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  #1  
Old 07-17-2011, 06:59 PM
Dimitris76 Dimitris76 is offline
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Default Linear actuator control circuit

Hi all! I am new in electronics and need some advice.

I need to build a circuit that will power a linear actuator with built-in end switches like this one:

http://www.firgelliauto.com/product_...products_id=31

for as long as I hold a push button depressed.

At the end of the actuator's travel the polarity to the motor should be reversed until the actuator retracts all the way to it's other end and then reverse polarity again to extend, and so on... until I release that push button.

The end switches are internal and not accessible and the actuator has only a cable with two poles.

I was thinking that measuring the voltage across the legs of a low value high wattage resistor that is connected in-line to the circuit output would be a nice way for the circuit to "sense" that the actuator has reached it's end of travel and switched off it's motor. No (high) current flow - no voltage drop across the resistor.

Simple PIC solutions are welcome too - you'll have to right the program for me though...

Thank you in advance!

Dimitrios
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Old 07-17-2011, 08:21 PM
Dimitris76 Dimitris76 is offline
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Do you thing that a 555 in bistable mode and either external NO reed end-switches or inline to the load resistors for trigger and reset would work?

The output would control a DPDT relay reversing the motor...

Comments?

Dimitrios
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Old 07-17-2011, 11:23 PM
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ifixit ifixit is offline
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Hi Dimitris76,

It seems like the internal limit switches turn off the motor when there respective ends are reached. So, all you need is a way to detect when no current is be used by the actuator and then reverse the polarity to the actuator, which will reverse the motion until the other end limit switch is opened and so on.

Let's see who comes up with a circuit to do as you requested.

Regards,
Ifixit
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Old 07-18-2011, 12:57 PM
John P John P is offline
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I looked at the company's wiring diagram and couldn't make sense of it. Yes, they could have wired the limit switches with diodes so you wouldn't have external access to them. Then you could only detect that the motor had hit a switch by whether it was drawing current or not. It sounds crazy, but maybe they did it that way to keep the user from damaging the unit (by making the motor overrun its safe travel).
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Old 07-18-2011, 01:37 PM
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SgtWookie SgtWookie is offline
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I'd taken a look at the site yesterday, and for some reason they have a "Click here for PDF Spec sheet" shown, but it's not a link. You can click until you're blue in the face, and nothing will happen.

John P, I'm in agreement with you about diodes with the limit switches; I figure that internally it's more or less connected like this:



For automation's sake, it's somewhat unfortunate that they wired it that way. However, it makes perfect sense from the aspect of a manually-operated system using a single DPDT switch to reverse direction; the limit switches make completing a manually reversing circuit simple.

But, to make this circuit reversible automatically, you'd really want to add external limit switches.
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File Type: png Firgelli linear actuator.png (6.5 KB, 105 views)
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Old 07-18-2011, 04:09 PM
Dimitris76 Dimitris76 is offline
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Thank you for the interest!

But let's forget those internal end-switches for a while...

I can easily mount a small magnet on the actuator arm and two reed-switches placed a few mm "inside" the travel endpoints.

Could you post a schematic of a circuit that would have the actuator running back and forth for as long as I press that push button?

Dimitrios
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Old 07-18-2011, 04:34 PM
Dimitris76 Dimitris76 is offline
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Can I use this bistable 555 circuit




with NO (normal open) end switches and a power on reset trigger with capacitor like this?



The 555 output would control a DPDT relay with a shunt diode across it's coil.

What do you guys think?

Also, if this does work, how can I prevent damage to the DC motor and the actuator's drivetrain by the sudden polarity reversal?

Dimitrios
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Old 07-18-2011, 09:36 PM
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ifixit ifixit is offline
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Hi Dimitris76,

I have attached a schematic circuit idea, very simalar to what you have suggested, but with more details. Using the 555 as a FF sould work also, but I have never tried it.

Regards,
Ifixit
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File Type: jpg actuator ctrl.jpg (61.8 KB, 40 views)
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Old 07-18-2011, 10:30 PM
Dimitris76 Dimitris76 is offline
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Great idea!

Your circuit uses 5 resistors, 2 capacitors, a transistor and an IC (4013D-FF).

What's the advantage over the above 555 based schematic that uses only 2 resistors and an IC (555) - plus the relay, diode and the two end-switches?

Also, how can I prevent punishing the motor/drive during polarity changes?

Dimitrios
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Old 07-18-2011, 10:42 PM
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The first step in limiting the "punishment" to the motor during a polarity change might be to require the current to have fallen to zero for some minimum period before reversing the polarity (you probably have to do that anyhow to avoid false triggering).

A minimum "dead" time will give the motor time to stop properly before applying the reverse voltage. Next, you might want to consider a current-limit or soft-start circuit to limit the motor peak current on restart.

Even if you do that, you may find that the motor overheats if you make it cycle continuously - is it continuously rated?
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