Issues with a stepper motor drive

[chetanthegreat]

Hello folks,

I am facing some issues regarding a stepper motor drive. The stepper
motor I have is a 20KG 12V DC L/R Full step driven one. I need it to
drive a Lead screw which eventually builds up pressures of the likes
of 400Kg cm in a cylinder.
I have attached a gear box of ratio 12:1 to the stepper and tried
boosting the torque. With the direct coupling the pressure used to
reach around 100-150 Kg cm but even with the gearbox which I presume
would boost the torque by a factor of 12 the pressure fails to rise
above 200.
And the stepper slips if I try to run it at high RPMs (15RPM ) even
without any load.
What can I do to find a solution to these 2 problems? What could be
going wrong in this whole exercise?
Thanking you in anticipation.
Regards,
Chetan.

 

[[email protected]]

Hello folks,

I am facing some issues regarding a stepper motor drive. The stepper
motor I have is a 20KG 12V DC L/R Full step driven one. I need it to
drive a Lead screw which eventually builds up pressures of the likes
of 400Kg cm in a cylinder.
I have attached a gear box of ratio 12:1 to the stepper and tried
boosting the torque. With the direct coupling the pressure used to
reach around 100-150 Kg cm but even with the gearbox which I presume
would boost the torque by a factor of 12 the pressure fails to rise
above 200.
And the stepper slips if I try to run it at high RPMs (15RPM ) even
without any load.
What can I do to find a solution to these 2 problems? �What could be
going wrong in this whole exercise?
Thanking you in anticipation.
Regards,
Chetan.

you dont say what you are driving the stepper with. its not so easy to
get performance from a stepper.

 

[Nico Coesel]

Hello folks,

I am facing some issues regarding a stepper motor drive. The stepper
motor I have is a 20KG 12V DC L/R Full step driven one. I need it to
drive a Lead screw which eventually builds up pressures of the likes
of 400Kg cm in a cylinder.
I have attached a gear box of ratio 12:1 to the stepper and tried
boosting the torque. With the direct coupling the pressure used to
reach around 100-150 Kg cm but even with the gearbox which I presume
would boost the torque by a factor of 12 the pressure fails to rise
above 200.
And the stepper slips if I try to run it at high RPMs (15RPM ) even
without any load.
What can I do to find a solution to these 2 problems? What could be
going wrong in this whole exercise?

Using a stepper is wrong in the first place. You'll need a proper
servo controller and a servo motor. The next thing you need to do is
determine the maximum amount of torque required to achieve your goals.
However, I guess the required torque depends highly on the amount of
friction in the lead screw so it may be difficult to calculate the
amount of torque precisely. You may want to get rid of the lead screw
and opt for a system with a more constant friction.

 

[[email protected]]

Hello folks,

I am facing some issues regarding a stepper motor drive. The stepper
motor I have is a 20KG 12V DC L/R Full step driven one. I need it to
drive a Lead screw which eventually builds up pressures of the likes
of 400Kg cm in a cylinder.
I have attached a gear box of ratio 12:1 to the stepper and tried
boosting the torque. With the direct coupling the pressure used to
reach around 100-150 Kg cm but even with the gearbox which I presume
would boost the torque by a factor of 12 the pressure fails to rise
above 200.
And the stepper slips if I try to run it at high RPMs (15RPM ) even
without any load.
What can I do to find a solution to these 2 problems?  What could be
going wrong in this whole exercise?

The problem with the gear box could be as simple as friction losses
thriugh the gear train.

The problem with the stepper motor seem to be that you haven read the
application notes carefully enough.

A stepper motor is just a synchronous AC motor. The torque you get out
of it is proportional to the AC current that you drive through motor
coils,and the faster the motor rotates, the harder it is to drive
current through the motor coils.

You have to allow for the back emf of the motor - this can be found
from the manufacturers data sheet, though some of them assume you know
that the back emf 0f a motor - in volts per radian per second - is the
same as the torque constant (when expressed in newton metres per
ampere) and you also have to remember that the AC impedance of the
coil is the vector sum (root means square sum) of the coil resitance
and the coil inducance. The inductive impedance increases in direct
proportion to the stepping rate.

This means that you have to apply a lot more than twelve volts to a
twelve volt motor to get its rated torque when it is spinning fast,and
the drive circuit has to be designed in such a way that the motor
won't burn out if you happen to stall it.

See

http://www.cs.uiowa.edu/~jones/step/

Nico Coesel is not entirely right to tell you not to use a stepper
motor for your job - the right stepper motor with the right drive
could do it - but a brushless DC motor (which is just a stepper motor
with a built-in stepper drive) is a lot easier to use.

 

[Benj]

Hello folks,

I am facing some issues regarding a stepper motor drive. The stepper
motor I have is a 20KG 12V DC L/R Full step driven one. I need it to
drive a Lead screw which eventually builds up pressures of the likes
of 400Kg cm in a cylinder.
I have attached a gear box of ratio 12:1 to the stepper and tried
boosting the torque. With the direct coupling the pressure used to
reach around 100-150 Kg cm but even with the gearbox which I presume
would boost the torque by a factor of 12 the pressure fails to rise
above 200.
And the stepper slips if I try to run it at high RPMs (15RPM ) even
without any load.
What can I do to find a solution to these 2 problems? What could be
going wrong in this whole exercise?
Thanking you in anticipation.
Regards,
Chetan.

Everyone has given you great advice. Steppers are difficult to drive
fast and tend to lose torque when you do. One simple way to improve
high speed performance is to use a much higher voltage power supply
and drop the current with a resistor. Lots of lost power but faster
performance. Generally for what you are doing, you don't want a
stepper at all! Get an nice strong DC motor and use some servo scheme
to monitor the shaft position. Usually it's some kind of digital
encoder. You can buy these as total systems. The beauty of this is
the DC motor can have huge torque and the encoder does not "slip".
Downside is complexity and expense. A stepper can be simpler and
cheaper but you really have to over-rate the torque spec to make it
reliable. I had to increase the motor size on a device I built that
only moved a drill X-Y with leadscrews and didn't build up any
pressure at all! It also used high speed, high voltage drivers so you
could get some speed out of the thing.

 

[Tim Wescott]

-- snip --

Nico Coesel is not entirely right to tell you not to use a stepper
motor for your job - the right stepper motor with the right drive
could do it - but a brushless DC motor (which is just a stepper motor
with a built-in stepper drive) is a lot easier to use.

Be careful with your terminology, though. Many brushless motors don't
come with a built-in drive, even ones marketed as "brushless DC".

I wouldn't characterize a brushless DC motor as 'just a stepper with
built-in drive' -- there are physical differences in the way they're
built that makes each one better for certain tasks. When you get down
to the bottom, though, a brushless motor and a stepper are both
synchronous AC machines.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

 

[Tim Wescott]

Hello folks,

I am facing some issues regarding a stepper motor drive. The stepper
motor I have is a 20KG 12V DC L/R Full step driven one. I need it to
drive a Lead screw which eventually builds up pressures of the likes
of 400Kg cm in a cylinder.
I have attached a gear box of ratio 12:1 to the stepper and tried
boosting the torque. With the direct coupling the pressure used to
reach around 100-150 Kg cm but even with the gearbox which I presume
would boost the torque by a factor of 12 the pressure fails to rise
above 200.
And the stepper slips if I try to run it at high RPMs (15RPM ) even
without any load.
What can I do to find a solution to these 2 problems? What could be
going wrong in this whole exercise?
Thanking you in anticipation.
Regards,
Chetan.

You've been given a lot of good advice. Something that has only been
alluded to, however, is that you need to figure out what torque is
necessary on your lead screw to develop the pressure you need, then
figure out what gear box to use to turn your available torque into the
torque you need.

