Satellite linear actuator control advice needed

Hi,

First, apologies if this has been answered before... I searched the forum but couldn't find a satisfactory answer...I did see emdee's heliostat project and still not sure...

So, I'm working on a single axis solar tracker using a satellite linear actuator with reed switch and built-in limit switches.

http://www.sadoun.com/Sat/Products/S/HARL-3618-TD-18-Moteck-Actuator.htm

The idea is to have an RTC hooked up and an algorithm/ lookup table to know solar position to control the linear actuator.

I'm trying to figure out the best/most robust option of control and have come through different opinions:

  1. Use the Official Motor Control Shield Rev3
  2. Use a simple TIP 12X H-bridge and connect the reed to an interrupt to have feedback.
  3. Use a combination of DPDT SPST relays and again connect the reed to interrupt.

I have read posts about the Motor Control Shield Rev3 saying that it easily overheats at high currents (close to the 2A max) and although I assume the "normal" current will be below 1.5A (see curve at the bottom of website link) most of the time, I'm concerned about it shutting down or getting damaged especially during the long morning west-to-east movement. (I'm not concerned about thermal shutdown in case of high winds as I would prefer the array to be stationary anyway)

In the other two cases I'm a little bit concerned about reliability in terms of properly handling startup currents, limit conditions, thermal protection...

In case I don't need sophisticated control (speed,PWM) but I do need to know actuator position what control would you suggest for this arrangement?

thanks in advance

Hi,

Since you don't need a lot of speed, think about running a lower voltage than 36V.

How accurate must the aiming be? If you're only going to change position like 25 times a day, relays would be OK..

Some how-to here: http://arduino-info.wikispaces.com/ArduinoPower

Simple and inexpensive relays like below probably will work, lots on ebay.

http://www.ebay.com/itm/Two-2-Channel-5V-Relay-Module-Expansion-Board-Arduino-ARM-PIC-AVR-MCU-DSP-/221009967173?pt=LH_DefaultDomain_0&hash=item3375387845

If you don't want to use relays then a mosfet h-bridge driver will run much cooler than motor driver based on bipolar technology such as the L298N or L293D. Something like http://www.pololu.com/catalog/product/1212 would do the job if you use PWM to ramp the motor current up slowly. Pololu also sell a range of higher power motor drivers.

Hi, many relays here also: http://goo.gl/8ZEQ8 DISCLAIMER: Mentioned stuff from my own shop...

Thanks for all your replies.

The project will serve as a student research platform so tracking will be varied between something like every minute to every hour so I guess the mosfet h-bridge will be the reliable answer.

Why would you want to "slowly ramp up the PWM" on the pololu driver? It should be able to handle 5A peak and the actuator current would be under 2A and most likely around 1A... would the startup current of the linear actuator be so much higher? And in that case how "slowly" could I go to 100% PWM?

quarentine: Why would you want to "slowly ramp up the PWM" on the pololu driver? It should be able to handle 5A peak and the actuator current would be under 2A and most likely around 1A... would the startup current of the linear actuator be so much higher? And in that case how "slowly" could I go to 100% PWM?

The data for that actuator you linked to does not specify the startup current. The graphs show current increasing to 3.6A as load increases to 3000N but does not show any figure beyond that. It could be that 3000N is the stall force and 3.6A is the stall current, or it could be that 3000N/3.6A is a thermal limitation.

If you can establish that the stall current is no more than 5A (perhaps by measuring its resistance and dividing that into the supply voltage), then you won't need to ramp up the PWM. However, if it is more than 5A then you should either use a higher current H-bridge or ramp up the PWM to limit the surge current when you power up the actuator. As for how quickly you can ramp up the PWM, that depends on how quickly the dish starts moving. Maybe you could go to 100% in half a second or so.

I haven't tried the below tle5206 h-bridge, but I would think it might be a good fit with the usual linear actuators, and the price is reasonable.

http://parts.digikey.com/1/parts/577889-ic-h-bridge-5a-dc-motor-to220-7-tle5206-2.html

OK That makes sense - I'll get the Pololu H-bridge, do some measurements and see if I need PWM for limiting the startup current.

Thanks for all your help. Greatly appreciated.

BTW Pololu have a number of higher current H-bridge motor drivers available as well. I think it would be best to measure the actuator resistance first, then decide which H-bridge is best.