remains fixed. The position is held for up to 20 seconds before moving to the next so then any internal leakage in the cylinder or valve is of no concern.

I'm very happy with the hydraulic system we're using...I just want to simplify the controls.

Thanks, OT