Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

I'm having trouble findind a USB linear actuator reverser

  • Please log in to reply
6 replies to this topic

#1 Coconuts

Coconuts

    Viking 1

  • *****
  • topic starter
  • Posts: 605
  • Joined: 23 Sep 2012

Posted 20 October 2020 - 05:30 PM

This is easy manually; it just requires a cross-wired DPDT ON-OFF-ON switch.  With the switch center poles hooked to +12 VDC, one switch "ON" position sends the DC brush motor powered linear actuator to one end of travel, and the other switch "ON" position sends it the other way.  My linear actuators have built-in limit switches that stop motion at each extreme.  But I would like to automate this function, which lowers a south facing flap in my observatory to allow low southern declination objects to be imaged.  I may also need one to advance soft tube closures when the OTAs are in the park position, to keep out spiders and geckos.  I had assumed that finding a USB controlled DC motor reverser would be easy, but so far, I am striking out.  I thought that Pegasus Astro products might support USB polarity reversal, but they don't currently.  They are working on something that does, but that is a ways off.  Despite my actuators having built-in limit switches, these aren't brought out, so the ability to monitor two pairs of limit switch inputs would be nice.  ASCOM compatibility would be spectacular, but that could be asking a bit much.

 

Any suggestions?  Thanks in advance.

 

All the best,

 

Kevin



#2 kathyastro

kathyastro

    Vanguard

  • *****
  • Posts: 2,408
  • Joined: 23 Dec 2016
  • Loc: Nova Scotia

Posted 20 October 2020 - 06:09 PM

There are plenty of USB relay devices.  If you can find one with a DPDT relay, you are good to go.  But even if you can't, a simple USB-operated SPST relay can operate a bigger DPDT relay to run the motor.


  • Stevegeo likes this

#3 rkinnett

rkinnett

    Viking 1

  • *****
  • Posts: 589
  • Joined: 08 Aug 2018
  • Loc: La Crescenta, CA

Posted 20 October 2020 - 07:29 PM

Hi Kevin, I envy your gecko problem!

 

You could use high-current normal-closed SPST switches as your limit switches to interrupt one control polarity but not the other, like this:

 

dc_polarity_control_with_limit_switches.png

 

You can drive the relay with anything that can supply enough current, including a USB-powered/controlled SPST relay as Kathy suggested, or a USB-powered DC motor driver.

 

You could instead get a 2-channel USB relay board on amazon for about $10, but you would need to work out a way to prevent closing both relays at the same time.  It's too bad there isn't a ready-made USB-powered/controlled DPDT relay (as far as I can find).

 

If you want something more intelligent, there are off-the-shelf dome controllers you might find interesting, or if you rather go DIY then look into open source Arduino dome control projects.  Quick google search shows multiple options.

 

Also consider driving your motor with an H-bridge DC motor driver circuit rather than relays.

 

 



#4 Coconuts

Coconuts

    Viking 1

  • *****
  • topic starter
  • Posts: 605
  • Joined: 23 Sep 2012

Posted 21 October 2020 - 06:22 AM

Thanks, both.  I am still surprised that a USB brush motor reverser isn't a thing, but I certainly haven't found one yet.  I don't really care if it is relay or H bridge based, but don't want to go down the Arduino path.  I'm not familiar with dome controllers, but if one was intended to operate a brush DC motor, that might be another option.  An ASCOM Pegasus Astro USB power controller driving a DPDT relay would work.  I already have a PA Ultimate Powerbox v2, but that is up above the mount.  Their Pocket Powerbox looks appropriate; one switched +12 VDC output could activate an external DPDT relay, and a second switched +12 VDC output could apply or withdraw power from the two center lugs of the relay.  But none of their products have any digital inputs to confirm full open or closed status.

