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'Stepper strategies--position control'
1999\09\16@211832 by Jon Petty

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Hi

I want to control a linear stepper motor to control the spring rate on a
diaphragm. The The position will change based on analog inputs , pressure and
flow rate. The system will learn the best postions at different analog
values. Most of my experience is with servos, so I've got a couple of
questions.

1. How do I keep track of the stepper position?

2. I want to program a default position on startup, is there a way to get the
same position each startup without the use of electronic position feedback?

3. I guess if you know the start position you can create  variable to count
steps, but what if an external event moves the position?

I know that GM uses an linear stepper for  idle air control. I know that it
has no position feedback and it can go to specific positions at startup. I
think it drives the motor to one position, either fully extended or retracted
and then counts from there. I don't know how it knows when it is fully
extended or retracted?

Any ideas?


Thanks

Jon

1999\09\17@055047 by Michael Rigby-Jones

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> Hi
>
> I want to control a linear stepper motor to control the spring rate on a
> diaphragm. The The position will change based on analog inputs , pressure
> and
> flow rate. The system will learn the best postions at different analog
> values. Most of my experience is with servos, so I've got a couple of
> questions.
>
> 1. How do I keep track of the stepper position?
>
By keeping a count of the steps you have sent it.

> 2. I want to program a default position on startup, is there a way to get
> the
> same position each startup without the use of electronic position
> feedback?
>
If the system has physical limits, the you can send the stepper enough steps
to ensure it reaches those limits and reset your count.

> 3. I guess if you know the start position you can create  variable to
> count
> steps, but what if an external event moves the position?
>
If that is a possibility, then a basic stepper system will fail.  You will
need some kind of feedback.  When positional feedback is added to a stepper
system, it's time to ask whether a servo system wouldn't be better and
cheaper.

Is it possible that the system could recalibrate itself periodically?  I
suspect not as this is will obviously upset whatever you are controling
during the recalibration.


> I know that GM uses an linear stepper for  idle air control. I know that
> it
> has no position feedback and it can go to specific positions at startup. I
> think it drives the motor to one position, either fully extended or
> retracted
> and then counts from there. I don't know how it knows when it is fully
> extended or retracted?
>
It dosen't, if the idle control get's sticky so that it won't return all the
way, the idle control will go to hell.  However, the stepper is moving a
good deal of the time so it dosen't get a chance to get sticky (hopefully).
When ever the ignition is switched on, the idle control is stepped over it's
full range and then back to it's reference point.  The ECU could also
recalibrate this any time that the engine isn't idling.

> Any ideas?
>
Linear servo motor?  Seriously, if you are expecting some kind of external
influence that can disturb the stepper position and you can't re calibrate
by moving to a reference position periodically, then some kind of positional
feedback is needed.

Regards

Mike Rigby-Jones

1999\09\17@060123 by Quentin

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Jon Petty wrote:

> 1. How do I keep track of the stepper position?
>
> 2. I want to program a default position on startup, is there a way to get the
> same position each startup without the use of electronic position feedback?
>
> 3. I guess if you know the start position you can create  variable to count
> steps, but what if an external event moves the position?
>
1 and 2: You will need a reference feedback like a sensor on the one end
of movement. On startup you move the stepper to make you sensor. Call
this Zero position. now move the stepper with the amount of pulses
needed to get your position. There is no other way of getting position
feedback just from a stepper.
3: Buy the correct rated stepper and trust it. If the stepper is
powerfull enough and you step it correctly (ramp up and brake) it will
hold it's position.

If it's realy that critical, then you just will have to use an encoder
which will take care of 1,2 and 3. With an encoder, you can use a small
electric motor controlled by PWM instead of a stepper, although it won't
hold position as good as a stepper.

Quentin

1999\09\17@132319 by Dave VanHorn

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> 1. How do I keep track of the stepper position?

Count steps, and hope it dosen't slip.

> 2. I want to program a default position on startup, is there a way to get
the
> same position each startup without the use of electronic position
feedback?

Apple did this in their apple II disk drives, you ram the assembly against a
stop a few times, then start stepping in the other direction.

> 3. I guess if you know the start position you can create  variable to
count
> steps, but what if an external event moves the position?

You're hosed.

> I know that GM uses an linear stepper for  idle air control. I know that
it
> has no position feedback and it can go to specific positions at startup. I
> think it drives the motor to one position, either fully extended or
retracted
> and then counts from there. I don't know how it knows when it is fully
> extended or retracted?

Think of a wall... :)

1999\09\20@085956 by Jon Petty

picon face
All the r/c type servos I have are rotational. Do they make a small linear
servo that is driven like a r/c servo?


JOn


In a message dated 9/17/99 11:43:07 AM US Mountain Standard Time,
spam_OUTmrjonesTakeThisOuTspamNORTELNETWORKS.COM writes:

<< Linear servo motor?  Seriously, if you are expecting some kind of external
influence that can disturb the stepper position and you can't re calibrate
by moving to a reference position periodically, then some kind of positional
feedback is needed.
 >>

1999\09\20@140300 by Andy Kunz

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At 06:29 PM 9/17/99 -0400, you wrote:
>All the r/c type servos I have are rotational. Do they make a small linear
>servo that is driven like a r/c servo?

They used to.  In fact, back when size didn't matter, they had both linear
and rotational outputs on the "standard" size servo.

But that was the 1970's.

If you are using it to control a servo, go with something like a sail winch
style head instead of the arm type.  Basically, you wind the wire around a
bobbin.  This will keep things linear for you.  If you go with the arm,
things aren't linear.

You can always try asking .....fredKILLspamspam@spam@fmadirect.com - he helped design the
encoder, decoder, and servo chips that made the original IC-based RC
systems.  He also stays pretty current on what's available.

You are probably going to be looking at "quarter scale" servos for torque
needed on a decent spring-loaded throttle.

Andy

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