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Every once in a while, I do some of the dumbest things
Last week ( mid July 2008) I was about to take my LX200 outside for the first
time since December 2007. I had brought it in to my work area ("my man cave" as
my wife calls it) so I could write and test the code for the advanced hand
controller. Before taking it out, I decided to take some speed measurements off
the Declination motor. I took the Dec. motor assembly off the telescope and
hooked it up to an external power supply. Just to see how fast the motor would
run, I ran the voltage up to 10 VDC. There was no speed change after 3 VDC. So I
put the assembly back in the telescope and took it outside to test the
prototypes. I discovered quickly that I had two
problems - declination was drawing way too much current and the scope would not
point for squat. Playing with the motor at slow speed it was pulsing like a
stepper motor. Looking at the display while moving the telescope I noticed that
when moving 90 degrees in Dec, the readout was correct in one direction but only
showed half the number of degrees in the opposite direction.
Back inside with the scope and off with the declination motor assembly. The
motor was stepping on the test power supply and drawing 1.5 amps. I removed the
gear and encoder disk from the motor shaft, removed and opened up the motor. It
has 3 field windings - one was shorted. Nice job Fansler. Now the story gets sad
and funny. I went to Radio Shack and found a motor with the same mounting hole
pattern rated at 3 VDC. The shaft was 1.87 mm rather than 2.0 mm - but what the
heck - I purchased it (under $4) and came home. After mounting the motor I
super glued the gear/encoder on the motor. Reassembled and spent HOURS
trying to get that mask to line up correctly with the encoder disk. It was a
pain, but I was successful. The Radio Shack motor drives the telescope - but it
is rated at a higher current. So I have a 5 ohm 10 w resistor in-line with the
motor to limit the current. Even so, I had to go to a 1.5amp fuse for the
time being. This works most of the time- unfortunately the current limiting
stalls the motor from time to time (I just reach up and turn the Dec knob
lightly and it takes off). I have two more 3 VDC motors on order from Jameco
that should be here soon - so I shall give them a try (they were about $2 each).
After the new 3VDC motors came in, I could not get any of them to work
without massive current being drawn - so I finally got out the oscilloscope and
made some measurements. Turns out that the motors are 12VDC.
Somewhere I got the impression they were 3VDC motors. Another order to
Jameco and a few more motors. After they came in I tested them for cut on
voltage and choose the one with the lowest. It is Jameco # 232005.
It is rated at12VDC, 0.6A and 9,000 rpm. It has the same mounting hole
pattern as the original motor. The holes were untapped, but screwing the
mounting screw takes care of that problem.
For my next LX200 project, I have decided to replace the drive motors on my
LX200 with something more substantial. According to measurements made by Doc G,
the one LX200 motor he tested has a torque of 1.55 oz-inch/Amp. Since each motor
seems to draw about ½ amp then the torque would be .78 oz-in or 5.5mNm (milliNewtonmeter)
in the metric system. The first decision I have made is to use an outboard power
supply and not use the L2724 motor driver. Brushless DC motors are the current
rage in the RC world, with higher torque and more efficient that brushed motors.
See
http://en.wikipedia.org/wiki/Brushless_DC_electric_motor for a
detailed explanation. Brushless motors come in two varieties - with Hall Effect
sensors and without sensors (sensorless). The RC world uses sensorless
motors - which cost any where from a few dollars to . . . well how much do you
want to spend. The sensors tell where the commutator is and therefore which
winding to turn on next. A sensored brushless motor would be ideal - except they
are expensive ~$190 each for the size needed for the LX200. Then there is the
driver - $250 - $425 per motor. So my approach is going to be a $24 sensorless
brushless motor with a home brew controller based on an
application note from MicroChip (http://ww1.microchip.com/downloads/en/AppNotes/00857a.pdf).
The motor will be here next week and I can start playing in my spare time.
My intention is to pick the drive signal from U16 (LM324) from the main board in
the telescope to drive my controller. The controller will then
drive the motor accordingly. I am considering adding optical feedback to the
motor to the driver - either using the current encoder, or another to help fake
the sensors. All under thought for now.
More as the scope turns.
July 28, 2008
More on new motors for the
LX200
It appears that brushless motors with sensors are out - they run $180-$200 for
the size needed. Before going to a brushless, sensorless motor, I looked
at a good brush motor. Maxon is a leader in that area. They have a
A-max 22 that would fit the bill at $65. Looking at doing more, I found
that they have encoders for the motor, as well as gear reductions available.
I wrote quick program in C that took the different gear ratios and
different encoder counts to give the final gears needed to drive the worm and
still have 5400 encoder pulses per revolution of the worm. Turns out that
the ideal combination uses the 4:1 gear reduction (actually 3.75:1) and either
the 128, 256 or 512 count encoder - along with a couple of new gears would give
the ideal desired result. The only catch is that the $65 motor with the
gear head and encoder is $295 dollar! The advantage of going that route
would be a quieter system with an encoder that would never get out of phase or
need trimming with the two pots on the encoder board. Oh well.
Still investigating,
August 6, 2008
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