robotics question (cable actuators) - Page 1
Forum section: electronics / projects

author: KTP
author_rank: Frequent Contributor
timestamp: January 06, 2013, 02:11:25 pm
content: Have any of you used really long cable actuators in a robotics application?This project idea is somewhat like a robotic arm, but needs to operate underwater. The precision needed is not great, but I would not want more than a few inches of slop in the position of the arm. I would like to keep all of the linear actuators, motors, electronics above the water and have cables inside of tubes (picture a typical bicycle brake or shifter cable but much beefier) to move the joints of the mechanical device underwater. The depth is a max of about 20 feet and the cables would need to be around 60 feet long leading back to the actuators/motors above water.Is this even remotely doable? What are the potential pitfalls?Thanks.

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 06, 2013, 02:43:36 pm
content: Quotedarn thats looks like an expensive puppet. how many actuators? is it salt or white water? you'll be in trouble if its salt/sea water.Quotenothing is undoable. it is just the price.

author: SeanB
author_rank: Super Contributor
timestamp: January 06, 2013, 02:48:48 pm
content: Loss in the cables for one. Just use pneumatic linear actuators and a manifold to drive them. Will work better and have lower loss, plus you only need one motor to drive the compressor. Makes for easy control, and feedback is also possible if you use position sensors and actuators. If it is salt water you use corrosion resistant shafts and bodies, or plastic housed actuators.

author: KTP
author_rank: Frequent Contributor
timestamp: January 07, 2013, 12:51:46 am
content: Quote from: SeanB on January 06, 2013, 02:48:48 pmWhat do you mean by loss? Friction in the cable? I don't care a lot about efficiency as long as the manipulator has enough force to get where it needs to go. If I am driving the cables with a 400 pound force linear actuator and I have 150 pounds of loss in the cable due to friction, that is still *ok*The environment is very cold salt water. I thought about using pneumatic actuators, but make them sort of hydraulic with an enviro friendly antifreeze. Yes I would need only one main compressor motor, but I would need 2 control valves per actuator.Something to think about though. I had the cable idea just because it would allow most of the guts of the system to be up out of the salt water and only the stainless steel cables exposed.

author: SeanB
author_rank: Super Contributor
timestamp: January 07, 2013, 04:56:33 am
content: Pneumatic actuators actually use one valve per cylinder. It operates both ports at once to open/close. If you want linear movement you can use a proportional valve that allows the pressure to be changed in proportion to the applied voltage (normally a 4-20mA or a 0-10V input) to give linear positioning. You can get IP68 cylinders and position sensors, very common in industrial use. The problem with cables is the loss and the friction that varies with time and corrosion in the cable. No need to use antifreeze as long as the operating pressure of the compressed air is higher than the water pressure at the required depth. You will still need to have an IP68 rated enclosure for the controls as otherwise sea water will destroy them in days.

author: Kremmen
author_rank: Super Contributor
timestamp: January 07, 2013, 09:42:43 am
content: But won't the compressibility of air be a problem? You will get significant load compliance if there is an appreciable lever ratio of the pneumatic actuators vs the tool end. Of course with very light loads this might not be an issue, but when some force and accuracy together is needed, the medium should be incompressible, i.e. a liquid.

author: johnwa
author_rank: Frequent Contributor
timestamp: January 07, 2013, 12:34:11 pm
content: If you want to try cables, have a look at this mob:http://www.speedscreen.com.au/. Expensive, but good, and also push/pull capable. I don't know how they would go underwater though.

author: robrenz
author_rank: Super Contributor
timestamp: January 07, 2013, 01:14:32 pm
content: Quote from: KTP on January 07, 2013, 12:51:46 amAll systems have losses, nothing is incompressible, nothing is infinitely rigid, etc.. A cable system should use the highest Young's modulus tension member practical like possibly Spectra fibre. Then you want to use the longest moment arm possible on your linear to rotary conversion to lower the change in cable tension from unloaded to max load conditions. You also should use the shortest cable lengths possible because the cable elongation under load is proportional to the loaded length. The elasticity of the system is going to be the main problem. Think of trying to write your signature at normal size with a pen tied to the end of a light fishing rod. The control loop is hopelessly incapable of making it work because the system is not rigid enough.All the principles above apply to any type of system, hydraulic, pneumatic, linkage, belt, electrical, etc. just to vastly differing degrees.Edit: fix quotes

author: KTP
author_rank: Frequent Contributor
timestamp: January 07, 2013, 05:17:36 pm
content: Thanks for the replies.I am not 100% stuck on using cables, I just thought it might be a way to keep most of the sensitive, corrosive parts out of the salt water while keeping the manipulator under the water pretty simple.Hydraulics do sound better than pneumatics because of the compressibility of air. I don't need thousands of pounds of force though, and I am not sure about the regulations when using possibly contaiminating oils in a marine environment.I need 24 actuators, so keeping the cost per actuator low would be nice.

