I am designing a control program for a dual axis solar tracking device for a final year project. I had planned to use stepper motors but have the following concerns:

I need to orientate the panel to a given compass bearing. Therefore 1º steps would be usefull but most motors I have seen only provide 0.9º steps.

Secondly I understand that stepper motors draw current continuously.Can power to the motor be cut without losing holding torque and position memory.

Maybe there is a better option?

 

The step size should not be a problem. You can always calculate the desired number of steps, even if the step size is not a whole number of degrees.

While steppers do need to be powered continuously to hold position, the needed holding current can be very low compared to running current. If you want to keep the heat and power consumption down, then design a two-level power driver that can be switched to a low-current mode when you are not moving. But don't cut the power completely.

Robert Scott
Real-Time Specialties
Embedded Systems Consulting

 

You probably don't want to drive your array directly but through a gearhead anyway, so the positioning resolution will be much finer than the motor resolution. Also, why drive to a numerical position? Why not put light sensors skewed a few degrees from the solar axis and drive toward the sensor with the stonger signal till the signals are equal? Then you will be pointed directly at the sun.

--
Steve Myres, PE
Automation Solutions
(480) 813-1145

 

If your gearboxes have a high gear ratio with enough internal friction, the devices may hold in place without any power on the motors. Many mechanisms do not "back-drive" because of this.

 

The positioning accuracy possible with steppers can be far greater than 1 degree if you use mimcrostepping in your design. Keep in mind, though, that you should size your stepper motor conservatively to reduce the risk of a stall. It may also make sense to include a feedback device for position verification.

If you are interested in reducing power draw you may want to consider completely power down your amplifier/drive to eliminate the power comnsumption required by any on-board logic deviced. Doing this would require that you also include an absolute feedback device (for power-on position detection) and a shaft brake on your motor (for power-off holding).

Options for "absolute" feedback devices can include a simple potentiometer, single pole-pair resolvers, and absolute encoders. The choice of device really depends on if you have gear reduction, the types of interfaces available in your control system, and your budget.

Another consideration to reduce power draw is to use either DC brushed motors or brushless servos. These motor technologies would eliminate the issue of your stepper system drawing a constant current. A feedback device would be necessary in either case for positioning, but there are very economic options on the market for feedback.

 

If you use gearing on the stepper and panel pivots, or toothed belting and different size pullys, you can achieve any step angle you wish on the driven end. If you use a worm gear on the driving shaft, it will therefore self-hold the other driven shaft, therefore no need for electrical force, only mechanical holding force.

 

Here I found a CPU with an interesting solution for tracking.

http://www.solartrack.it

 

Hi Steve,

Just ran accros your astute response. I am a R.O. manufacturer in San Diego/Mexico. I am looking at solar, just for fun, so to speak.

I would like to build a consentrator and related parts for my own facination, as well as possible marketing a unit. I might consider building up the hardware. Our reverse osmosis equipment utilizes A-B PLCs, so I have a general notion of PLC application. How does a PLC fit into this tracking question?

(Has anyone writen a program that would position according to mathmaticaly cqlculated position? Perhaps you have detected that I know little about elecronics.)

CARL ROCHTE
HYDRODYNE INTERNATIONAL

 

I might be interested in helping you get this tracker built. I might contribute some money and research time for components and component acquisition. I ran across this PC-based tracking system made at a university in the Phillipines. Interesting.

http://digital.ni.com/worldwide/singapore.nsf/web/all/52A3AE2820CDC81386256EC800346E5F

I asked this very question to Steve Myres about 15 minutes ago, when I posted a comment to his message, which was a response to your original question. I am a manufacturer of large industrial RO water systems, with an interest in getting involved with solar collectors/hi temp equipment.

Carl Rochte
Hydrodyne International