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Author Topic: Gun turret rotation - servo or stepper motor?  (Read 13489 times)

Norman Castle

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Gun turret rotation - servo or stepper motor?
« on: January 07, 2016, 06:34:38 PM »

 I've only ever used a servo controlled by Arduino (and then only the once) and I know nothing at all about stepper motors, but might I be better off with one on the next build?   What are the pros and cons of each?
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essex2visuvesi

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Re: Gun turret rotation - servo or stepper motor?
« Reply #1 on: January 07, 2016, 09:12:40 PM »

Im looking into the use of a stepper motor and arduino on my next turret


Using servos does seem to limit the amount of movement to a little under 180 degrees which in the case of my current tirpitz build was sufficient and also with the size there was not enough room


I'm planning on using one of these
http://www.hobbyking.com/hobbyking/store/__26925__Stepper_Motor_and_Driver_Board_5V.html
and a mini Arduino board
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Subculture

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Re: Gun turret rotation - servo or stepper motor?
« Reply #2 on: January 07, 2016, 09:23:00 PM »

Servos can give about 270 degrees of movement if the mechanical limits are removed. The limit then becomes the feedback pot. Another way to get more movement is to gear up the servo say 1:2. That will give you plenty of movement at the expense of torque, but most servos will have more than you need anyway.

Martin [Admin]

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Re: Gun turret rotation - servo or stepper motor?
« Reply #3 on: January 07, 2016, 10:45:45 PM »


I've seen many turrets mounted on 'standard' servos, they look very 'jittery' after a few years.
I would recommend either using much bigger servos than you would expect, bigger servos have bigger bearings or mounting the turrets on a
proper shaft with ball-bearings and then drive the turret through belts, gears, etc.

However, CNC controlled stepper motors do look really cool!  O0

https://youtu.be/cR-YlZ9NdIA

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dreadnought72

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Re: Gun turret rotation - servo or stepper motor?
« Reply #4 on: January 07, 2016, 11:27:50 PM »

I'm planning on using one of these
http://www.hobbyking.com/hobbyking/store/__26925__Stepper_Motor_and_Driver_Board_5V.html


Highly recommended. The stepper is geared down - that is, you can't drive the stepper by turning the output axle - a Good Thing for turrets. These are also super-cheap. I got five for about a tenner off the 'bay. Just make sure that the Arduino you're choosing has enough outputs for the steppers.


Andy

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C-3PO

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Re: Gun turret rotation - servo or stepper motor?
« Reply #5 on: January 08, 2016, 12:01:54 AM »

You could always use i2c motor controllers, you could then control loads of them using just 2 wires/pins from Arduino!
C-3PO




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derekwarner

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Re: Gun turret rotation - servo or stepper motor?
« Reply #6 on: January 08, 2016, 12:45:29 AM »

Missile launchers on real war ships move with surprisingly fast motion in both train and elevation motions...when we scale this down the speed of motion is literally snap..snap... <*< bang...to the point if you blinked you miss the movement .....having said this, there is ZERO jerkiness in any movement

Gun mounts on real war ships are a different kettle of fish....older vessels say WWII and 12"  mounts were very slow and graceful in motion....having these move with lightning fast speed would look rather out of place....more modern gun mounts...say 76 mm are much faster in all motions, however far slower that the missile launchers

Stepper motors [and the associated controller's] are designed to enable fast and repeatable positional accuracy. That said, this repeatable positional accuracy is only possible if the supporting structures are robust enough to cope with the mass being accelerated and de-accelerated to the stop position

Martin is eluding to the point of robustness which is the key for smooth operation :-))

I think the jury would need to consider the cost verse's the outcome with this topic.... :o ..... Derek
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Brian60

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Re: Gun turret rotation - servo or stepper motor?
« Reply #7 on: January 08, 2016, 02:40:11 AM »

Stepper motors are designed for that purpose maybe, but that is not to say they can't travel slowly. The xy gantry on my laser can be driven at 1mm per second or in imperial terms, in 1 minute it would travel a little over 2 1/4inches.
How slow do you want to go?

Norman Castle

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Re: Gun turret rotation - servo or stepper motor?
« Reply #8 on: January 08, 2016, 07:23:38 AM »

Hmmm.  Interesting.  In my case we're talking about a very light weight turret/gun mount and a slow rotation, by which I mean something like 10 per second.  I can get by with 180, and my first try at one actually works well enough considering with the turret mounted directly onto the servo.  It does tend to be a bit jittery/jerky though if I slow it right down.

I was just wondering whether a stepper motor might give me a smoother slow-speed rotation (assuming the turret mounting/bearing is good), and might also be cheaper than a decent standard servo.
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C-3PO

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Re: Gun turret rotation - servo or stepper motor?
« Reply #9 on: January 08, 2016, 08:49:09 AM »

Check out the spec of the stepper motor you intent to use. The Hobby King one mentioned by essex2visuvesi has 64 steps of 5.625 degrees.

So movement from one step to the next will be a swift movement of 5.625 degrees and therefore possibly jerky. I'll try it later and see what happens.

Some stepper controllers allow micro stepping. Some stepper motors have a high "step" number of say 200 and combined with 1/8 microstepping you can achieve 1600 steps per revolution, which equates to about 4 revolutions per minute free running without any programmed delays.

For anybody that gets into micro stepping - I can't remember the detail but you need to investigate limiting the current to the motor as this is a one factor in achieving smooth movement.

Fascinating
C-3PO

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Norman Castle

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Re: Gun turret rotation - servo or stepper motor?
« Reply #10 on: January 08, 2016, 09:03:27 AM »

D'oh ... I now realise what the limitation is i.e. the size of the step!

So how about the other alternative - a small geared dc motor?  Can I program a Uno to run a loop of various sequences of rotation direction, amount of travel and traverse speed in the same way that I can with a servo?
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C-3PO

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Re: Gun turret rotation - servo or stepper motor?
« Reply #11 on: January 08, 2016, 09:45:45 AM »

Hello Norman,

Yes you could use a small DC geared motor under Arduino control. To an extent that's what the servo is!.

A shot in the dark - experiment reducing the voltage to the servo - haven't given this much thought so probably end up with egg on my face but worth a shot!

If you want to control DC motors with the Arduino then best get an Arduino shield that will allow stepper(both types) and DC etc. Google Arduino Motor Shield.

Also try Googling Arduino Robot Supplies as you will find lots of gears/pulleys/geared motors/mounts etc on these sites - an example in general

https://www.coolcomponents.co.uk/robotics/motors and then have a look at this (DC motors only but cheap) https://www.coolcomponents.co.uk/drv8835-dual-motor-driver-shield-for-arduino.html

Also www.pololu.com do some cool bits of kit
C-3PO



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inertia

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Re: Gun turret rotation - servo or stepper motor?
« Reply #12 on: January 08, 2016, 10:03:28 AM »

I would suggest a small DC motor (stepper or otherwise) fitted with a worm gear and driving a large ring gear which is fitted to the bottom of the turret. That will slow down the speed of rotation considerably and should iron out any visible jittering or stepping. Control by Arduino, twin-relay switch, whatever. This has the ability to give you 360 movement and the speed can be altered by changing the supply voltage to the motor.
I think Technobots and Model Radio Workshop both do dedicated turret servo controllers. These will be restricted in rotation by the end stops inside the servo - unless you remove them (in which case good luck!).
DM
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Re: Gun turret rotation - servo or stepper motor?
« Reply #13 on: January 08, 2016, 10:07:19 AM »

Standard analogue servos e.g. those pulsed at about 50hz tend to get a bit 'grainy' or jittery when slowed down. Digital servos work much better, and I found Hitec servos, which tend to be pulsed at a higher rate than some other makes, are particularly smooth.

Finally if the turrets are going to be shuttling about a lot, then you might want to look at the Hobbyking 'Mi' range of servos. These use a magnetic encoder in place of a feedback pot, meaning there are no wipers or carbon tracks to wear out. This is generally the main reason for jittery positioning.

malcolmfrary

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Re: Gun turret rotation - servo or stepper motor?
« Reply #14 on: January 08, 2016, 10:11:14 AM »


A shot in the dark - experiment reducing the voltage to the servo - haven't given this much thought so probably end up with egg on my face but worth a shot!

C-3PO
You can't really do a lot with voltage to the servo - the internal electronics are designed around 5 volts plus or minus not much.  Digging inside the servo and inserting a resistor of about the same value as the motor resistance might do the job of slowing the motor and allowing it to start under load.


The idea of a worm and ring is pretty much what happened on model railway turntables - they rotated slowly and smoothly (thinking of the old Tri-ang one)
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C-3PO

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Re: Gun turret rotation - servo or stepper motor?
« Reply #15 on: January 08, 2016, 10:31:33 AM »

Cheapest solution by far Arduino 5 and a few bits of cable! Can also run an almost unlimited automated sequence of movements (varying degrees and speed), switched on and off by radio channel all  to make it look realistic. I'm sure close up the servo jitter is noticeable - 5 metres out of the boating pond is it an issue?

PS Struggled to type this post as my vision obscured by egg!
C-3PO
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Martin [Admin]

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Re: Gun turret rotation - servo or stepper motor?
« Reply #16 on: January 08, 2016, 10:51:21 AM »


OK, come on C-3P0, write up an article how it's actually done!



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john44

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Re: Gun turret rotation - servo or stepper motor?
« Reply #17 on: January 08, 2016, 01:36:49 PM »

With wiring diagrams to please C-3PO for dummies like me.


John
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Norman Castle

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Re: Gun turret rotation - servo or stepper motor?
« Reply #18 on: January 08, 2016, 01:48:39 PM »

I'm sure close up the servo jitter is noticeable - 5 metres out of the boating pond is it an issue?
 
Nope, but I'm one of those people who when something does what I wasn't expecting it to do (in this case, jitter), I want to know why it's doing it  :}
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C-3PO

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Re: Gun turret rotation - servo or stepper motor?
« Reply #19 on: January 08, 2016, 04:58:59 PM »

Wow,
I have just returned from "Gods own Junkyard" - could have sat in there for hours, shame I couldn't afford any of the lights - it's even better than the pictures on their website!

http://godsownjunkyard.co.uk/

I never knew how much I liked coloured lights/neon etc. ( better to google gods own junkyard and look at images...)

Norman - I understand the curiosity - good on you - I think the jitter is down to the way the varSpeedServo library works/ is being used.

I like a challenge - I think I can do away with the jitter on the 5 Arduino solution - get those eggs ready!

Hello Martin/John44 - yes happy to do diagrams etc but to do it justice and help save lots of questions I'll do it properly which will take some time to prepare so watch this space.

C-3PO





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dreadnought72

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Re: Gun turret rotation - servo or stepper motor?
« Reply #20 on: January 08, 2016, 06:59:18 PM »

So movement from one step to the next will be a swift movement of 5.625 degrees and therefore possibly jerky.


That model is geared down - it takes 4096 steps (in an 8-step sequence) to do 1 revolution at the output shaft. It's great (and smooth) at slow speeds, and <0.1 accuracy is surely enough for us?


Andy
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tsenecal

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Re: Gun turret rotation - servo or stepper motor?
« Reply #21 on: January 08, 2016, 07:00:25 PM »

having seen others build working turrets on their ships, the "elephant in the room" that nobody has brought up, is this:


in all the turrets i've seen, jerkiness was not caused by the speed of the servo, or the types of gears, or the use of stepper motor or DC motor.


it has to do with how the turret itself is mounted on the deck. don't just bolt it to the servo horn and expect it to be flawless.  support the turret.  you need to design a system that has as little friction as mechanically possible. it matters not if you use ball bearings, or bushings, or teflon.  just make sure you can rotate the turret by hand with no "sticking" or other issues, and whatever "motor" you use will be able to rotate the turret without issue.
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Re: Gun turret rotation - servo or stepper motor?
« Reply #22 on: January 08, 2016, 07:03:30 PM »

if you do go with a stepper motor, even if it is the cheap geared job, i use a "real" stepper driver that basically requires 2 pins from the arduino (and can even be used to power the arduino).  arduino sends a direction and a "step" command. adds cost, but reduces the amount of work needed in the actual arduino code.  https://www.sparkfun.com/products/12779 is the one i have used in the past
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C-3PO

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Re: Gun turret rotation - servo or stepper motor?
« Reply #23 on: January 09, 2016, 08:32:57 AM »

Supply line of eggs secured for an future information mishaps - sorry I got this one wrong - should have read further down the spec sheet and I should have realised it had a geared output as it is offset


dreadnought72 thank you for correcting my mistake re stepper motor - it does exactly as Andy states


C-3PO
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Norman Castle

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Re: Gun turret rotation - servo or stepper motor?
« Reply #24 on: January 09, 2016, 09:23:20 AM »

My brain hurts now.

So going back to square one, if the turret/platform/roundabout/whatever weighs no more than 50gm, 170 rotation is enough for this application and control is with an Arduino Uno, what's my best bet for slow, smooth traverse?  I don't have the facilities to make anything more sophisticated by way of a mounting than a brass tube pivot turning in a brass tube sleeve.

Picking up tsenecal's point about jitter being down to the mounting of the turret itself, in this particular case the jitter's still there if I just stick a 4" piece of balsa strip to the servo horn.  This is with a (genuine) HiTechS-311.

BTW, that micro metal 100:1 geared motor at 12.39 that C-3PO kindly linked to above from Cool Components looks remarkably like the one that's on Ebay and from Component Shop at well under a fiver ...
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