Slow mains-connected linear actuator? - Page 1
Forum section: electronics / projects

author: brainwash
author_rank: Frequent Contributor
timestamp: March 30, 2015, 04:50:41 pm
content: Since I cannot really explain in a short phrase what I'm after, here's what I bought:Helios EVK100http://www.heliosventilatoren.de/mbv/evk100_150_90741_0314.pdfThe unfortunate thing about this unit (and the 20yr old one before it) is that it leaks air even when shut off (closed position). The suction caused by the forced ventilation system causes the unit to 'squeal'. I assume there are some big fans on top of the building, shared between all apartments. The suction power of the 100mm vent is enough to cause a 200-400g cardboard to stick without slipping down.The principle of operation is that when 220V is fed into a bimetal strip it causes that strip to bend up and rotate the shutters to an open position. Its movement is about 1cm at the extreme position, over the course of ~1 minute. After power is removed, it slowly springs down to its original position, after 1-2 minutes. The force at the extreme position is probably <100 grams.I've tried designing some other kind of actuation with servos, but then it requires battery power and some kind of detection (which I fail to do properly). I do not want to mix 3-5V and 220V, out of principle if nothing else. I'd be fine with, for example, hardwiring a USB charger or some other kind of PSU.I'm looking for a way to make a travel of 3-10cm with peak forces at minimum 200g (2N). It should however spring back within 1-5mm of its original position if power is removed. The remaining slack will be taken up by the suction force.I'm sure there are some things that achieve this, like 'muscle wire' or the things that scooters use for the automatic choke.

author: Marco
author_rank: Super Contributor
timestamp: March 30, 2015, 05:37:43 pm
content: Do you want to replace the actuator on that unit? Or do you just want to move a plate away/across the hole and let suction do the rest?

author: brainwash
author_rank: Frequent Contributor
timestamp: March 30, 2015, 06:15:27 pm
content: I'd go with the second option.I would likely replace the unit completely with a hinged cover that swings away. I already tested that with a cardboard and a servo and it worked better than the commercial unit.

author: Richard Crowley
author_rank: Super Contributor
timestamp: March 30, 2015, 06:39:56 pm
content: I have seem implementations of HVAC vent "valves" that simply use a rubber "bladder" (baloon). When you want to block the vent, you simply put air into the baloon and it completely blocks the air. The nice thing is that it is non-conductive, passive, non-electrical, so you can actually run the thin control tubing through the same vent.Alternatives would be using a linear or rotary solenoid to actuate the vent.Another solution I have seen is to use a motor-operated "gate valve"...

author: Marco
author_rank: Super Contributor
timestamp: March 30, 2015, 07:02:53 pm
content: If it's in the wall (and not the ceiling) I'd just try a small motor on a transformer to pull the hinged plate up and away from the vent with a wire wound on the motor shaft, put some weight on the bottom of the plate and gravity will pull it back when the power is removed.

author: LukeW
author_rank: Frequent Contributor
timestamp: April 08, 2015, 02:29:49 pm
content: Microwave oven turntable motors are geared for fairly high torque / low speed, and run from mains AC.

author: brainwash
author_rank: Frequent Contributor
timestamp: April 08, 2015, 02:57:46 pm
content: I like the ballon idea.A motor would have to have a limit switch and some way to make it reliably return to the rest position after power is removed, but still have enough torque to overcome the spring/weight.Probably something with an eccentric cam and a solenoid latch: the motor turns the eccentric cam for some predetermined time, which raises the 'hatch'/flap, which catches on a latch/hook. The motor then stops rotating. When power is removed the latch/hook is released which causes the flap/hatch to spring back in place. But this sounds so complicated. Perhaps there are mechanism available in some appliances that do this.

author: Ian.M
author_rank: Super Contributor
timestamp: April 08, 2015, 03:57:21 pm
content: Google: electric vent openerHere's one site that looked possible:http://rollertrol.com/actuator-for-vent-control-or-window-opener

author: ajb
author_rank: Super Contributor
timestamp: April 08, 2015, 04:16:16 pm
content: I've seen electrically actuated ball valves that just use a motor and a spring. The motor is simply allowed to stall against the end stop, and when power is removed the spring returns the valve to its idle position. The trick is sizing the motor and spring so that the stall current is sufficiently low that you don't cook the motor.If you wanted to get fancy, you could do the same arrangement but add a switch at the end stop that reduces the power to the motor to just the amount required to hold against the spring when it reaches its open position. Could be as simple as a switched series cap.

author: Marco
author_rank: Super Contributor
timestamp: April 08, 2015, 04:21:11 pm
content: Quote from: brainwash on April 08, 2015, 02:57:46 pmI have no feel for the magnitudes of the forces involved ... but fora couple bucksyou can get a geared electric motor, Id just try letting it stall (maybe add some current limiting, can be done with two NPNs and two resistors).

author: brainwash
author_rank: Frequent Contributor
timestamp: August 10, 2015, 11:00:45 am
content: Hi, it's been a while since I last wrote here.The idea I've went with (tested for over a month) is to use a supercap as the power source after power-down. I've also tested some other solutions but basically I went back to my initial implementation: servo motor driving a flap.Implementation:I did some back-of-the-napkin calculations and I needed somewhere between 1F and 2F. The circuit is now powered by a small usb charger, which, through a Schottky, charges the 1F/5.5V supercap.The MSP430 is powered through an LED: this limits the current, drops the voltage by ~2V and prevents the buffered voltage going back to the main/power circuit. There is also a relatively large capacitor (470uF/6V or so) after the LED that is able to power the MSP430 for minutes.The servo is powered from the supercap terminals while the signal is taken from the uC.The uC senses the 5V usb power, before the Schottky, through a divider/LED combination.On startup - when power is applied - the capacitor in parallel to the uC gets charged, the supercap gets charged, the uC senses power and after ~1s orders the servo to go to the open position.On shutdown - the uC senses the 5V is missing, orders the servo to go to the closed position.Really easy in principle but there are a few caveats:The 1F cap is barely enough - I could have put 2 of those in parallel but I thought the sucking force of the vent would do the rest of the job - and it does so 90% of the timeUpon closing I drive the servo as fast as possible. I tried to do it slowly so that the supercap has time to regenerate but if the voltage on the servo goes too low it starts going backwards. It seems like 2.5V is the tipping point. However I could not reproduce this reliably on the bench supply.Sometimes (1/10-1/20 cases) the supercap seems like it does not have the complete capacity available - the flap only goes halfway down. Maybe I should have derated the cap more but I think 4.9V compared to 5.5V maximum should have been reasonable. I have no explanation for this.Powering the uC through an LED is a good idea - it lets you see the current draw and activity of that circuit portion. The LED is rated for ~20mA, it might sink more on startup (while the 470u capacitor charges) but only for a really brief time. During activity bursts the uC is drawing ~1mA which is visible on the LED.That's a lot of words for a really simple project but hopefully there are some lessons learned.