Mechanized TV Mount - Page 1
Forum section: electronics / projects

author: JiggyNinja
author_rank: Regular Contributor
timestamp: December 22, 2016, 07:03:05 pm
content: So my new year's resolution is to actually do this project I thought about way back in the summer. The first thread I made about this was on Arduino forum (Actuating existing hinge), but I think I have a better chance of getting higher-level help here. I'll excerpt relevant points here.Picture of the wall mountThe basic summary of this project is that I want to build some motors into this TV wall mount so that I can make it move with something like an IR remote control. The software and electronics side of this project are within my capabilities, my major learning curve is with the mechanics. I have basically no first-hand experience with actuators, gears, and linkages, so there's a lot of things that I don't know what to buy, where to buy it, or how I would use it.I want this to be an aesthetically pleasing build, so I'm hoping to be able to build as much of the hardware as possible (at least the motors and control boards) into the inside of the tubes. The problem with that is that the tubes have the sameoutsidewidth as a standard size servo. The tube walls are about 2-2.5mm thick. Inner dimensions are as follows:2 x wall plate tubesWidth: 17.3 mmHeight: 27.0 mm1 x 2nd link barWidth: 16.4 mmHeight: 35.5 mmThe large bolt going through the center of the middle hinge is 9.86 mm think. The two spacers on it as 19.2 mm tall (including the washers) and 20.0 mm wide.I have a few different ideas for how I might gear things, but the most important thing is the position feedback that I can get from a servo's potentiometer. I'm wondering what help I could get to learn how to mount a small enough motor into the tubes, gear them down enough to be useful, and attach a pot for position feedback.Servocity and pololu are about the only websites I know for motorization parts like this, but I'm still floundering about not knowing what kinds of things I would need.My speed requirements are not great (I'll be fine if a hinge takes 10 seconds to go from stop to stop), but the torque requirements might be a little high. I estimated with a luggage scale maybe 1 or 2 ft-lbs.

author: ebclr
author_rank: Super Contributor
timestamp: December 23, 2016, 08:01:28 pm
content: I guess this kind of support isn't the best choice for motorization, you will be surprised on how many troubles will appear, Just test what torque is necessary, I believe that 80 % of chance to fail for this idea, The best option is to have two arms one fixed who will handle most of the TV weight, and other arm as a linear actuator who will move the angle you want.

author: JiggyNinja
author_rank: Regular Contributor
timestamp: December 27, 2016, 06:49:09 pm
content: I don't think I will be surprised about how troublesome this will be, because I am already expecting a lot of trouble. And I did attempt to measure the necessary torque:QuoteThe motors won't have to hold the TV up against gravity, the motors' force is all applied orthogonal to gravity. The only thing they need to overcome is friction. Once it reaches position (or one motor draws too much current) I intend to have the motors all cut out so that it's held in place by just normal friction and I can re-position it manually if I so choose.

author: ajb
author_rank: Super Contributor
timestamp: December 27, 2016, 08:15:07 pm
content: 1-2 ft-lbs translates to 192-384 oz-in. A generic high-ish torque hobby servo probably has <100oz-in. Trying to keep everything inside the tubes is going to be very difficult. Consider that with a 17mm wide tube you have a maximum radius of <8mm from the axis of rotation, and at a radius of 8mm that 384oz-in of torque becomes 1220oz or 76lbs of force, which your linkages or gears will need to sustain. Doing external linkages will allow you to have a largermoment arm, which reduces the forces involved.You might consider servo gearboxes likethese. Some of them are in the right ballpark, torque-wise, and have provisions for feedback either via a gearbox-mounted pot (direct feedback of the output shaft position) or via an integral pot in a multi-turn servo (indirect feedback, but allows you to use the servo's internal feedback system).

