hey guys I thought
I'd show you my
latest project this is a microstepping
a driver and motor here so what I've got
is an SL a 7:06 to chip this is an
entire stepper controller and it's wired
up
there's no circuitry on the breadboard
is just to keep my wires straight and
it's controlling this stepper motor over
here so as you can see it's going around
nice and slowly the goal of this project
is to have two stepper motors well they
don't have to be steppers but two motors
that are operating in synchronicity and
they're moving very slowly so ultimately
these will go into an old Wurlitzer
jukebox where there's two motors that
control the colors that come out the
front of the jukebox they turn physical
color wheels so the original motors are
just AC synchronous motors well not
really synchronous they're like clock
motors but the problem is that the color
wheel loads the clock motor down enough
so that they don't spin in synchronicity
anymore and then the colors become out
of sync on the front of the jukebox so
the improvement here is to replace those
Clough motors that's something that
actually is synchronous like this
stepper motor then the problem I ran
into I mean normally I just would just
drive this with my own transistor driver
circuit factors one that I built here
it's just a an AVR microcontroller with
some output transistors to drive the
phases of the stepper motor so the
problem with this is that the stepper
motor would sort of click around it was
not a very smooth operation at low speed
one of the problems is that this stepper
motor is I think seven and a half
degrees per step or whatever the course
common stepper size is and I chose this
motor because it's going to fit into an
assembly that's already in the tooth box
so another requirement of this project
is that it be easily inserted into the
jukebox with minimal modification of the
hardware there so the size of this motor
and the shaft length and all that stuff
were pretty much perfect so I really
want to use this particular motor anyway
so I've got the
they hooked up with the chip and as you
can see it is actually going around you
know pretty smoothly and it's got enough
torque it really doesn't need much
torque at all the existing clock motors
were just really really weak so any
loading it always was too much for them
I think I've got this thing pretty much
all figured out the remaining problem is
that this thing is noisy in and of
itself I mean I think you can hear that
from here on the camera but let me just
bring it close to the microphone it's a
real squeaker and so I I think I might
be able to fix that by adding just a
little bit of capacitance to these motor
windings to keep down some of the the
sharp transients and the ringing that's
probably causing some of that squeaking
noise anyway let me zoom in on the chip
and tell you a little bit about how that
works so here's a block diagram of the
chip
by the way I'll put a link in the
description to where I bought this since
they aren't particularly easy to find
the chip has h-bridge drivers for each
of the phases of the stepper motor this
is meant to drive a unipolar stepper
motor and the inputs to it are of what
it calls a clock which is really just
going to increment the step or increment
the micro step and if we have it
configured to do that and then it uses
some current sense resistors and it uses
a reference voltage so I'll get into the
details a little bit later but the idea
is that this wants to control the
current going through the motor windings
not the voltage so it really needs to
know it needs to have a current sense
resistor in series with the motor
winding and then it checks the voltage
across that sensor resistor and compares
it to the reference voltage so that it
knows when to cut off the the the motor
current so by changing the reference
voltage you can control how much current
you want to go through each phase the
the chip has a few other settings up
here m1 and m2 define how many micro
steps it's going to make so let me show
you what that looks like a typical
stepper I guess they're not going to
show half-stepping
a typical stepper would just have either
the current is positive zero or negative
for each phase and for a unipolar
stepper motor you're really just
switching which wire it's going through
so in a bipolar stepper motor that has
four wires what you can do is just swap
the phase or swap the the direction of
the current going through the coil in a
unit polar stepper motor you have at
least five wires usually six and
sometimes eight and what you do is you
just switch which ones are connected so
for the a phase that might take three
wires and I can choose to put current
through one wire no wires at all or
choose the other wire and the third wire
is a common so typically like I say we
with normal half stepping or full
stepping the current is either on zero
or off but with microstepping
the controller has the ability to
modulate the current going through each
phase so in it's sort of high resolution
mode it can really approximate a sine
wave which is really what we want
stepper motors would actually operate
perfectly well on ninety degrees out of
phase sine waves I think I might even
try this
I just just as an experiment if I had a
real sign what if generator then I could
get ninety degrees out of phase feed
those into the into the stepper motor
and see how quiet and smoothly it runs
then anyway I'm pretty happy with the
performance of this chip so I'll
probably stick with it if I can make it
acoustically just a little bit quieter
these higher speeds like this it sounds
quiet I mean it just has mechanical
noise out here that's the squealing from
the PWM on the motor windings there's
another interesting thing the PWM
frequency that that SLA chip puts out is
not really stated I think somewhere in
the datasheet it says there's a seven
microsecond off time or something so it
waits for seven microseconds turns on
the power and then waits for the current
to reach the reference voltage so
basically it turns on full power and
then as the current starts flowing
through the motor winding the voltage
across those sense resistors will rise
and when it reaches the reference
voltage then it cuts off and waits for
another seven microseconds I believe
that's how it works so I have it set up
with one ohm cents resistors these nice
ceramic guys here and so with a one volt
reference voltage and a one ohm
a sense resistor we'll get one amp
through the motor for I guess I think
that's the target
so one amp and that cuts off for seven
microseconds and then it builds up to an
amp again or something I've looked at
the waveform and it's actually very
difficult to see what's going on in
there and I've tried putting some small
capacitors on the output of the chip to
see if I could kind of smooth it out a
bit and that doesn't work very well at
all that confuses the the circuitry I
think it's anticipating an inductive
load and that's basically how the chip
was designed to work so maybe I'll do
another video when this thing is all
done and see how the jukebox looks from
the front okay
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