Need to replace DC gearmotor with a stepper one

[frazelle09]

We have finally decided to go more "electronic" with our parabolic solar hot water heater project and use some python scripts to determine the Sun's position and send the commands to a motor to move the parabola. We were using a small LED circuit before which sent the signals to the 1Amp, 12V gearmotor. However our LEDs were always getting confused by stray light from everywhere, so we've decided to try something else.

We were going to use a Raspberry Pi to record the temps anyway, so we thought that it wouldn't be too much more for the Pi to run the python programs and send the signals (H-bridge) to the motor. However we now understand that the python program is expecting a stepper motor but we understand that the one we are using is capable of generating some 40 ft-lbs of torque (which we would like to have since we would like to gang some 5 or 6 of the parabolas in a row) and we haven't been able to find a stepper gearmotor of this capacity.

Do they exist? This looks like the one we are using...

https://www.grainger.com/product/DA...m/rp/s/is/image/Grainger/2L003_AS01?$smthumb$

Here's a link to our project's site http://www.appropedia.org/Parabolic_Solar_Hot_Water_Heater

Have a great morning!

 

[Harald Kapp]

Why don't you change the program to operate the motor already in place instead of changing the hardware?
If the programm is wel written, all that should be required is changing the stepper motor driver subroutine to a (probably much simpler) subroutine driving the DC gearmotor (no stepping required).

 

[frazelle09]

"Why don't you change the program to operate the motor already in place instead of changing the hardware?
If the programm is wel written, all that should be required is changing the stepper motor driver subroutine to a (probably much simpler) subroutine driving the DC gearmotor (no stepping required)."

Wow, Harold! i had thought of that but no one could give me any feedback as to whether it would be possible or not. i've got a friend in Denmark who helped debug Jay Dosher's script (which is the one that combines the info from the IMU and the pysolar script and drives the motor) because he did it in Python2.x and i, not being a programmer, installed Python3.x on my Pi, causing all kinds of headaches. However we have finally debugged it. i'll ask him if he can help convert the stepper motor parts of the program to regular gearmotor ones. Boy, that would help a lot!

Thanks again for the great advice and have a wonderful afternoon!

 

[Minder]

The one problem I can see going from stepper to DC motor is the feedback that will be required, a stepper motor is an open loop servo/positioner.
A stepper, all you have to do is supply step/dir signals, and providing the load is within the capability of the stepper it will position to a predictable point with no feedback.
OTOH a DC servo motor requires a PID loop for precise positioning, you can control the position.
Although you mention you are using a 12v gear motor now, so how are you confirming the intended position is correct?
M.

 

[frazelle09]

Thank you, Minder. That was the thought we also had.

At the present we are (were) using a small LED electronic sun seeker board purchased from Ebay. Like we mentioned above, we never could get this thing to behave - it was always confused by stray light from everywhere which is the reason we'ver been working on the Pysolar and Jay Dosher's programs (+ the IMU and H-bridge). i imagine the LEDs provided the feedback and the circuit on the board made the comparisons until the values of the LEDs were equal.

Thanks again for posting and have a super afternoon!

 

[73's de Edd]

.



Sir frazelle09 . . . . . . . .





First of all . . . . for mere simplification of the problem . . . .

Did you utilize. light restricting, long flat black light tubes to channel in your sampled light source . . . .thereby eliminating your
"getting confused by stray light from everywhere."


Further refinement . . . . .
After viewing E-bay offering and its sensor design and additionally seeing the quite complete, supplementary u/p incorporation.

Initially try flat black heat shrink tubing on the LEDS that could be trimmed down in overall length as needed.

As is being depicted on the one far left unit.

If that doesn't work pull them and try:

Secondarily . . .thin, flat black, semicircular discs could be fabricated with one going on very bottom and the other spaced up the distance of LED diameters up and it being the top cover unit.
Pattern profiled as being depicted with the RED/GREEN outlining.

An added flat black baffle at the rear at the RED line seals off that end from any ambient light entry , optional thin separational baffles cemented in at the BLUE lines if needed.

The overall resultant effect is overall ambient light shielding from all but the select frontal semicircular slit.

73's de Edd

 

[frazelle09]

73's de Edd, thank you for posting! Yes, we tried what we thought was everything to reduce the stray ray problem.

Have a great afternoon!

___________________________________________________________________________________________

"The earth is one country and mankind its citizens."
Bahá'u'lláh
"La tierra es un sólo país y la humanidad sus ciudadanos."

 

[73's de Edd]

Recheck above . . . .as I might have been prepping that drawing and adding on, in the time interim.

 

[frazelle09]

That was a very beautiful description of the many things that we tried. The original seller of that item should include your update. It might work for others!

