# Need help with step motor info

#### xtal_01

Joined May 1, 2016
169
I am just starting to research a project ... steering a car with a step motor ... a fly by wire if you will.

My issue is getting a very high torque 12 volt stepping motor.

I understand how to use one ... I have put one on a project and used a 36 volt supply ... but not on something as low as 12 volts.

I looked at one motor ... a frame 23 ... it said 12volts in the advertisement (906 kg-cm at 2.8 amps). I can't use this exact one because it has permanent gearbox attached rated at only 240 kg-cm. But when reading it said it could be 24 or 36 volts and would perform better.

Is there just a curve attached to each motor that it will deliver so much torque at so many amps and so many volts ?

Can I use any step motor on 12 volts even if it says 36 volts?

Just a bit confused .... thanks

Joined Jul 18, 2013
28,692
A stepper motor should always be used at the plate rated current, What you will see is that some stepper drives use various voltages, that are higher than the motor rated value.
But what they do is maintain the rated motor Current as the motor inductive reactance increases with RPM.
So the bottom line is, ensure the stepper rated Current is maintained.

#### xtal_01

Joined May 1, 2016
169
A stepper motor should always be used at the plate rated current, What you will see is that some stepper drives use various voltages, that are higher than the motor rated value.
But what they do is maintain the rated motor Current as the motor inductive reactance increases with RPM.
So the bottom line is, ensure the stepper rated Current is maintained.
Thanks ...

I assumed the current is most important as a higher current would cause the windings to fail.

Can I assume that there is some kind of curve that a motor (say rated at 2.8 amps and 36 volts) would product "X" torque at 2.8 amps and 12 volts?

Assuming a linear curve (and I have no idea what the real curve is) ... 2.8 x 36 = 100 watt (.14 hp) and 2.8 x 12 = 33 watt (.045 hp) ???

Thanks!

#### LowQCab

Joined Nov 6, 2012
4,075
There are many Late-Model Cars that are being produced with Electric-Power-Steering,
it's becoming a popular upgrade for many Custom-Cars and Hot-Rods.

These Motors / Gear-Boxes are easy to convert to other Cars, and have plenty of Reserve-Power.

I think that going this route would be much more productive than trying to adapt a Stepper-Motor.
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#### xtal_01

Joined May 1, 2016
169
There are many Late-Model Cars that are being produced with Electric-Power-Steering,
it's becoming a popular upgrade for many Custom-Cars and Hot-Rods.

These Motors / Gear-Boxes are easy to convert to other Cars, and have plenty of Reserve-Power.

I think that going this route would be much more productive than trying to adapt a Stepper-Motor.
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You are correct and I did a little looking into this.

It might be best to just find an electric steering box and build some kind of controller.

I was trying to find a more universal solution ... a motor I can gear to (chain to ... belt to ...) an existing steering shaft.

For example, if this worked I would like to fit it to an older motor home I have.

It obviously has power steering to it doesn't need 100's of lbs of force but the steering is firm and would need a fair bit of force.

There is add on power steering kits to reduce the effort of a manual steering car ... no reason I would think you could not put it onto a car with power steering to reduce the steering effort ever further but I would still need to come up with a motor/controller so you could control the wheel with a small remote wheel.

Thanks!

#### LowQCab

Joined Nov 6, 2012
4,075
Sounds like a dangerous proposition.
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#### xtal_01

Joined May 1, 2016
169
Sounds like a dangerous proposition.
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There is a loooong back story.

My wife is a quadriplegic, she had a diving accident when she was 12. She has no use of her hands, only her arms.

We live in the country, no transit of any type ... especially not for a wheelchair.

We have a van equipped for her .. no drivers seat ... her electric wheelchair locks in. She drives with a small auxiliary wheel. Low effort mechanical system ... very complicated. We have had lots of issues ... lost steering a few times.

The State had a program ... if you have a job, they will pay for the modifications (we had to buy the van) ... I think it cost them $80K ... OUCH !!!!!!!!! I looked at an electrical system at a show. Done exactly as I said ... auxiliary motor geared to the steering shaft. You can even engage and disengage the motor manually. FYI ... it had lots of issues ... they tried to do everything with one controller (steering, brakes, gas, heater, lights, doors, shifting, ....... ). I would like to spit everything up into small individual units. The nice part is you don't actually modify the van ... all stock parts for the steering ... super reliable ! The problem is, once she retires or if the program ends, I can't afford$100K (the new going price for mechanical or electrical).

