Stepper motor circuit

[zalzon]

Hi,
i have a pic microcontroller putting out signals to 4 pins. I
need to turn a unipolar stepper motor (6 wires). Can someone suggest
a circuit to do this?

I don't want to use an allegro or whatever else controller chip. I'd
like to build the circuit myself. I hope its not too hard?

Would I need to build an H-bridge circuit and what wires from the
motor go where? Specifically I would like to know if the center tap
needs to be used or should it just be wired up as a bipolar stepper
motor would.

I've been fiddling around with npn and pnp transistors on a
breadboard. So far I have only succeeded in exhausting myself without
getting the motor to turn.

Some suggestions or diagrams if u please.


a very merry christmas & a happy nu year.

 

[koool~]

Hi,
i have a pic microcontroller putting out signals to 4 pins. I
need to turn a unipolar stepper motor (6 wires). Can someone suggest
a circuit to do this?

<Snip>

Hi Zalzon,

This topic and its variants have been previously discussed here at
great length. You really should do the research before asking the
group for information so easily found at Google.

A Google search of "microcontroller "stepper motor"" turns up over
6300 hits. On the first page of links, this one appears:

www.imagesco.com/articles/picstepper/01.html

(NOTE: no personal affiliation with the site)

There is information here on interfacing the controller to the motor,
and some sample code. I hope this gets you going, Zalzon.

M/C & H/N to you as well.

koool~

 

[Spehro Pefhany]

Hi,
i have a pic microcontroller putting out signals to 4 pins. I
need to turn a unipolar stepper motor (6 wires). Can someone suggest
a circuit to do this?

I don't want to use an allegro or whatever else controller chip. I'd
like to build the circuit myself. I hope its not too hard?

Would I need to build an H-bridge circuit and what wires from the
motor go where? Specifically I would like to know if the center tap
needs to be used or should it just be wired up as a bipolar stepper
motor would.

It's *easier* to use the center tap. An H-bridge is more complex, and
generally has higher losses, but is required to drive a bipolar motor.
If you try to drive a unipolar as a bipolar (which I don't recommend)
you'll have to reduce the coil current from nominal.

I've been fiddling around with npn and pnp transistors on a
breadboard. So far I have only succeeded in exhausting myself without
getting the motor to turn.

Some suggestions or diagrams if u please.

Google for them and you'll find LOTS of them. You just need 4 NPN
transistors (or, better, MOSFETs or darlingtons) to drive the ends of
the two coils. Then sequence them one way or the other (4 states) to
get the motor to run one way or the other.

Unless your required performance is very low, you'll probably want to
implement a chopper drive to control the current through the coils to
get fast response without burning up the motor coils at slow speeds.
If your application does not require high static torque you can also
reduce the coil current from the maximum shortly after the motor has
stopped. This will help it run cooler when it's just sitting there.

a very merry christmas & a happy nu year.

Best regards,
Spehro Pefhany

 

[John Popelish]

Hi,
i have a pic microcontroller putting out signals to 4 pins. I
need to turn a unipolar stepper motor (6 wires). Can someone suggest
a circuit to do this?

I don't want to use an allegro or whatever else controller chip. I'd
like to build the circuit myself. I hope its not too hard?

Would I need to build an H-bridge circuit and what wires from the
motor go where? Specifically I would like to know if the center tap
needs to be used or should it just be wired up as a bipolar stepper
motor would.

I've been fiddling around with npn and pnp transistors on a
breadboard. So far I have only succeeded in exhausting myself without
getting the motor to turn.

Some suggestions or diagrams if u please.

a very merry christmas & a happy nu year.

There are quite a few different ways to drive stepper motors, some
simple and some quite complicated, though all of them can be built of
simple components no more complicated than transistors and
comparators. The point is that motors (and anything that makes use
the force of magnetic fields produced by current in coils of wire)
needs an increasing voltage to make it go faster. This is a result of
the basic inductor formula that relates voltage applied to the rate of
change of current through an inductor: V=L*(di/dt).