Gear boxes (and lead screws) have gear ratios (or lead/turn ratios), and
ideal gear boxes will, indeed, multiply torque by the gear ratio while
an ideal lead screw will transform a torque into a force in a
predictable way (force = 2 * pi * torque / pitch, if I'm not mistaken,
but check).

But gear boxes and lead screws also exhibit friction. Lead screws (and
worm gears) have _lots_ of friction. You need to check not only the
gear ratio or lead, but the efficiency of the gear box and lead screw --
basically, an 80% efficient gear box will deliver your input torque,
times your gear ratio, times 0.8.

So check the efficiency of the lead screw, or better yet get an
indicating torque wrench and _measure_ the torque it takes to develop
the pressure you want. Then use a gearbox with a known efficiency and
enough of a ratio to develop the torque you need with the amount of
torque that you can trust to get out of whatever motor you use, whether
you stick to your stepper or go to another sort of motor (a brushed DC
motor is much lower tech, by the way, and may be easier to implement if
you're doing the drive electronics).

Good luck. If you succeed, you'll come out of this smarter than you
were going in.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

 

[chetanthegreat]

Thanks for the help people!
Thing is I am kind of stuck with the stepper I bought for this
application. And for the first few rotations I don't need precise
movement or much torque but the last revolution is highly demanding as
far as torque and precision is concerned.
Today, I tried hanging weight on the cross bars handle attached to the
lead screw and found out that at around 20-25 Kg weight the lever
moves in order to develop the ultimate 400 Kg/cm pressure in the
gauge. As the distance from the centre was 16 cm, the calculated
torque needed would come around 20 / 25 x 16 Kg cm. Thus I am hoping
that a 60 Kg Stepper with a 12:1 gearbox will be suffice to drive the
lead screw in.
Please comment if you could point out a flaw in my logic.
Regards.
Chetan.

 

[Nico Coesel]

Thanks for the help people!
Thing is I am kind of stuck with the stepper I bought for this
application. And for the first few rotations I don't need precise
movement or much torque but the last revolution is highly demanding as
far as torque and precision is concerned.
Today, I tried hanging weight on the cross bars handle attached to the
lead screw and found out that at around 20-25 Kg weight the lever
moves in order to develop the ultimate 400 Kg/cm pressure in the
gauge. As the distance from the centre was 16 cm, the calculated
torque needed would come around 20 / 25 x 16 Kg cm. Thus I am hoping
that a 60 Kg Stepper with a 12:1 gearbox will be suffice to drive the
lead screw in.
Please comment if you could point out a flaw in my logic.
Regards.

Perhaps you could use a counterweight which is equal to half the
maximum pressure you need on the rod (assuming the cylinder is
standing). This way the stepper engine needs to deal with half the
load.

 

[[email protected]]

Thanks for the help people!
Thing is I am kind of stuck with the stepper I bought for this
application. And for the first few rotations I don't need precise
movement or much torque but the last revolution is highly demanding as
far as torque and precision is concerned.
Today, I tried hanging weight on the cross bars handle attached to the
lead screw and found out that at around 20-25 Kg weight the lever
moves in order to develop the ultimate 400 Kg/cm pressure in the
gauge. As the distance from the centre was 16 cm, the calculated
torque needed would come around 20 / 25 x 16 Kg cm. �Thus I am hoping
that a 60 Kg Stepper with a 12:1 gearbox will be suffice to drive the
lead screw in.
Please comment if you could point out a flaw in my logic.
Regards.
Chetan.

First you need to fing the torque needed at the motor shaft at the
speed you need. That means you'll need to measure it or try guessing.
Stepper performance depends on the drive, you seem to keep ignoring
that, so you need a motor/drive pakage, ie. you have to buy one or
guess. Then look at the torque/speed curve published for said package
to see if it exceeds your minimum requirement. Repeat untill you have
the best match.