How to set up a braking system for a heavy duty turntable?

[mgv9694]

Hello, everybody!

I am interested in adding an electrically-controlled braking system for the project I'm including in the link below (think human-sized Lazy Susan).

The main goal is to have a remote control that will not only determine the DC motor's speed and direction, but also stop the system safely. It's important to note that "stopping" the DC motor will not be sufficient for this design since the pulley system leaves the rotational surface some independence of movement - which is where the human targets would be standing.

My hope is to have a brake design that can be applied before and after the system is in motion as well as an interrupting emergency brake of sorts.

As you can see in the images of the link below, the original builder included a mechanical foot brake - this would work if it could be automatically programmed to work via remote control.

I am very unfamiliar with this type of mechanically-intensive hardware design so I was hoping I could get some suggestions as to how to best approach solving this problem.

So far I am considering a linear actuator underneath the mechanical footstop in question (which may be a bit too large for the height space available - and a bit pricey too) or some sort of electrically stimulated clamp on the exterior (similar to that of a hard drive design but with the ability to hook/release based on voltage application).

If you find I am not explaining the problem properly or if my solution options don't seem feasible, I will be more than happy to answer any more specific questions you can bring up to me. I am open to any and all suggestions.

*** Heavy Duty Turn-Table ***

https://www.thingiverse.com/thing:28454

Thanks in advance for your help!
- MGV

 

[itsmoked]

There are many DC motors that have integral brakes that must be released before motion can happen. Why don't you just use one of them as your motive source.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[OperaHouse]

If you can show that, I don't feel bad about making this suggestion. Years ago I was playing with braking induction motors with a DC field. I was quite impressed. This might work for somebody. Use a good size three phase motor just for for the bearings and apply quite a few amps to the windings for braking. A large motor will be able to take the heat. Quick and dirty.

 

[jraef]

As I see it the problem with trying to do this with electronic motor braking is that you are apparently trying to use a small little DC motor with a large pulley ratio. Even if you were to use a brake on that little motor, the stopping time would be relatively long. To know how much, observe how long it takes to accelerate the main platter to full speed using that motor with the expected load on it. You will NOT be able to brake it any faster than that, and likely not even at twice that time (1/2 the rate), depending on what the motor can take and how powerful the braking chopper is.

I have to admire the simplicity of that original design however:
1) Step on stick.
2) Stick presses on disc.
3) Disc stops.

If you insist on over killing it however, a simple taut band style brake could be fabricated and operated with a simple pulling solenoid. The band wraps around a disc or drum but just barely not touching it, with a small gap between the ends. One end is fixed in place to a rigid frame, the other is attached to the solenoid. When the solenoid is energized it pulls the band tight around the disc/drum.




"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[BrianPetersen]

There are many ways to do this, depending on how nicely you want it to operate and whether positioning accuracy is of any relevance.

The easiest way, as already mentioned, is to use a motor that has a built-in spring-activated brake (you have to apply power to release the brake). This ought to be simplest and cheapest. It will have next to no positioning accuracy and there is only "ugly" (on/off) control of the brake - in a power-off condition, it will slam to a stop. If there is stuff piled up on the turntable, there is a fair chance that the turntable will stop and the stuff that's on the turntable will keep on going and make a big mess. You haven't told us the application, so at the moment, only you know whether that's a relevant issue.

At the other end of the spectrum are the turntable applications that I usually deal with (robotics) and if it is anything like that ... it would ordinarily be driven through an AC motor with VFD or a servo drive. If you have a stop situation, you can have the drive ramp down to a stop in a controlled fashion (using the motor in regenerative-braking mode), and *then* apply the mechanical brake.

Keep in mind that if this is a high-inertia application, the drive mechanism will have a torque rating that will limit the deceleration rate. If most of the inertia is on the load side, it is quite possible to break stuff (gearbox internals, etc) by slamming the motor to a sudden stop.

 

[racookpe1978]

You indicated "human" people as the load (riders) on the rotating platform.

People can be relatively "flexible" about stopping and starting (like on a trolley or airport railroad/subway car). But - Do you need a very gradual slowing force that comes accurately to a stop at that single point on the rotation? Or is a more sudden brake force OK, as long as the rotation stops "exactly" at the right place each time?

 

[itsmoked]

Just us a pawl that engages a notch on the table perimeter, it's positive, visual, simple, fatal.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[mgv9694]

Thank you very much to all of you that replied!

I am sorry if I didn't explain the application very well: I want to get people (~100 - ~300lbs_max) to be on top of the platform and have them spin around at a slow, constant rate for a given period of time.

The reason I'm not using a servo is because accurate positioning isn't of vital importance as long as the pulley system does its job right. The reason I'm not using a more powerful motor is because my target speed is >1-3rpm, any faster and people may start getting dizzy! All I'm looking for is for the turntable to rotate at a given rpm for as long as I (or users) decide via a programmed micro-controller attached to the system.

It's intended for human subjects to either stand or sit on it. Thus, when they make a motion (be it by getting on/off or making any movements) the platform may wobble. Additionally, when the motor stops outputting toque (i.e. stops/brakes), the momentum of the pulley - as jraef mentioned - would keep the platform spinning for a while (i.e. overcoming the "brake" command that I send through the uController).

I am sure there are better ways of designing such a system but I already have a working unit of the mentioned design, so I just want to make a simple addition for now - I'll consider an upgrade in the future.

As for the solenoid brake that jraef mentioned, which is pretty close to what I'm looking for:
Wouldn't the system overheat quickly due to the large current draw of the solenoid? This would imply that braking times would be reduced to probably a few seconds (possibly up to a minute at most), right?

The idea does have a lot of merit, I was just hoping that there was a solution that had the effect of the manual foot-brake for a more extended period of time (like, when people are getting settled in/climbing on and off the platform). We get elderly ladies trying to climb on top and I am looking for a more automatic way of assuring them the platforms won't wiggle (Assuming that maybe the person with the uController is unable to use the footbrake perhaps due to injury or disability. Eventually, we want the people standing on the platforms to be able to use the device by themselves.) Hence my idea of inserting a small linear actuator under the footstep.

I understand that there are plenty of limitations to the design itself, but I really appreciate all your help and suggestions in trying to make it just a little bit better.

Once again, thanks!
-MGV

 

[BrianPetersen]

Why do you need a brake at all? Why not just let it coast to a stop?

A reduction gearbox with such a high reduction ratio to get to your target speed, while keeping the motor speed in a decent range, won't back-drive very easily. At 3 rpm, if you simply switched off the motor, it's not going to coast very far.

 

[btrueblood]

You are already using an auto wheel hub. Get the hub from a disc-brake car. Plumb the caliper line to a hydraulic cylinder (or better a hydraulic accumulator). Plumb the other side of the cylinder or accumulator to a solenoid valve, and connect a pressurized air source to the solenoid valve inlet. A suitable relay to drive the solenoid, and presto...

...though I too like the simplicity of the pedal brake. You might think of a way to put the pedal (and a safety switch) on the rotating platform, such that a user must stand on the pedal to release the brake and start the motor.

 

[3DDave]

Servos can be velocity controls as well as position controls.

I would look at a strap or band brake. They are self-exciting so they amplify the input, but are easy to control. They typically act in only one direction, but could be set as a pair.

You could add it by cutting a groove in the edge of the platform. A piece of wire could serve as the band. As with any friction brake they need to be adjusted for smooth operation - this is done most easily by moving the anchor point.

http://en.wikipedia.org/wiki/Band_brake

 

[ScottyUK]

Expanding on btrueblood's idea, have a look at the mechanism of the electric parking brakes fitted to many modern cars. It may be possible to modify it slightly so it is less aggressive in its action.

 

[dicksewerrat]

To stop your 'Wobble' issue, you may have to install a track under the platform near the edge and install wheels to run in the track to stabilize the platform

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[tygerdawg]

In a previous life, I spent a substantial amount of time at a well-known gearmotor manufacturer as an Applications Engineer. My solution would be a gearmotor rig with appropriately-sized motor and gear ratio. You would have to figure out the mechanics of how to drive the table, either gearing, belts, chains, or direct. Add one of the simpler variable frequency drives that will allow you to program all the accelerations/decelerations/stop positions with I/O signals if needed. Include a regenerative brake (during deceleration, allows the inertia to work that rotational energy through the motor windings and that energy is dissipated as heat through heat exchanger fins). And finally, add a brake on the motor to hold it place once stopped. All in one package from one supplier.

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering

http://www.bluetechnik.com

 

[jraef]

Actually, heading down the gearbox rabbit hole, I think if you used a worm gear drive, it will be inherently braking! So redesign your platter with a large ring gear cut to match up to a worm gear on a horizontally oriented drive motor. When you remove power from that motor, it will stop very quickly, almost immediately. So then, you simply control the Decel profile of your little DC motor.

A sudden power failure may be problematic with that however, you might need a magnetic clutch in there as well so that if power fails, the worm gear is not engaged and it coasts to a stop rather than fling the old ladies off into the weeds...


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington