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Old Feb 15, 2013, 03:14 AM
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Hi David,

I ordered some steppers with driver board, does that still need some sensor to check it's position or is that sensor incorporated in the driver board?

The code you wrote, would that work on an Arduino board?

Greetings Josse
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Old Feb 15, 2013, 04:31 AM
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Hi Josse,
The aircon louver motors and driver boards don't have any indexing but will have quite high torque which will greatly reduce the chance of slipping steps. I suspect the original application probably had a clutched drive or possibly limit switches.

If you have enough torque you may decide to drop the sensor. The worst that would happen is something like a twig jamming the rotation briefly then coming free but leaving you with an offset on that drive. On a direct drive system you would force the drive back to center with the controls also centered. The motor will simply slip. With a highly geared system this may be difficult or even destructive.

The code is really pseudo code and would need to be adapted to suit the port and bit addressing of the micro chosen. With the compiler I use something like PortB=3 means the first two bits (1 + 2) of Port B are set (high) to drive the logic FETs or bipolar devices. You are simply pushing numbers to an output register, waiting a short time them pushing more numbers out to sequence the coils.

Cheers,
David
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Old Feb 15, 2013, 05:08 AM
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Hi David,

I thought this is the stepper motor I ordered (might be in today!!): http://www.goodluckbuy.com/dc-5v-4-p...rd-module.html
Would this be likely to need some kind of sensor?
And if the answer is yes, could I use the pot meter from a servo?
Got a few of does somewhere around here which have been dismantled so it would not be a problem to use they're pot meter.

Greetings Josse
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Old Feb 15, 2013, 10:48 AM
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Josse

The stepper motor you linked to is very small and has a large step angle. Listed step angle is 5.625 degrees per step or 64 steps per rev.
The motors I plan to use are nema 11 with 200 steps per rev at 1.8 degrees per step.
The Z drives we are building have a 1 to 1 ratio steering input to drive rotation.

Yes you will need to have a home switch on the drive and a was to trigger it on the input shaft so when you have the drive pushing astern that the switch would be tripped.
Then when you center you stick the drive will automagicly rotate to the home position.

Dave
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Old Feb 15, 2013, 01:08 PM
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Hi,
The link shows the same type of motor I've used for stepper projects. Please note they have an integral gearbox so the motor's step angle of 5.6 degrees is divided down by 64 making extremely fine movement. They need around 2020 steps for a full rotation.
Check this out also. http://arduino-info.wikispaces.com/SmallSteppers
Ideally it would require a sensor to give it an index position. Perhaps a Hall effect sensor and small magnet?

I must admit I would be prepared to give it a go without a sensor relying on the torque to ensure that it kept in step but you would have to accept that it might need occasional alignment following any obstruction. I guess it would be similar to a twin rudder boat having the coupler to one rudder slip out of alignment - confusing rather than devastating.
You would also have to ensure the motors were in a neutral position when powering off so that next time you start the code can presume the motors are in the home position.

I'm not really familiar with your mechanical requirements and have never used this type of propulsion so it's quite possible I'm missing some of the finer points here.

Cheers,
David
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Old Feb 15, 2013, 01:16 PM
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Hi David,

Thanks for the link, that's just what I needed!
If you think the sensor is not really needed I think I won't use them.
If I find, at a later point, that the sensor is needed to keep it straight at all times I can always build it in later on.
A model with a system as complex as I'm building should always have as easy excess to it's internals as possible so when something fails it can easily be fixed.

Greetings Josse
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Old Feb 15, 2013, 01:23 PM
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Hi guys...

Call it just me (ex QA type) - I'd definitely build in a sensor to give a verifiable 'zero position' (aligned with the hull axis) to the system. Yes, this adds complexity, but if you plan for it from scratch, it is always easier to do than engineering it back in later on all fronts.
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Old Feb 15, 2013, 01:27 PM
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Hi Craig,

You certainly have a point there, I already have trouble building stuff into my boat, to such a degree that I decided to equip my next boat with all it's electronics and fit all parts into the hull and test that before placing the deck.
This is also due to my new strategy of completing something before placing it on the model itself.

Greetings Josse
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Old Feb 15, 2013, 05:12 PM
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Hi,
Serviceability is important given the complexity of both electronics and mechanics so there is no way I would "deck" something in to place if I couldn't replace it following a failure.

I did use one of the little steppers for a turntable for reversing model locos. I had a switch to select either a slow, automated 180 reversal or a manual adjust using fwd/rev switches. The latter allows manual rotation through any angle (e.g. a siding line) but it also provides for manual indexing should the system get out of alignment. So far it's been well behaved but a key difference is that I only go forward and reverse by half a turn and this side of a serious jam it can repeatedly align itself. The software always knows where it should be pointing and it's only the mechanical stall that will mess it up.

Cheers,
David
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Old Feb 15, 2013, 05:39 PM
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Hi,
This is the same type of one turn (GWS) servo I mentioned before-
http://www.pololu.com/catalog/product/522

While it provides a full 360 degree movement on a standard servo pulse width range (and up to 3 turns on full range control) it is not the same as a free, continuous 360 rotation. I'm sure it may do it but the wiper contacts of the feedback potentiometer will be riding over rivets at some point and would wear out.
For the price and ease of use this would do most (but not all) of what the fully blown stepper would do. Think of it as a +/-180 degree rudder centered on straight ahead. Off the shelf, fully compatible, drop-in fit but lacking the cool factor of the drive in the video.

Cheers,
David
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Old Feb 15, 2013, 10:35 PM
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I think hall effect sensors might be a sensible approach here. Perhaps a system like that used in IC engines, where we have both a camshaft position sensor and a crank shaft position sensor, one to indicate the cam lobe positions relative to the cylinder that is current at Top Dead Center.

Except in our case we could use two toothed wheels (cnc cut from thin guage steel? or maybe even standard metal gears for which we know the number of teeth?) on the shaft of our stepper motor, one for counting the number of teeth in a revolution from the home position, with as many teeth cut into it as we like depending on the resolution in degrees that we want.

The other being a plain disc with a single tooth to indicate the home position to the controller. That way all we have to do is count the number of teeth in either direction from the home position, particulary since we will know the number of teeth from home to 90 degrees, to 120 degrees and so forth.... the controller will be able to tell when the motor hasn't reached, or has overshot the position in degrees from home that we want, and adjust accordingly...

If the drives aren't in the home position already, have a section of code before the main loop, that has the controller rotate the steppers until it finds HOME, then begin the main loop.

EDIT:
No mechanical wear and requires two hall effect sensors per stepper.


good article on the usage of a hall effect sensor from camshaft position. When reading it, replace camshaft with stepper motor....

http://people.physics.anu.edu.au/~am...or_trigger.htm
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Old Feb 16, 2013, 01:44 AM
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What is a Hall effect sensor?
Would it have to be in contact with the moving part or can it measure from a distance?
If the latter it the case it would make it much more easy.

Greetings Josse
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Old Feb 16, 2013, 01:55 AM
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Jargon Watch: Hall Effect Sensor & Transducer

Quote:
Originally Posted by Wikipedia
A Hall effect sensor is a transducer that varies its output voltage in response to a magnetic field. Hall effect sensors are used for proximity switching, positioning, speed detection, and current sensing applications.
Quote:
Originally Posted by Wikipedia
A transducer is a device that converts one form of energy to another.
It can measure at a distance as it senses a magnetic field generated, usually, by a small, powerful magnet, on the armature or rotating surface. Sometimes the magnets are inserted into holes in a rotating shaft.


from wikipedia
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Old Feb 16, 2013, 02:01 AM
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Ah, that sounds very useful.
With a Hall sensor only minor modifications would be needed thus making things easier and my accuracy determines how well it works.
There will also be no fine tuning, no trying to find out what value means what angle as all I need to do is make sure that I place a magnetised part at the correct angles.
I think I'll design a plate for this which I will have photo etched with some other parts, just to make sure it really is accurate enough.

Greetings Josse
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Old Feb 16, 2013, 03:45 AM
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Quote:
Originally Posted by craig_c View Post
It can measure at a distance as it senses a magnetic field generated, usually, by a small, powerful magnet, on the armature or rotating surface. Sometimes the magnets are inserted into holes in a rotating shaft.


from wikipedia
It can also sense the difference in a magnetic field naturally made by a tooth metal disk without requiring magnets as in the car engine applications. A Hall effect sensor can pick up the difference in magnetic fields that different quantities of metal can produce, sensitive enough to pick up the difference between a tooth, and the gap between teeth.
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