author: robrenz
author_rank: Super Contributor
timestamp: January 07, 2013, 05:51:33 pm
content: There are existing water based hydraulic fluids and some for under sea applications. You will have to google for them. I don't remember where I saw them. They have additional maintenance issues compared to petroleum based fluids and require specialized components for long term success. Control valves close to the actuators would be ideal for response if you can get waterproof versions then you would only have 2 hydraulic lines to the manifold that supplies all the valves and smaller cheaper umbilical cord for the electrical control of the valves.

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 07, 2013, 06:09:47 pm
content: Quote from: KTP on January 07, 2013, 05:17:36 pmlets call it engine. and the arm is an arm. is it the engine is significantly heavier than the arm? and making all of them in one single water tight compartment will greatly increase the cost? have you consider the cost of bracing rigs for both the engine and the arm? and also the maintenance cost?

author: dermitza
author_rank: Contributor
timestamp: January 09, 2013, 04:04:03 pm
content: Quote from: robrenz on January 07, 2013, 01:14:32 pmYou should keep in mind that all polymer-based cables have elongation creep too. If you require your system to be positionally repeatable over a number of cycles you will have to go for steel cables.

author: nctnico
author_rank: Super Contributor
timestamp: January 09, 2013, 04:13:37 pm
content: I'd put some encoders on the joints to have a proper positional feedback. Even when using cables one could use standard DC servo systems for positioning. The control loop will need extra dampening due to stretching in the cables to prevent oscillation.

author: Unknown
author_rank: Unknown
timestamp: January 09, 2013, 05:51:41 pm
content: Cables could be used to transmit rotational movement to operate screw jacks etc underwater, much like the tach cable in old cars.Servo controlling hydrolics does not sound easy, especially in salt water.

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 09, 2013, 06:09:21 pm
content: Quote from: HAL-42b on January 09, 2013, 05:51:41 pmunder 400lb of load at 20ft?

author: Unknown
author_rank: Unknown
timestamp: January 09, 2013, 06:23:36 pm
content: Quote from: Mechatrommer on January 09, 2013, 06:09:21 pmSure. Just apply appropriate reduction at the end.I am not saying to use an actual tach cable of course. Obviously there must be some engineering done.

author: robrenz
author_rank: Super Contributor
timestamp: January 09, 2013, 07:03:37 pm
content: A M6x1 setscrew only needs 12 in*Lb torque (1.3558 N*m) to generate 400 Lb force (1779 N)

author: Unknown
author_rank: Unknown
timestamp: January 09, 2013, 07:10:57 pm
content: There ya go. Sorry about the crap picture, lighting is bad atm.All the complicated bits out of the water. No need for pressure stuff. No need for lubrication (with suitable bearings). No worries about pollutant liquids.

author: robrenz
author_rank: Super Contributor
timestamp: January 09, 2013, 07:29:19 pm
content: Quote from: HAL-42b on January 09, 2013, 07:10:57 pmNot a bad idea but the flexible shaft still has torsional windup specified in torque/degree. It cant be as stiff as a pull-pull cable system containing the exact same amount of piano wire as the flex shaft. Unidirectional flex shafts are stiffer than bidirectional flex shafts but you need bidirectional here. They also need a properly constrained outer casing to not turn into a pretzel when loaded.

author: Unknown
author_rank: Unknown
timestamp: January 09, 2013, 07:37:51 pm
content: Quote from: robrenz on January 09, 2013, 07:29:19 pmAgreed.Lets just say that solid linkages, piano wires and flex shafts are good enough repertoire to render hydrolics unnecessary.

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 10, 2013, 03:47:16 am
content: Quote from: robrenz on January 09, 2013, 07:03:37 pmi wonder how long it will complete the job esp for this OP application. he needs to calculate and figure that one out himself and decide whether a slow robot is acceptable.edit: and btw he's going to need all stainless steel gear reduction mechanism down there if he opt for non-sealed arm. good luck on that.