 

All the best,

 

Kevin



#5 kathyastro

kathyastro

    Vanguard

  • *****
  • Posts: 2,408
  • Joined: 23 Dec 2016
  • Loc: Nova Scotia

Posted 21 October 2020 - 07:48 AM

I operate my dome with an Arduino.  I use two relays to power the shutter: a DPST relay that switches the power on and off, and a DPDT relay that reverses the polarity.  This arrangement prevents open and closed polarity from being applied at the same time.  The Arduino also handles dome rotation.  However, if you are only interested in shutter operation, a dumb USB relay box would handle it.

 

I built a shutter sequencer based on relays and diodes to operate the motors in the correct sequence and to accept inputs from the limit switches.  The sequencer rides in the dome.  The interface from the rotating dome to the stationary controller consists of three wires: two for motor power, and one returning that indicates completion of the current operation. The limit switches ensure proper sequencing of the doors in either direction, and disable motor power when the open or close completes.

 

Here is the schematic, if it is any help to you.  If the upper main conductor is positive with respect to the lower one, the shutters open.  If the lower one is positive with respect to the upper, the doors close.  The completion signal changes polarity depending on the direction of movement, so another relay (which must have a non-polarized coil) in the main control box provides the actual completion signal to the Arduino.

 

WD-6-ShutterController.png


Edited by kathyastro, 21 October 2020 - 07:49 AM.

  • rkinnett likes this

#6 rkinnett

rkinnett

    Viking 1

  • *****
  • Posts: 589
  • Joined: 08 Aug 2018
  • Loc: La Crescenta, CA

Posted 21 October 2020 - 02:24 PM

Not sure if this was clear from my earlier post, but the idea was that you can implement limit switches without digitally observing the limit switches.  The normal-closed SPST switches in my sketch disconnects the motor from one polarity branch when the shutter reaches that switch, still allowing the motor to be driven away from the limit switch via the other polarity branch.  That's a common scheme for home and basic industrial automation, given the benefit of not relying on digital logic to stop motion.  Changing those limit switches to SPDT type will open some easy ways to provide digital state indication (for awareness sake) and/or drive a bright red contact light.  I can sketch that out if useful.  

 

Note that with the DPDT relay approach you would need a means to control it via USB (likely via a USB relay module) to set direction, but you would also need yet another relay to turn power on and off (stop/go), so 3 relays in total.

 

Instead, it turns out you can use 2 separate SPDT relays (see "Second Solution):  https://www.instruct...With-Two-Relay/

 

Contrary to my earlier statement, there is no risk of shorting your motor power supply in any switch states.

 

Here's a 2-channel USB relay controller, rated for 10A, for $10:  https://www.amazon.c...r/dp/B07CFQMDJ3

 

Consider this (plus inline limit switches configured to directly interrupt power) to be your cheapest, simplest option.

 

You will need flyback diodes around your motor.

https://electronics....rectional-motor

 

----

 

Instead of relays, you could use a generic DC brushed motor driver:  https://www.pololu.c...tor-controllers

 

See this thread about connecting limit switches directly to the simple motor control board, allowing control away from the limit switches.

https://forum.pololu...ontrollers/3296

 

----

 

For an off-the-shelf solution, take a look at Astrometric's DomePro2 and similar.

https://www.astromet...ome-controller/

 

----

Check out this thread and search for other ASCOM roof control options, if you haven't already:

https://www.cloudyni...sgp-economical/

 

----

I hear your regarding avoiding Arduino, but it's worth noting there are open source projects out there that make this quite easy.  That's the cheapest approach toward ASCOM integration.  That's what I would do.



#7 Coconuts

Coconuts

    Viking 1

  • *****
  • topic starter
  • Posts: 605
  • Joined: 23 Sep 2012

Posted 25 October 2020 - 06:43 AM

Kathy and Ryan:  Sorry for the delay in getting back; crazy schedule at work this past week.  Thanks a lot for the valuable advice.  That "no bad states" two relay solution is great.  My actuator has built-in limit switches and auto-cuts off power by itself, although those sensors aren't brought out for interrogation.  But easy enough to do that separately.

 

All the best,

 

Kevin




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics