Reliability of servos in commercial use rather than hobby - Page 1
Forum section: electronics / projects

author: pratiken
author_rank: Contributor
timestamp: October 09, 2015, 12:30:15 am
content: Hi!I'm designing a simple mechatronic device that will need to be somewhat reliable as it will need to rotate 360 degrees constantly 24/7 for up to a full week at a time. It'll rotate a plate with a 5lb weight on it. Fortunately, it doesn't have to be fast at all at a max of 1 rpm.I was looking at this system from ServoCityhttps://www.servocity.com/html/spg805a-bm-cr_continuous_rotat.html1700 oz-in should be plenty but I'm wondering about the servo itself. Can these servos handle longevity like I need or do I need to look into more robust, industrial applications?? I'd hate to have to replace the thing after 3 months of use.I know these little digital servos are great for drones and such, but what's their reliability really like?Thanks!!!

author: LabSpokane
author_rank: Super Contributor
timestamp: October 09, 2015, 02:17:54 am
content: Vibration & noise issues aside, that sounds like a good application for a stepper motor.

author: BradC
author_rank: Super Contributor
timestamp: October 09, 2015, 03:04:22 am
content: Mechanically if you get the metal gearbox models they are pretty ok. If you dismantle them you'll find pretty average shaft/bush configurations and a relatively cheap DC motor. They are not really rated for continuous use and I would think they'd wear out considerably faster than a properly designed unit.As LabSpokane said, I'd be looking at a small stepper coupled to a suitable gearbox. You'll kill the servo quickly putting a static axial load onto the spindle though. They are only designed for radial loads.You could use a small thrust bearing arrangement to take the load of the table, and a worm/gear combo to gear it up. Use an appropriate bearing and it'll last considerably longer than you need it to.As an alternative, a thrust bearing for the table and a high reduction belt drive (like used in a hi-fi turntable) would give a simple and low cost solution.

author: Richard Crowley
author_rank: Super Contributor
timestamp: October 09, 2015, 03:19:55 am
content: Perhaps you did not reveal ALL the pertinent requirements, but nothing you mentioned sounds like it needs a SERVO motor, either the conventional kind OR the "continuous" modification.Do you need control of on/off?Do you need control of speed?Do you need to control specific POSITION?Do you need speed/position feedback?And if you DO need position control and/or feedback it is not clear that a SERVO would be better than a stepper.Furthermore, even the largest and most "heavy duty" servos are rated for "hobby use", not for continuous commercial/industrial applications.Even the largest and most "heavy-duty" servo uses plastic gears and bearings at one or more points in the drive-train.NONE of them are rated for anything like the continuous use you describe.Is there some reason a more conventional (and "industrial" rated) AC or DC gearmotor wouldn't do the trick?Or a stepper motor if you need position control.It is not clear from your description how the 5lb load is applied?That would have a strong "bearing" (no pun intended) on what kind of bearings you would need.

author: pratiken
author_rank: Contributor
timestamp: October 09, 2015, 04:18:34 am
content: You guys are awesome, thanks for the tips.I apologize for lacking details, I was in a bit of a hurry.I need to be able to control the speed of the rotation, direction of rotation, and relative position via computer. I.e. Arduino controlled interface that can accept a command like, rotate 15 degrees every 30 seconds or rotate continuously for 8 hours at 0.25 RPM. The (up to) 5lb load will be sitting centered on a circular platter about 12 inches in diameter. Basically any electronic device from a cell phone to a small 13" notebook computer will reside on the plate.I've worked with servos before but only for fun. Are stepper motors as easy to interface with as servos? Same thing basically?Thanks again!!

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 09, 2015, 04:38:30 am
content: the bottleneck in dc motor is in its brush wiper, thats where you need to think about...stepper can track position as long as there is no miss steps...if you need robust solution i think you'll need to sort out brushless motor (or stepper) + encoder.

author: BradC
author_rank: Super Contributor
timestamp: October 09, 2015, 05:12:06 am
content: If you are going to use a stepper, don't worry about the encoder. Just use a zero reference (hall effect, optical, switch, whatever) and it'll correct any mis-steps each time it passes the zero.

author: Ian.M
author_rank: Super Contributor
timestamp: October 09, 2015, 05:32:38 am
content: It sounds like most of the problem is going to be the mechanicals. Small DC motors can be good for thousands of hours of operation - e.g any good quality cassette player.I'd go for a large diameter thrust bearing consisting of two circular tracks milled into the turntable and the base, loaded with Torlon balls. To prevent jamming, use two different sizes and alternate them so that between every pair of load carrying balls, there is an idler ball separating them. You'll need a central shoulder bolt in a loose clearance hole to positively retain the turntable to prevent loss of the balls if it is tilted or anyone attempts to lift it.Use belt or friction drive (with a rubber tire on the shaft) from a gear motor, and print out an optical encoder disk to use with an optical quadrature sensor to close the loop for accurate positioning and speed control. Add an index pulse mark and sensor if absolute positioning is required.

author: LabSpokane
author_rank: Super Contributor
timestamp: October 09, 2015, 07:20:41 pm
content: Interfacing with a stepper is pretty trivial if you have something like an Allegro A3967.

author: lincoln
author_rank: Regular Contributor
timestamp: October 11, 2015, 08:12:10 pm
content: I would also point out that there are proper industrial servos + drives, The interface can be the same as a stepper with but with better torq and resolution(but will added cost). The industrial servos will also have a proper motor/ shaft mounting.

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: October 11, 2015, 08:41:29 pm
content: reversing direction can be a problem if there's uch inertia and rapid change required you get large transient torque requiring higher current, peak powerslow, smooth reversal keeps the dynamic requirements low - steppers will lose steps if you demand too much torquesteppers can be microstepped for smoother motion but simple controllers will give torque rippleDC, Hall Effect or Brushed Servo rated motors should have low torque ripple from internal construction - some other electric motors may not

author: FreddyVictor
author_rank: Regular Contributor
timestamp: October 12, 2015, 07:17:54 am
content: maybe abrushless motor servowould be more reliable long-term

author: Fungus
author_rank: Super Contributor
timestamp: October 12, 2015, 07:41:45 am
content: Quote from: LabSpokane on October 09, 2015, 07:20:41 pmInterfacing with steppers is easy. Making sure they never lose a step? Not so much.