Have a great evening!

P.S. i located this http://www.robotshop.com/en/12v-17a-416oz-geared-bipolar-stepper-motor.html but it's only 416 oz./in. and i think i need a larger one for the 40 in./lbs. that we presently have -- almost there!

 

[73's de Edd]

OK then . . . . .how about a timer to power up the system only in incremental time slots.
The use of a photo cell to initially detect valid sun presence and then the timer only supplies power every 15 minute period, needing JUST AN INITIAL 15 second servo micro tracking correction, then no power present until the start of the next 15 minute period, wherein it corrects again .

Then a photo cell sensed shutdown until the next day, when it all starts again.

By that powered down procedure you can only have "stray" ambient light problems in 15 second time periods every 15 minutes


Of your opted drive trains, I still like the simple Grainger unit, pending its having adequate torque, as this unit only needs to make fast micro adjustments thru the day.

 

[hevans1944]

Infrared-transparent black caps over the photo sensors might help with the stray light problem. Most plastics are transparent to infrared radiation. Or just use a black plastic strip in front of all the sensors and mount the circuitry in a light-proof box, maybe covered with aluminum foil on the outside exposing only the infrared "window". Does this contraption mount on the parabolic reflectors and move with them? It should, but it appears to be mounted on a pole separate from the trough reflectors.

 

[frazelle09]

Darn, didn't see that last post, hevans1944! Sorry about that.

We had mounted the LEDs on the parabola itself until we began to get light interference problems - as well as the concentrated beams melting some of our mounting hardware - lol.

So we mounted the LEDs on the pole to which you are referring which moves with the parabola but never could get it out of the stray rays...

But, our original question...

"We were going to use a Raspberry Pi to record the temps anyway, so we thought that it wouldn't be too much more for the Pi to run the python programs and send the signals (H-bridge) to the motor. However we now understand that the python program is expecting a stepper motor but we understand that the one we are using is capable of generating some 40 ft-lbs of torque (which we would like to have since we would like to gang some 5 or 6 of the parabolas in a row) and we haven't been able to find a stepper gearmotor of this capacity."

was more about finding an equivalent stepper motor.

Thanks again for posting and have a great day!

 

[BobK]

You do not need a stepper with the equivalent torque, you would gear it down (just like the original motor was) to achieve the needed torque.

Bob

 

[hevans1944]

What?! Forty foot-pounds of torque to move a few dozen linear feet of solar reflectors?! Someone has not properly engineered this project. If the reflectors are mounted on ball-bearing saddle-block end supports, and properly counter-balanced, you should be able to rotate them with your finger.

Back in the last century, shortly after graduating with a brand-new BEE degree, I flew to Tuscon AZ to interview with Hughes Aircraft for an electrical engineering position. I had some time left over after the interview to do a little sight-seeing, so I drove out to the foothills of the mountains nearby and visited a model home that had a solar-heated water system installed in the backyard. The parabolic trough reflectors were installed with their long axis tilted in a vertical direction (the tilt being a function of the latitude of Tuscon, presumably) surrounding the coaxial water tubes. The inner tube was black and carried the water, while the outer tube was clear. There was either a dead-air space or a vacuum between the inner and outer tubes to minimize heat loss to the environment.

Anyway, perhaps twenty or forty of these reflectors, each about four feet long, were mounted with bearings to a common frame, and each reflector had a linkage to a single gear-motor driven arm that would rotate the mirrors to follow the sun from sun-up to sun-down. The tracking mechanism was a pair of photocells mounted on either side of an "I" beam shaped piece of aluminum. When this was pointed at the sun, each photocell received the same illumination. If it was pointed not at the sun, then one photocell received more illumination than the other photocell and the difference in output was used to drive the motor that rotated the mirror reflectors in a direction that would make them and the sensor mechanism point at the sun. I don't know what happened at night, or how they got the mirror array to reset to facing east in the morning, but it was working really well the day that I visited and producing plenty of hot water.

 

[frazelle09]

Thanks, guys. Back to the drawing board!

Have a great evening!

 

[hevans1944]

Here is an interesting page by a Do-It-Yourself solar-energy builder that might give you some inspiration.

 

[frazelle09]

hevans1944 / Thanks for the link. A couple of years ago i even purchased his set of instructions, but by then we had already started and were basically only interested in how to get the parabola to swing (the motor part). If you check out our Appropedia area you should probably easily see the similarities.

Thanks again for the link - we are now proceeding along two paths (why not make it even more complicated than it already is!): one, trying to make the LEDs work and two, continuing to try and put together the programs, IMU, Raspy B, H-Bridge (or replacement) and stepper motor.

Have a great day!

Edit: It appears that i'm so addled that i've got another post on the same subject...