author: JiggyNinja
author_rank: Regular Contributor
timestamp: January 01, 2017, 06:46:49 pm
content: Quote from: ajb on December 27, 2016, 08:15:07 pmI don't expecteverythingto be built into the arms, just as much as possible. I'll be fine with having the last pair of gears or a set of swing arms be outside the bar, I just don't want to have the servos and gearboxes glued to the outside of the arms like a hideous wart.The problem is that standard servos are just a bit too big, so those gearboxes you posted are way too big also. The idea I've been looking into is to DIY the servo using something like amicrometal motor. I could take one of the 100 RPM high power motors, gear it down 10x again, and couple that to the arms. Then it's just a matter of figuring out how to hook up the potentiometer to it for feedback. I don't need a very sophisticated control algorithm, so I can probably program my own motor controller with an ATtiny or PIC.

author: DTJ
author_rank: Super Contributor
timestamp: January 03, 2017, 05:12:05 am
content: As ebclr suggested, linear actuators will make it easier.http://www.peopleoflava.com/wp-content/uploads/header_6013.jpgPlenty on eBay.

author: ajb
author_rank: Super Contributor
timestamp: January 03, 2017, 12:46:26 pm
content: Quote from: JiggyNinja on January 01, 2017, 06:46:49 pmBe aware that any additional reduction stages will incur additional losses, which can be substantial. Friction increases with tooth pressure, so later stages where torque is higher incur greater losses, as do smaller gears where the radius (moment arm) is shorter and thus the force on the tooth is higher. Note that on that page, you have one motor with 2oz-in @ 6000rpm = 12,000 oz*in*r/min, and the same motor with higher reduction has 125oz-in @ 32rpm = 4,000 oz*in*r/min. In other words, the additional 200:1 reduction in the latter gearbox is only 33% efficient!Also note that the torque rating for those motors is thestalltorque, in other words the torque at which they don't turn any more. I'm not sure if there's a good rule of thumb for stall torque vs required running torque, but stalling is very hard on a motor and ought to be avoided (as Pololu note on that page). Bottom line, if your required torque is towards the higher end of your estimate, then the 100:1 motor plus 10:1 gearing may still wind up being marginal after you account for reduction loss.

author: JiggyNinja
author_rank: Regular Contributor
timestamp: January 12, 2017, 08:41:37 pm
content: Thank you very much ajb, that isexactlythe kind of knowledge I was hoping to see.Do you have a reference for the claim that tooth pressure reduces efficiency? By my intuition the higher tooth pressure would certainly cause higher friction, but it would be exerted over a shorter distance so the actual work wasted would be about the same.I did some calculations with the table of HPCB 12V motors on the Pololu page. The ungeared speed (RPM * gear ratio) is pretty consistent at 30k RPM for all the motors, give or take a few percent. However, the ungeared stall torque (torque / gear ratio) is not so consistent. When I graph the normalized stall torque against the gear ratio, it changes in clear steps. There's a step at 0.4 oz-in, another at about 0.3 oz-in, and another at 2.4. The 1000x geared motor is an outlier with 0.125 oz-in normalized stall torque.The suggests to me that the efficiency of a gear box is more dependent on the number of gears in the train, and not the overall reduction ratio. The 30x and 100x motors have the same normalized stall torque, even though the rations are more than 3x apart.So the additional gear stage I'd need to use will certainly have power loss, but I don't think the ratio of that stage would make a difference.I can probably reduce the torque I need by loosening the bolts a bit and adding a little lube, though I would like enough static friction to hold the mount in the selected arrangement when the motors are not energized.I am planning to use some method to sense a stall condition on the motors, so that if the arm catches on something or I decide to stop it with my hands, all the motor controllers will de-energize the motors.

author: nickcres13
author_rank: Newbie
timestamp: April 16, 2017, 05:47:20 pm
content: Yo JiggyNinja I know you said you have the electronics part figured out, but I just wanted to share my latest triumph of servo control with the attiny85, it seems to be just the thing you need. Let me know if it helps! (or if you already knew all that and this post was a waste of your time)https://github.com/electricsheeplabs/attinyServo

author: retrolefty
author_rank: Super Contributor
timestamp: April 16, 2017, 10:15:49 pm
content: My gut thought would be to use threaded rods connected to stepper motors using threaded followers connected to arms needing to be moved relative to stationary plate. A simple led/sensor can give astarting reference point for the software to calculate position without analog feedback signal.