Whats the application? Float switch for water level, or some other type of mechanism that just happens to be in the water. If the later, it may be wise to remote the switch above water by use of push rods, cams or other means. Just for a maintenance point of veiw.
Application is a hydraulically operated lift mechanism to position a large (4x8 foot) mirror for underwater photography in a towing tank used for scale model testing. The mirror normally sits flush with the tank floor, but is hinged along one of the long edges, and can be raised up to a 45 degree angle by a hydraulic motor and acme screw "scissor jack" mechanism recessed into a shallow pit under the mirror. The tank itself is essentially a swimming pool 8'deep, 16'wide, and 312'long.
Linkages to the surface will be difficult, as nothing can be run through the water column above the mirror, as it would interfere with the tests being run. There is a 3" PVC conduit between the pit and the outside of the tank (above waterline), to be used as a chase for the hydraulic hoses and limit switch wiring.
I need limit switches to detect the end of travel in both directions, to avoid overtravelling the device and twisting the frame/cracking the mirror.
This mirror mechanism is just ONE of the strange projects I am involved with these days. The hydrodynamics laboratory I work for is in the process of a major renovation/upgrade, and myself and our machinist are responsible for just about all the specialized apparatus needed. This includes a large servohydraulic wavemaker, 125 HP towing carriage drive with .01% speed repeatability, 300' long flexible cable carriers for power and instrumentation to a moving carriage, and racks of support electronics/signal conditioners, all tied together with literally miles of low voltage signal and control cabling.
Hope to get some pics of the neater stuff and post them here on ECN.
PS, as far as maintenance is concerned, we have SCUBA divers on-staff for the minor stuff, and the tank gets drained about once a year for regular PM.
[This message has been edited by NJwirenut (edited 06-23-2006).]
Wow, sounds like you get to play with some fun stuff! Was some of this used in prior attempts to kill a Roadrunner? Is there any chance of coupling position pulses off of the screw drive? Wouldn't there be pressure spikes in the hydraulic lines at either end of travel. I believe that is one way that some systems determine when tools of various lengths contact the work surface. Joe
First: Fasten an inclinometer to the backside of the mirror. This will give you an analog signal proportional to mirror tilt. Then use any standard process controller for set point control for upper and lower limits. This also allows the option of ramping the travel speed up or down for gentler starts and stops, and shorter travel time.
Second: Use any IP68 sealed proximity switch adjusted to operate at desired location.
Third: Clippard along with the rest of the pneumatic and hydraulic component manufacturers make manually operated control valves. Use a piston style switch to supply or release fluid pressure when the mirror carriage is at its desired position.
Fourth: Attach a sealed shaft encoder to the acme-threaded shaft and generate pulses to count revolutions to determine upper and lower travel limits. Omega has all sorts of controllers to handle the pulse counting and limit set points.
Can you replace the hydraulic motor with a hydraulic cylinder?
If so... why not duplicate the internal cylinder with a cylinder on the outside of the tank. Pipe the two together, fill the system with oil and bleed all air out. Apply force to the cylinder outside of the tank and you will get a corresponding motion inside the tank. Apply your control sensors to the cylinder outside the tank and you are good to go.
[This message has been edited by Rick Kelly (edited 06-23-2006).]
Re: Submersible limit switches?#66956 06/24/0603:37 AM06/24/0603:37 AM
I also need reliable continuously submersible switches, for a model robot submarine. We have been using some IP-68 rated switches sold by McMaster ( http://www.mcmaster.com )as part# 6944K11. These have proven to be reliable, and cheap enough, but don't have much in the way of mounting options.
I've been wondering if 'float type' liquid level switches could be used, with the float removed and a different mechanical actuator in place. Many of these switches seem to be rated to several atmospheres, but I don't know the details of their operation or sealing. Judging by the pictures in the mcmaster catalog, many of these look magnet based, but when you have a well defined and enclosed mechanical system using magnets I bet they are pretty reliable.
What I would really like to buy is a completely waterproof lanyard switch, where you have a NO switch held closed by some sort of pin or clip, with the pin or clip on a lanyard, so that you can open the switch with a yank on the lanyard.
Re: Submersible limit switches?#66958 06/24/0610:56 AM06/24/0610:56 AM
It looks like they might have something that will fit my application, and maybe yours, Jon. No idea about price yet...
Replacing the motor and screw with a simpler cylinder isn't an option because of the limited depth of the pit under the mirror (about 8"). There may be an inclinometer installed on the mirror for setting intermediate angles, but I wouldn't want to rely on that type of electronic system as a failsafe limit device.
[This message has been edited by NJwirenut (edited 06-24-2006).]