So, the simplest circuits (4 power switches and 4 catch diodes to a
zener voltage clamp) that use the resistance of the coils to limit the
current are the slowest, since they stabilize the coil currents via
the L/R time constant, with L and R being the properties of the motor
coils themselves. This simple approach can be sped up a bit by adding
more resistance external to the motor to lower this time constant
(L/(Rmotor+Rexternal). Of course, this requires more supply voltage
to get the same current, and this extra voltage goes back to the first
point in explaining how it can make the motor go a bit faster.

If this is not fast enough, you will have to either use even bigger
resistors and supply voltage, or change to a switching regulator
approach that applies the full supply voltage to the motor coils till
the current reaches the desired value (based on the motor L/R time
constant) and then short circuit the winding so the current circulates
while running down. By the time you build in the ability to perform
this current regulation trick and also get the current to go either
way through the two windings, you have reconstructed the H bridge
chips that Allegro makes.

 

[PlucknGro]

Thanks all. That was one of my problems. I was trying to use an
H-bridge to drive a unipolar stepper motor! yikes.

I now use 4 NPN transistors. When I switched it on, the shaft gently
rocked back and forth. I could not understand why this was happening.
It should be turning from step to step not be stuck rocking back and
forth on the first step. This is a low current stepper motor by the
way. 0.17A, 5.3V DC

I found if i gave it a slight nudge it would move forward and then
stalled again. I surmised that the rotor did not have enough inertia
to get to the next step. I fiddled with the time delay between each
pulse from the PIC port. Finally it managed to turn. But if it faces
even a little resistance, its stalls into rocking back and forth.

The motor does not seem to have sufficient inertia when moving from
step to step. Is there any way of overcoming this? Right now it has
enough torque to maybe turn a CD and not drive a small robot car which
is what I was hoping to use it for.

I'm using this pulse sequence : 0001, 0010, 0100, 1000

 

[A.M.]

Does it seem to 'chatter' when it runs? It may be that you have the
windings mixed up. Try systematically varying the connections to
determine the correct order.

You might also try half-stepping...

1100 0100 0110 0010 0011 0001 1001 1000


PlucknGro wrote:

 

[John Popelish]

No, a UNIPOLAR Stepper, (two pairs of three leads on each of the
center-tapped motor windings), only needs four sinking transistors
connected to your quadratured ON signals from the PIC.

An H-bridge is ONLY for BIPOLAR Steppers (or changing the motor
direction of plain DC brush-motors), the steppers the with only
four wires, two for each coil, and not two sets of three.

But if you have a six wire motor (two center tapped coils, you can
certainly drive it from an H bridge by using either half of each of
the center tapped coils, or by using the whole coils. One gives more
speed per volt, and one gives more torque per amp. The only real
problem for dual H bridge operation occurs in motors with the two
center taps tied together, internally (5 wire motors).

 

[John Popelish]

Thanks all. That was one of my problems. I was trying to use an
H-bridge to drive a unipolar stepper motor! yikes.

I now use 4 NPN transistors. When I switched it on, the shaft gently
rocked back and forth. I could not understand why this was happening.
It should be turning from step to step not be stuck rocking back and
forth on the first step. This is a low current stepper motor by the
way. 0.17A, 5.3V DC

I found if i gave it a slight nudge it would move forward and then
stalled again. I surmised that the rotor did not have enough inertia
to get to the next step. I fiddled with the time delay between each
pulse from the PIC port. Finally it managed to turn. But if it faces
even a little resistance, its stalls into rocking back and forth.

The motor does not seem to have sufficient inertia when moving from
step to step. Is there any way of overcoming this? Right now it has
enough torque to maybe turn a CD and not drive a small robot car which
is what I was hoping to use it for.

I'm using this pulse sequence : 0001, 0010, 0100, 1000

That is not a valid pattern for a two phase motor. The full step
pattern (assuming the first two bits are the drivers for the two
halves of one coil) would be:

0101 1001 1010 0110

One half of each winding on at all times, with each pair toggling
alternately. There is also a half step mode that takes 8 steps to
complete a cycle that alternated between having one coil powered and
two coils powered.