I don't want her to be stranded at home. Think of loosing all your independence .. not being able to go out yourself when and where you want.

So I figure I have a few good years to play and figure it all out.

I worked for the nuclear industry for years (industrial electrical work ... PLC's and such). I am use to doing things right .... I still do some small projects for them. Everything has a redundant system. The braking system in the van has an emergency backup ... small battery and actuator so if you pull the lever back all the way it engages (just in case the main system fails).

Just FYI ...I built an elevator out of the mast of a forklift. Again, lots of little safely items like a limiter so if a line brakes, it stops the flow of fluid to it does not drop.

Mike

#### MisterBill2

Joined Jan 23, 2018
18,538
Steering and braking certainly need at least redundant systems, possibly triple redundant so that there is a second backup. Driving without brakes is a challenge, , but possible in reasonable traffic. And one time the steering box failed, 4 turns of freeplay. Those 5 miles were tough and slow. So backups are important.

#### xtal_01

Joined May 1, 2016
169
Steering and braking certainly need at least redundant systems, possibly triple redundant so that there is a second backup. Driving without brakes is a challenge, , but possible in reasonable traffic. And one time the steering box failed, 4 turns of freeplay. Those 5 miles were tough and slow. So backups are important.
Her steering has not backup and it was a close call when it failed.

The only real backup is the braking.

The biggest problem with the steering was the parts all needed modifying to make it low effort.

I think the electrical steering will be way more reliable. I am looking at things like a stepping motor because not chance of brushes failing and such. Way over size everything.

One possible thing to do is to make it the same way we do safety PLC's. Two individual systems and a third to monitor the output of both and make sure they agree.

Nothing is perfect ... like I said, I have now had two systems ... have friends who have one ... and have seen two at a show. All had problems! That was not reassuring to me! The dealer demo we took for a drive kept having issues and would make you pull over and restart the system.

I am one of those "if it is not done right, don't do it" type of guys. I am building a new workshop ... at 64 ft long, my walls are out less than 1/8".

This is why I want to start playing now so I have a few years to work out all the bugs.

Mike

#### LowQCab

Joined Nov 6, 2012
4,075
When You take apart one of the new Electric-Power-Steering-Units,
I'm sure that You will appreciate the over-kill engineering that goes into them.

They're doing the same thing with Electric-Power-Brakes, also recommended.
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#### MisterBill2

Joined Jan 23, 2018
18,538
servo-motor power steering would be the way to go. A stepper system is much more complex, and for the same torque it uses more power. Servo systems with two motors in tandem, driven by separate drivers driven in unison, would be the safest approach. They would not even need to be linear servo systems, but simply provide fast and slow application speeds, provided that the control system response was adequate . Electrical steering has been used in some larger vehicles for a very long time.
Hydraulic-assist power braking would be a simpler to implement choice, and would not need to depend on engine operation for the power, nor even on battery power if an accumulator is provided as backup.

#### LowQCab

Joined Nov 6, 2012
4,075
I do like my Hydro-Boost-Brake-Booster, but
I would say that a Hydraulic-Accumulator will not last anywhere close to as long as a Starter-Battery.
Maybe ~3 applications ?, and then nothing,
as opposed to maybe ~20 minutes of Battery operation with no working Alternator,
and in which case control would tend to gradually "fade-out" over a much, much longer period of time.
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#### xtal_01

Joined May 1, 2016
169
servo-motor power steering would be the way to go. A stepper system is much more complex, and for the same torque it uses more power. Servo systems with two motors in tandem, driven by separate drivers driven in unison, would be the safest approach. They would not even need to be linear servo systems, but simply provide fast and slow application speeds, provided that the control system response was adequate . Electrical steering has been used in some larger vehicles for a very long time.
Hydraulic-assist power braking would be a simpler to implement choice, and would not need to depend on engine operation for the power, nor even on battery power if an accumulator is provided as backup.
I think we are on the same path. I have used servos but they were all DC with the positioning pot built in. I was really thinking of building a servo but with a stepping motor. This way there is no brushes to go bad. I still planned on some kind of pot for feedback (positioning) of the steering but I was going to put it into the steering system and not on the motor.

