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PICList Thread
'Optical Tachometer'
1998\03\04@222611 by wayne galaugher

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This my first request to the piclist

I have designed an optical tachometer using a stepper motor with a
slotted wheel through which one can look and stobe rotating objects by
adjusting the rpm of the motor.  I can count the pulses to the stepper
motor and measure the RPM with a 4 digit 8Hz frequency counter.  This is
all done with discrete IC's.  It works great! However, this project
screams for a PIC such as a 65F84. I'm sure this is an old question
answered many times but I'll ask again. I need to design a 4 digit 7
segment LED  or LCD display to read the stepper motor pulses at 8 Hz.
We are only measuring RPM to 2000 RPM so the frequency is low.  Some
sample code for a frequency counter in this range would be helpful.
Also, it would be nice to pulse the stepper motor chip with the PIC.

Regards
Wayne Galaugher
Vancouver, Canada

1998\03\05@055924 by Russell McMahon

picon face
This is off the top of my head. If you want more details or
it doesn't make sense email me. I'll post this reply to the
list as it may be of general interest.
There is a method which may be simpler than building a
frequency counter per se with a PIC. Briefly, don't measure
what the motor is doing - instead, tell the user  what YOU
are MAKING the motor do.

Driving a stepper motor directly from a PIC is easy. You
simply output each phase pattern in turn. For most steppers
(eg unipolar units with 2 centre tapped coils) you need 4
PIC pins and there are 4 phase sequences for full stepping
and 8 for half stepping. There are a number of circuits
around for driving bipolar steppers and unipolar steppers
are able to be driven with simple buffers directly by the
PIC.
Typical small 12 volt steppers draw around 150ma so you can
drive them with simple octal buffer ics like the ULN2803.

With 2 more pins you input up/down speed control directly to
the PIC - you vary the delay between phase changes to vary
speed. Now you don't have to MEASURE frequency of rotation
as you are generating the frequency yourself - you simply
have to convert the delay time to a frequency. This can be
by division (f = 1/t) or by lookup table. If you are using
the timer for phase change timing you will have ample time
for calculations between changes but this task could be the
main task with phase changes interrupt driven as the speed
will not vary rapidly compared to phase changes. (At 2000
RPM a 1.8 degree stepper has a new phase change every 150
microseconds). Display could be via multiplexed LEDS or any
of a number of LCD displays - there have been several
discussions on driving LCDs on the list in the recent past.

Total device is now:    PIC, driver IC, Stepper motor, LCD
display & a few buttons.
Jameco have suitable stepper motors for $5-$10 US.

   regards    Russell McMahon

{Original Message removed}

1998\03\09@032334 by TONY NIXON 54964

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>There is a method which may be simpler than building a
>frequency counter per se with a PIC. Briefly, don't measure
>what the motor is doing - instead, tell the user  what YOU
>are MAKING the motor do.

How does the controller know if the stepper is actually turning
at 'any' speed ;-)

Tony



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1998\03\09@144035 by Russell McMahon

picon face
Blind faith my son.

Actually, blind faith in engineers' specifications.
Stepper motors .have well defined conditions under which
they will "follow" the "move-along-now" signals given to
them. As long as the stepping signals stay within the speed
range specified for the applied load then you can
confidently expect the motor to revolve at the stepping
rate. In this application the load is (or should be) a well
balanced and quite light weight (hence low inertia) disk so
the loading is almost purely inertial. As long as the disk
is "light" (which is relative to the power of the motor
used) then it will track properly. Acceleration rates need
not be vast in this application and can, if necessary be
limited to ensure lock is not lost.

{Original Message removed}

1998\03\09@194716 by Steve Baldwin

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face
> Blind faith my son.
>
> Actually, blind faith in engineers' specifications.
> Stepper motors .have well defined conditions under which
> they will "follow" the "move-along-now" signals given to
> them. As long as the stepping signals stay within the speed
> range specified for the applied load then you can
> confidently expect the motor to revolve at the stepping
> rate. In this application the load is (or should be) a well
> balanced and quite light weight (hence low inertia) disk so
> the loading is almost purely inertial. As long as the disk
> is "light" (which is relative to the power of the motor
> used) then it will track properly.

If your load is light, you have to watch that your speed range doesn't
include a motor resonance.
Some motors have a very marked bite out of the speed/torque curve. If you
try and start it at that speed, it will go buzz instead of whirr and
nothing much will happen. Once started, the motor construction itself
usually has enough inertia to overcome it.

Steve.

======================================================
 Very funny Scotty.  Now beam down my clothes.
======================================================
Steve Baldwin                Electronic Product Design
TLA Microsystems Ltd         Microcontroller Specialists
PO Box 15-680                email: spam_OUTstevebTakeThisOuTspamkcbbs.gen.nz
New Lynn, Auckland           ph  +64 9 820-2221
New Zealand                  fax +64 9 820-1929
======================================================

1998\03\10@042428 by Josef Hanzal

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face
>If your load is light, you have to watch that your speed range doesn't
>include a motor resonance.
>Some motors have a very marked bite out of the speed/torque curve. If you
>try and start it at that speed, it will go buzz instead of whirr and
>nothing much will happen. Once started, the motor construction itself
>usually has enough inertia to overcome it.

This resonance effect is much less signifficant when using half step phase
drive sequence. But it needs twice more pulses to achieve the same RPM.

Josef

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