0001 0101 0100 0110 0010 1010 1000 1001

The full step mode has higher peak torque, assuming all coil currents
are the same, but it is easy to add series resistors to the center
taps to make the torque smooth for the half step mode. This consists
of a common resistor to the supply that feeds a pair of resistors that
connect to the two center taps. With the right choice of resistors,
the break away torque with one coil holding (and sharing the common
resistor with no one) can be made equal to the break away torque when
two coils are on, but sharing the common resistor.

I Found the right two currents for one motor by making a lever out of
steel wire ( a coat hanger) that was wound into a tight fitting coil
at one end to be pushed on to the motor shaft. I hung a small weight
on the lever that was near the break away torque at normal motor
current for on coil half. Using a current limited supply, I slowly
lowered the current till the motor would not hold the weight with the
lever sticking out sideways and noted what the minimum hold current
was. Then I energized two coils in series and repeated the
experiment. This gave me the two coil current that had the same
holding torque as a larger single coil current. With the resistors
adjusted to produce these two currents, the motor torque was smooth
more drop put resistant than when having the same coil current,
whether one or two coils were in operation.

 

[Don Bruder]

Thanks all. That was one of my problems. I was trying to use an
H-bridge to drive a unipolar stepper motor! yikes.

I now use 4 NPN transistors. When I switched it on, the shaft gently
rocked back and forth. I could not understand why this was happening.
It should be turning from step to step not be stuck rocking back and
forth on the first step. This is a low current stepper motor by the
way. 0.17A, 5.3V DC

I found if i gave it a slight nudge it would move forward and then
stalled again. I surmised that the rotor did not have enough inertia
to get to the next step. I fiddled with the time delay between each
pulse from the PIC port. Finally it managed to turn. But if it faces
even a little resistance, its stalls into rocking back and forth.

The motor does not seem to have sufficient inertia when moving from
step to step. Is there any way of overcoming this? Right now it has
enough torque to maybe turn a CD and not drive a small robot car which
is what I was hoping to use it for.

I'm using this pulse sequence : 0001, 0010, 0100, 1000

Not clear on what you mean by using it as a pulse sequence...

Are you saying you're broadside-loading a decoder that puts a 1 on the
output corresponding to the value you load?

Or have you got something else in mind, and I'm just confused?

FWIW:
When I did this little tinker a few months ago, I used a hex counter IC
with parallel outputs and a clock source as the heart of the beast. I
set things up with some logic gates to make a "mod 4" function on the
counter's outputs, and set things up so the counter just kept ticking
over as long as there was power. I decoded the output from the mod 4 via
some more logic gates to give me four output pins, 0, 1, 2, and 3. At
any given moment, the output pin corresponding to the current value of
the counter mod 4 was high, while the other 3 outputs were low. From
there, it was just a matter of tying into the four transistors to switch
them on and off to make the motor run.

I had a 0Hz-25Khz variable clock source handy, and found that my motor
tended to go into fibrillation at around 16KHz, give or take a little.
Slower was fine, and would turn steadily, with or without a load, but up
in the neighborhood of 13-14KHz, it started losing torque in a big way,
and if I kept turning up the frequency, eventually just sat there and
buzzed as the critical frequency (somewhere above 16KHz, but below
20KHz) was reached. Memory is foggy, and I don't have my notes in front
of me to figure out exactly why/how, but I seem to recall figuring out
that running the clock at about 7KHz was "optimal" for what I was trying
to do - That would turn the motor fast enough to be useful, and do it
under the load I was wanting it to work for, without making much noise
or "letting the smoke out".

 

[Don Bruder]

Thanks all. That was one of my problems. I was trying to use an
H-bridge to drive a unipolar stepper motor! yikes.