The input needs to be a pot not an encoder. I say this because you need to know where the steering wheel is in relation to the wheels. I drove in one van that used an encoder .... it literally went through a calibration (moving the van wheels right and left ... then putting the steering wheel where you want it and telling it "set".

Is there a reason I should not use a stepping (or some other kind of AC brushless motor) for a servo type application?

Thanks .... Mike

Joined Jul 18, 2013
28,692
I think we are on the same path. I have used servos but they were all DC with the positioning pot built in. I was really thinking of building a servo but with a stepping motor. This way there is no brushes to go bad. I still planned on some kind of pot for feedback (positioning) of the steering but I was going to put it into the steering system and not on the motor.
The input needs to be a pot not an encoder. I say this because you need to know where the steering wheel is in relation to the wheels. I drove in one van that used an encoder .... it literally went through a calibration (moving the van wheels right and left ... then putting the steering wheel where you want it and telling it "set".
Is there a reason I should not use a stepping (or some other kind of AC brushless motor) for a servo type application?
One advantage of a stepper is it does not require any feedback or PID loop control as would a DC/BLDC servo with encoder FB.
There is no concern with brush wear etc, if a BLDC servo is used, however.

#### xtal_01

Joined May 1, 2016
169
One advantage of a stepper is it does not require any feedback or PID loop control as would a DC/BLDC servo with encoder FB.
There is no concern with brush wear etc, if a BLDC servo is used, however.
I think I would need feedback for positioning ...otherwise you would not know where the wheels are when you start. You would need to go through a calibration each time (unless it remembers the count) ... hitting a limit switch at each end of the travel and recording the counts between (divide by two for the center ... straight).

Would you consider feedback via a pot or ??? more reliable than keeping track of counts or ?

Thanks ....

Joined Jul 18, 2013
28,692
In servo positioning, an incremental or similar encoder is used, a pot is not quite so accurate and durable, as a quadrature encoder, but even encoders require a power source in order to keep record of position, unless the unit is ran to a zero ref point at power up.
How much torque is demanded for the motor?

#### xtal_01

Joined May 1, 2016
169
In servo positioning, an incremental or similar encoder is used, a pot is not quite so accurate and durable, as a quadrature encoder, but even encoders require a power source in order to keep record of position, unless the unit is ran to a zero ref point at power up.
How much torque is demanded for the motor?
And maybe that is why on the two electric versions I looked at (one at a show and the other we test drove) they "calibrated" the system every time you started driving. As I said, the wheels would move till they hit the ends of travel ...then you put the small separate steering wheel where you thought "straight" should be and hit set.

I understood what was going on with the system but thought it odd to do this each time you drove (even if power was left on the system ... probably just a safety measure).

I was just thinking positive feedback from a pot or ??? would save doing this each time.

But as you said, definitely not as accurate and then you have a contact item (pot) to wear out.

And this is why I love bouncing ideas of others!

Mike

Joined Jul 18, 2013
28,692
You can also set up a calibrating point at power up etc, anywhere on the point of travel, the operating system would then record this position and know its position from here on.

#### xtal_01

Joined May 1, 2016
169
You can also set up a calibrating point at power up etc, anywhere on the point of travel, the operating system would then record this position and know its position from here on.
Much better idea!!!

You can imagine trying to turn a cars tires while standing still each time you drive.

The travel (number of pulses) would not change ... just need to know where you are.

Sooooo many little things to think about.

This is why I wanted to start this project early.

I can even test it on an off road vehicle first (we live in the middle of nowhere ... every day two or three farm tractors drive by).

Thanks so much for all the input !!!!!!!

Mike

#### MisterBill2

Joined Jan 23, 2018
18,538
There are also "absolute" encoders that output an exact position so that the actual position is known at power-up. It is the incremental encoders that only supply pulse trains. The limitation of absolute encoders is resolution. How many bits
Actually, I was thinking of an open loop steering system where the driver is the feedback link. Like in a regular car. You turn the wheel and see where you go. MUCH SIMPLER and much less to go wrong. And no position feedback except thru the driver.