Linear Actuators 101

Quixote_1 · Sep 3, 2008

BACKGROUND:
I'm building an automatic door entry system that will use keyless entry and incorporate a door-strike, two electric deadbolts and a linear actuator to open and close the door. The system will run on a marine deep-cycle battery so that I will still be able to get in if there is a power failure and will communicate wirelessly with my computer to keep track of who is arriving and departing so that it can arm my alarm automatically when there are no occupants in my flat.

QUESTIONS:
If anyone here is familiar with the inner workings of linear actuators (not any model in specific), I would really appreciate any advice that you can offer before I make my order. I've pretty much figured out everything else, but I'd like to know if and how I can use (built-in) limit switches and a (built-in) potentiometer. Can I use the limit switches to see what position the actuator is in? Can I use the potentiometer somehow to stop the current to the actuator so that I don't accidentally cut off someone's fingers in my door?
I've already found a circuit to use a relay to reverse the current the the actuator that I will use with a ratchet relay so that I can get movement in both directions... so far, so good.

Thanks for the help!

beelzerob · Sep 3, 2008

(Not to derail the topic too much ) Is it an electric actuator? I'd be curious what you are using or how you found it. I've been considering a linear actuator to use as a shutoff for my propane valve, should it be needed.

Quixote_1 · Sep 3, 2008

beelzerob said:
(Not to derail the topic too much ) Is it an electric actuator? I'd be curious what you are using or how you found it. I've been considering a linear actuator to use as a shutoff for my propane valve, should it be needed.

Yes, it's an electric actuator. I don't think hydrolics or pneumatics would lend themselves too well to this scenario.

I've been looking at firgelliauto.com and it looks like they have good hardware at reasonable prices.

Don't they sell valves specifically for controlling gas? http://stcvalve.com/Process%20Valve.htm?gc...CFRcXsgodLwn2AA

Quixote_1 · Sep 3, 2008

So apparently the feedback device provides a variable resistance which can be used to determine the position of the actuator. I wonder how I can make use of this while keeping the system independant of my computer?
Any ideas, guys?

beelzerob · Sep 3, 2008

Quixote_1 said:
Don't they sell valves specifically for controlling gas? http://stcvalve.com/Process%20Valve.htm?gc...CFRcXsgodLwn2AA

I guess that'd be the more direct and reliable approach....but I'd have to have someone out to install it. I don't suppose the electric actuator is any cheaper...

amirm · Sep 3, 2008

beelzerob said:
(Not to derail the topic too much ) Is it an electric actuator? I'd be curious what you are using or how you found it. I've been considering a linear actuator to use as a shutoff for my propane valve, should it be needed.

On boats, we use electric valves like this: http://www.westmarine.com/webapp/wcs/store...mp;classNum=691

Kind of pricey though but it does have a saftey shut off and sensor.

There is a cheaper one ($50) here but no description: http://www.boatersworld.com/product/337410195.htm

Quixote_1 · Sep 5, 2008

OK, so apparently I'll need to use a microprocessor to use the feedback mechanism. The feedback is given in the form or resistance.

Anyone know anything about microprocessors and how to use them without having to rely on a computer?

CollinR · Sep 5, 2008

If you use pneumatics you could control the power by simply regulating the pressure to the cylinder and it's bore.

Plus you would have that cool Star Trek whoosh sound effect if you put an unloader and the end of the cycle.

You could also do a normal door closer and use a single acting cylinder to open it. Plus an air tank can store more energy for longer then a battery as you could use a giant upright tank in the garage or whatever. You could even use a 20lb CO2 bottle and just refill it every couple of months. It all depends on the volume the cylinder goes through, but a 20lb CO2 bottle has boatloads of stored energy I use them on my Jeep to inflate the tires at the end of the day and run an impact wrench when it's required.

rbroders · Sep 6, 2008

I have used linear actuators in a number of projects. I like the Danahermotion line: danahermotion.com.

These actuators are all quite simple. There is a motor at one end which drives a nut along a long piece of all-thread. There are lots of options, bearings, gearing, potentiometer or limit switches (sometimes adjustable). If limit switches are provided, they are generally wired into the motor so the system will auto-stop at the limit.

Sometimes these devices are noisy. For my HVAC vent damper system the thing kept waking me up at night until I reduced the supply voltage. It slowed the travel time and max force, but the noise went down dramatically.

The actuators can be VERY powerful and could easily take off or trap a finger. The limit switch or potentiometer will not save you. I think there are three solutions to this problem:

1) Breakaway link - a mechanical connection to the actuator which will break away if excessive force is applied - magnets are a good choice here (perhaps an electro magnet which is energized with the actuator so the door behaves normally if not using the auto-open option).
2) electrical edge contact detector - this needs to be wired to reverse the motor if a contact is detected. Simply stopping is not enough - you usually cannot move an actuator when there is no power applied to it.
3) Power spike detector. System reverses when it detects greater than normal current draw from the actuator. I have not seen this implemented, but it should be possible.

--Bob

Quixote_1 · Sep 6, 2008

rbroders said:
I have used linear actuators in a number of projects. I like the Danahermotion line: danahermotion.com.

These actuators are all quite simple. There is a motor at one end which drives a nut along a long piece of all-thread. There are lots of options, bearings, gearing, potentiometer or limit switches (sometimes adjustable). If limit switches are provided, they are generally wired into the motor so the system will auto-stop at the limit.

Sometimes these devices are noisy. For my HVAC vent damper system the thing kept waking me up at night until I reduced the supply voltage. It slowed the travel time and max force, but the noise went down dramatically.

The actuators can be VERY powerful and could easily take off or trap a finger. The limit switch or potentiometer will not save you. I think there are three solutions to this problem:

1) Breakaway link - a mechanical connection to the actuator which will break away if excessive force is applied - magnets are a good choice here (perhaps an electro magnet which is energized with the actuator so the door behaves normally if not using the auto-open option).
2) electrical edge contact detector - this needs to be wired to reverse the motor if a contact is detected. Simply stopping is not enough - you usually cannot move an actuator when there is no power applied to it.
3) Power spike detector. System reverses when it detects greater than normal current draw from the actuator. I have not seen this implemented, but it should be possible.

--Bob

Thanks for all the great advice! I never realized that noise could be a problem with these.
I think I'll look into the power spike detector and have a safety button or other mechanism close to the door.

Linear Actuators 101

Quixote_1

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beelzerob

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Quixote_1

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Quixote_1

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beelzerob

Senior Member

amirm

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Quixote_1

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CollinR

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rbroders

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Quixote_1

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