Turntable troubles

Alan Turner
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Re: Turntable troubles

Postby Alan Turner » Wed May 31, 2017 3:09 pm

All this shows you that for a practical turntable you need to go to a stepper motor.

A stepper motor accurately steps (typically 1.8deg). This can be reduced with a gear box and using a typical 1:64 gearbox this gives 0.028deg steps.

This means a 300mm diameter turntable can step to a precision of 0.073mm. The stepper motor also holds its position.

So no encoding disks, stops or micro switches.

regards

Alan

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Guy Rixon
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Re: Turntable troubles

Postby Guy Rixon » Wed May 31, 2017 4:19 pm

Alan Turner wrote:All this shows you that for a practical turntable you need to go to a stepper motor.

A stepper motor accurately steps (typically 1.8deg). This can be reduced with a gear box and using a typical 1:64 gearbox this gives 0.028deg steps.

This means a 300mm diameter turntable can step to a precision of 0.073mm. The stepper motor also holds its position.

So no encoding disks, stops or micro switches.

regards

Alan


Except, of course, the zero-set switch so that the stepper can find its reference position when it's first powered up. Which is usually the part of the system that gives most grief.

Alan Turner
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Re: Turntable troubles

Postby Alan Turner » Wed May 31, 2017 4:44 pm

Guy Rixon wrote:
Alan Turner wrote:All this shows you that for a practical turntable you need to go to a stepper motor.

A stepper motor accurately steps (typically 1.8deg). This can be reduced with a gear box and using a typical 1:64 gearbox this gives 0.028deg steps.

This means a 300mm diameter turntable can step to a precision of 0.073mm. The stepper motor also holds its position.

So no encoding disks, stops or micro switches.

regards

Alan


Except, of course, the zero-set switch so that the stepper can find its reference position when it's first powered up. Which is usually the part of the system that gives most grief.


No need with a proper programme.

We have a turntable on Ellerton Road without any servo-set switch and it works fine.

Regards

Alan

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Jol Wilkinson
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Re: Turntable troubles

Postby Jol Wilkinson » Wed May 31, 2017 6:13 pm

Alan Turner wrote:All this shows you that for a practical turntable you need to go to a stepper motor.

A stepper motor accurately steps (typically 1.8deg). This can be reduced with a gear box and using a typical 1:64 gearbox this gives 0.028deg steps.

This means a 300mm diameter turntable can step to a precision of 0.073mm. The stepper motor also holds its position.

So no encoding disks, stops or micro switches.

regards

Alan


But isn't the gearbox likely to have some backlash which will introduce a possible error?

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Paul Townsend
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Re: Turntable troubles

Postby Paul Townsend » Thu Jun 01, 2017 10:33 am

Jol Wilkinson wrote:
But isn't the gearbox likely to have some backlash which will introduce a possible error?

Definitely and that defeats the aim.
I am planning to use a micro-stepping motor drive for my turntable and a final drive of about 5:1 ratio using toothed belt.
I anticipate no backlash so the motor holding position will work. A similar technique has been proven on my traverser.

Alan Turner
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Re: Turntable troubles

Postby Alan Turner » Thu Jun 01, 2017 11:21 pm

Jol Wilkinson wrote:
Alan Turner wrote:All this shows you that for a practical turntable you need to go to a stepper motor.

A stepper motor accurately steps (typically 1.8deg). This can be reduced with a gear box and using a typical 1:64 gearbox this gives 0.028deg steps.

This means a 300mm diameter turntable can step to a precision of 0.073mm. The stepper motor also holds its position.

So no encoding disks, stops or micro switches.

regards

Alan


But isn't the gearbox likely to have some backlash which will introduce a possible error?


How much "backlash" does your printer or scanner have? Both have stepper motors with gearboxes. If there were any appreciable so called "backlash" in these they would not work. Stepper motors and their gearboxes are used for precise positioning and at far greater precision than you are ever likely to require for a turntable.

Regards

Alan

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Jol Wilkinson
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Re: Turntable troubles

Postby Jol Wilkinson » Fri Jun 02, 2017 7:10 am

Alan Turner wrote:
Jol Wilkinson wrote:
Alan Turner wrote:All this shows you that for a practical turntable you need to go to a stepper motor.

A stepper motor accurately steps (typically 1.8deg). This can be reduced with a gear box and using a typical 1:64 gearbox this gives 0.028deg steps.

This means a 300mm diameter turntable can step to a precision of 0.073mm. The stepper motor also holds its position.

So no encoding disks, stops or micro switches.

regards

Alan


But isn't the gearbox likely to have some backlash which will introduce a possible error?


How much "backlash" does your printer or scanner have? Both have stepper motors with gearboxes. If there were any appreciable so called "backlash" in these they would not work. Stepper motors and their gearboxes are used for precise positioning and at far greater precision than you are ever likely to require for a turntable.

Regards

Alan


Alan,

I don't know the answer, which is why I asked the question.

Do scanners need to stop at an exact position, or is a stepper motor used because it provides a consistent speed for the scanner "head"?

I know of one turntable drive mechanism that uses a stepper motor, but at £175 it is possibly more than most people might want to pay. Can you suggest a stepper motor and gearbox, together with a controller that will do the job? I've had a look at RS Online, etc. but don't know enough about it to identify that which will do what is required.

Jol

nigelcliffe
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Re: Turntable troubles

Postby nigelcliffe » Fri Jun 02, 2017 7:28 am

Jol Wilkinson wrote:
I know of one turntable drive mechanism that uses a stepper motor, but at £175 it is possibly more than most people might want to pay. Can you suggest a stepper motor and gearbox, together with a controller that will do the job? I've had a look at RS Online, etc. but don't know enough about it to identify that which will do what is required.

Jol


Jol,

Two suggestions:

search internet on "stepper motor and ardunio" and start reading up how to make controllers, sorts of stepper with gearboxes, etc.. I think you could get the budget to nearer £17.50 than £175. I think the lower end of motors for robotics should be adequate for turntables.

Have a chat with Graham Maslin (Bury St Edmunds way, big P4 model of Safron Waldon under construction/restoration). He's done quite a bit with stepper controls for signals with arduino. Converting that knowledge to a larger stepper to drive a turntable would be simple.


- Nigel

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Jol Wilkinson
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Re: Turntable troubles

Postby Jol Wilkinson » Fri Jun 02, 2017 8:09 am

Thanks Nigel, but I don't need a TT drive as I have no turntable.

I was asking simply for the sake of knowledge and to help those, including Andrew who started this topic, with a way to find a way to accurately control a turntable.

If I needed to, I think I would use a simple drive mechanism like the LRM version and separate locking, along the lines of Russ's suggestion with a solenoid or servo driven latch. However that doesn't fix Andrew's problem and hopefully Hubert will be able to provide a satisfactory fix.

Jol

Alan Turner
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Re: Turntable troubles

Postby Alan Turner » Fri Jun 02, 2017 7:16 pm

There are many stepper motors available by Googling Stepper motor.

However the cheapest, and whilst not up to the precision of the NMEA range of motors but more than adequate for turntables, are these:

Stepper 1.jpg



Made in their millions for Air Conditioning units they are cheap (circa £2) and adequately powerful for N and OO turntables. They are easily driven from an Arduino. They have an integral 1:64 gear box giving 12800 steps per full rotation.

regards

Alan
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Andrew Bluett-Duncan
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Re: Turntable troubles

Postby Andrew Bluett-Duncan » Tue Jun 06, 2017 12:34 pm

Hello everyone
Firstly I'd like to thank you all for your experiences and ideas. I think if I knew when I started out trying to motorise a turntable what I do now, I'd probably have gone for something much simpler or maybe the stepper motor route. The Chatham mechanism however is not without its good points, but I think it needs more development work. The locking mechanism is a must I think, the size of the indexing disc (much bigger would be much easier to set up), and lastly the difficulty in achieving 180 degree stops opposite each other(although this last point may just be inability to measure accurately on my part!) for me needs looking at.

Having said that I've been making some progress with a number of difficulties I've been having. They included the problem that I first wrote about which was the turntable going out of sync when I put the weight of an loco on it. This turned out to be the screw that locked the turntable to the motor drive shaft was slipping. If I screwed it down hard the nut soldered to the outside fractured the soldered joint( see the photos).
Central drive with soldered nut.jpg
So I drilled two 1.8mm holes on the other side of the outer shaft and tapped them 8BA. I also replaced the brass bolts with steel. Now the turntable doesn't budge on the joint and slipping appears to be a thing of the past.
Central drive with holes tapped 8BA..jpg


The other problem I had was with a loco on board it sometimes jammed at a couple of points in the revolution. This turned out to be entirely of my doing. I'd failed to pay enough attention to the positioning of the Turntable Well rail and the wheels were coming off the rail and slipping down inside and then jamming up completely. So I made a simple jig that hinged off the central drive shaft and so described circle accurately and I relaid the rail to that diameter. This worked well.

So now I have a turntable that runs and stops in all 4 of the right places going clockwise and this may have to be good enough. Because, try as I might, I can't achieve reliable indexing going anti-clockwise. I've feeling this maybe due to either the tiny amount of sideways movement left in the index locking plunger or something happens rather as Mark described on page 1 of this thread when the plunger comes to rest at a subtly different point on the disc when going in the opposite direction. But in that regard I'm still a bit puzzled as I thought that Ive taken the majority of the slack out of mechanism by turning the stop points which are normally a wide-ish U shape into V's. (See the photo). The V's are the same shape as the locking plunger. Do the V's have to be very evenly sided? mine aren't?
Indexing disc with alterations.jpg


Any further ideas of how I might achieve turntable nirvana would be very welcome.
Andrew
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junctionmad

Re: Turntable troubles

Postby junctionmad » Mon Jun 12, 2017 9:02 pm

Alan Turner wrote:There are many stepper motors available by Googling Stepper motor.

However the cheapest, and whilst not up to the precision of the NMEA range of motors but more than adequate for turntables, are these:

Stepper 1.jpg


Made in their millions for Air Conditioning units they are cheap (circa £2) and adequately powerful for N and OO turntables. They are easily driven from an Arduino. They have an integral 1:64 gear box giving 12800 steps per full rotation.

regards

Alan



My own experience has shown that stepper controls, even with belt drive systems, are prone to mis-location, furthermore the movement of the Loco can cause issues. Furthermore zeroing the turntable on startup is not easily achieved . The simplest solution is the conical locator pin idea. Its easy to implement even without a lathe and works reliably over time.


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