I now use 4 NPN transistors. When I switched it on, the shaft gently
rocked back and forth. I could not understand why this was happening.
It should be turning from step to step not be stuck rocking back and
forth on the first step. This is a low current stepper motor by the
way. 0.17A, 5.3V DC

I found if i gave it a slight nudge it would move forward and then
stalled again. I surmised that the rotor did not have enough inertia
to get to the next step. I fiddled with the time delay between each
pulse from the PIC port. Finally it managed to turn. But if it faces
even a little resistance, its stalls into rocking back and forth.

The motor does not seem to have sufficient inertia when moving from
step to step. Is there any way of overcoming this? Right now it has
enough torque to maybe turn a CD and not drive a small robot car which
is what I was hoping to use it for.

I'm using this pulse sequence : 0001, 0010, 0100, 1000

Not clear on what you mean by using it as a pulse sequence...

Are you saying you're broadside-loading a decoder that puts a 1 on the
output corresponding to the value you load?

Or have you got something else in mind, and I'm just confused?

FWIW:
When I did pretty much this same little tinker a few months ago, I used
a hex counter IC with parallel outputs and a clock source as the heart
of the beast. I set things up with some logic gates to make a "mod 4"
function on the counter's outputs, and set things up so the counter just
kept ticking over as long as there was power. I decoded the output from
the mod 4 via some more logic gates to give me four output pins, 0, 1,
2, and 3. At any given moment, the output pin corresponding to the
current value of the counter mod 4 was high, while the other 3 outputs
were low. From there, it was just a matter of tying into the four
transistors to switch them on and off to make the motor run.

I had a 0Hz-25Khz variable clock source handy, and found that my motor
tended to go into fibrillation at around 16KHz, give or take a little.
Slower was fine, and would turn steadily, with or without a load, but up
in the neighborhood of 13-14KHz, it started losing torque in a big way,
and if I kept turning up the frequency, eventually just sat there and
buzzed as the critical frequency (somewhere above 16KHz, but below
20KHz) was reached. Memory is foggy, and I don't have my notes in front
of me to figure out exactly why/how, but I seem to recall figuring out
that running the clock at about 7KHz was "optimal" for what I was trying
to do - That would turn the motor fast enough to be useful, and do it
under the load I was wanting it to work for, without making much noise
or "letting the smoke out".

 

[zalzon]

That is not a valid pattern for a two phase motor. The full step
pattern (assuming the first two bits are the drivers for the two
halves of one coil) would be:

0101 1001 1010 0110

alternately. There is also a half step mode that takes 8 steps to

0001 0101 0100 0110 0010 1010 1000 1001

Right on John. The above two sequences worked though I don't quite
have a good understanding why the sequence is such.

With the right choice of resistors,
the break away torque with one coil holding (and sharing the common
resistor with no one) can be made equal to the break away torque when
two coils are on, but sharing the common resistor.

Its way over my head John I'm was just hoping to get the thing
turning. It does 'stick' a little as you say. I would think that the
current to the leads has to vary with the load in order to prevent
this sticking. I guess that's what you were trying to describe to me.

Thanks all.

 

[zalzon]

Not clear on what you mean by using it as a pulse sequence...

Are you saying you're broadside-loading a decoder that puts a 1 on the
output corresponding to the value you load?

Or have you got something else in mind, and I'm just confused?

Definately confused

Well I'm using a microcontroller, not digital IC/logic chips which I
presume would be hell to successfully get working with stepper motors.

 

[Bill Garber]

: On Mon, 29 Dec 2003 03:39:57 GMT, Don Bruder <[email protected]>
wrote:
:
: >> I'm using this pulse sequence : 0001, 0010, 0100, 1000
: >
: >Not clear on what you mean by using it as a pulse sequence...
: >
: >Are you saying you're broadside-loading a decoder that puts a
1 on the
: >output corresponding to the value you load?
: >
: >Or have you got something else in mind, and I'm just confused?
:
: Definately confused
:
: Well I'm using a microcontroller, not digital IC/logic chips
which I
: presume would be hell to successfully get working with stepper
motors.

Dumb question here, but why not simply use a stepper
motor controller IC?

Bill @ GarberStreet Enterprizez };-)
Web Site - http://garberstreet.netfirms.com
Email - [email protected]
Remove - SPAM and X to contact me

 

[Don Bruder]

Definately confused

Well I'm using a microcontroller, not digital IC/logic chips which I
presume would be hell to successfully get working with stepper motors.

Actually, it was a piece of cake for me. Hardest part was trying to
locate the right output transistors in the scrap-pile that serves as my
"parts store".

A bi-directional 4-bit synchronous counter (a ripple counter would
undoubtedly have worked just as well, but I had a synchronous version
sitting near the top of the junkbox, and stopped looking when I found
that one) with parallel outputs was wired so that it just sat there
counting from 0 to 16 (or 16 to 0, dpending on rotation direction
desired) and rolling to start. I used a handful of ANDs and NANDs to do
a "mod 4" (0 mod 4 = 0, 1 mod 4 = 1, 4 mod 4 = 0, 5 mod 4 = 1, etc)
operation to give me 4 sequences of 0, 1, 2, 3 (or 3, 2, 1, 0) per full
count. I then fed that output into another group of ANDs and NANDs whose
final output was on four independent wires I called (originally
enough...) Phase 0 through Phase 3. When the counter mod four was 0,
Phase 0 went high, and the rest went low, when the counter mod 4 was 1,
Phase 1 went high, and the rest went low, and so on. Each of the phases
controlled one of four transistors, which were in turn wired to the
phases of the motor as on/off switches. Ignoring the transistors and the
clock source, I think it was a total of 6 chips.

Simple as pie!

 

[Don Bruder]

: On Mon, 29 Dec 2003 03:39:57 GMT, Don Bruder <[email protected]>
wrote:
:
: >> I'm using this pulse sequence : 0001, 0010, 0100, 1000
: >
: >Not clear on what you mean by using it as a pulse sequence...
: >
: >Are you saying you're broadside-loading a decoder that puts a
1 on the
: >output corresponding to the value you load?
: >
: >Or have you got something else in mind, and I'm just confused?
:
: Definately confused
:
: Well I'm using a microcontroller, not digital IC/logic chips
which I
: presume would be hell to successfully get working with stepper
motors.

Dumb question here, but why not simply use a stepper
motor controller IC?

In my case, it was because a stepper-motor controller IC would have cost
money, and this was a "Can we do it out of the junkbox with a budget of
exactly US $0.00?" project. Besides... It was fun figuring out how to
make it happen

Seems I recall Zalzon mentioning that he was wanting (for reasons either
unspecified by him, or forgotten by me) to do it "from scratch" for some
reason.

 

[John Popelish]

Right on John. The above two sequences worked though I don't quite
have a good understanding why the sequence is such.

Think of the coils as hands lined up passing a bucket full of water
along the line. The first pattern has two hands carrying the bucket
at all times. As one hand lets go on one side, a new hand grabs on on
the other side, shifting the bucket from between the first two hands,
to between the second and third. Etc. All stable positions consist of
the bucket hanging between a pair of hands.

The next pattern has just one hand holding the bucket, then the next
hand in line grabs on, and the bucket shifts from under the first
hand, to between these two hands. Then the first hand lets go, and
the bucket hangs under the second hand. Then a third hand grabs on
and the bucket hangs between the second and third hand, etc. So the
bucket alternated between hanging on a single hand, to hanging between
a pair of hands.

Fairly obviously a positions where two hands support the bucket
(resist torque that would drag the rotor out of lock) is stronger than
a position held by just one hand. So the second pattern generally has
strong and weak positions.

Its way over my head John I'm was just hoping to get the thing
turning. It does 'stick' a little as you say. I would think that the
current to the leads has to vary with the load in order to prevent
this sticking. I guess that's what you were trying to describe to me.

Using the above analogy, I was just trying to make the single hand
positions stronger, and weaken the two handed positions, so that the
strength of the torque was constant at any step position. As long as
you have more torque than you need at all positions, and are not
concerned with two acceleration rates at alternate half steps, it
makes little difference. Just trying ot get your mind inside the
mechanism.