Servo vs. Solenoid vs. Stepper

[sailgene]

Hello Group.

I've been working on a project to supplement a marine refer using cold outside air (a winter scenario only) in lieu of the 6 amps required to run the refrigeration compressor. So far I've had help getting my Picaxe 08m to take to temp. readings (both outside and in the refer) from a couple of DS18B20's and then to turn on a small fan which would pump the outside air into the refer. I've got that part working pretty well.

But one problem I still need to overcome is to build a small duct "valve" which will only open (90 degrees) when the fan turns on (to avoid the problem of a warm breeze accidentally blowing into the refer when the outside air is too warm).

I've been looking at various motors/actuators with the goal of rotating a shaft 90 degrees to open the valve and then back 90 degrees to close the valve. I don't want the motor to draw amperage after the valve has been rotated in either position.

I also may be pushing things with a single 08m chip but I can deal with that later.

But given this, has anyone thought of a way to power a small motor/actuator on-off to accomplish this feat? Solenoids seem to draw significant amperage when "on" and few are suited for continuous duty. Servos seem to need PWM for all positions, unless I'm mistaken and can automatically return to a preset position when powered down.

To complicate things, I'd love to do this with only one output pin on the Picaxe. Am I crazy?

Thanks for any ideas or help.

Gene Darby

 

[Andrew Cowan]

A servo, definitly.

Servos only 'hold' their position if supplied by a servo signal - otherwise they can be moved by hand. However, this should not be a problem is the duct cover is light (or counterweighted).

A

 

[hippy]

Providing you can use some sort of catches or weighting to hold the flap open and closed which don't need too much force to overcome them, you only need a mechanism which will force the flap open or closed and only power it while doing so.

Something other than rotation may also do the job - Car central locking solenoids may be one way to go with some linkage arms.

You could perhaps also take inspiration from the typical retracting ball-point pen which has a mechanical mechanism so all you need is a push to 'turn it on' another push to 'turn it off'.

 

[eclectic]

Just throwing in a couple of ideas.

A transistor > small latching relay, then a
Latching Solenoid?

Eg
http://cgi.ebay.com/5-LATCHING-Miniature-Signal-RELAY-5V-Coil-2-TYPES_W0QQitemZ370179199197QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item370179199197&_trksid=p3286.c0.m14&_trkparms=72:1205|66:2|65:12|39:1|240:1318|301:1|293:1|294:50


http://au.farnell.com/jsp/search/browse.jsp;jsessionid=QBCLIRFO40CWGCWNBBSLY4Q?N=0&Ntk=gensearch_001&Ntt=latching+solenoid&Ntx=&suggestions=false&_requestid=2119

e
Lots of other sources, I'm sure.

 

[Wrenow]

As noted, plain old hobby servos are probably ideal, assuming the valve, when closed, will stay closed on its own, and when opened, will stay open. If not, a small magnetic catch bight ensure it stays where you want it. Properly set up, the servo will draw little electricity when at rest in either the open or closed position (it will only be powering the circuitry in most, though some may actually be applying a brake on the motor). If the valve latches in place, you could depower the servo after it does its business, and it will stay where it was last until acted on by an outside force.

If you will go to the http://servocity.com site, and then to Servos, you will find. listed by the servos, several types of adapters to attach shafts, disks, etc. to the servo's splined hub. They also ave a geared servo that uses an external pot that will give you large amounts of torque if you need more oomph.

Most servos do not return to any particular position when powered down - they stay where they were last. While the positioon command is based on a pulse width, do not confuse wit with regular PWM - it is not quite the same thing. It is a pulse of about 1.0ms to 2.0ms every about 20ms. You are not driving the motor with it, you are signaling the servo's electronics o indicate the commanded position, and it takes that information and drives the motor to the appropriate position. If you have not, you might want to look at the Servo tutorial recently posted here. http://www.picaxeforum.co.uk/showthread.php?t=12120&highlight=servo or some of the discussions I linked in this thread: http://www.picaxeforum.co.uk/showthread.php?t=10033&highlight=servo+basics

Cheers,

Wreno

 

[BCJKiwi]

The simplest mechanical 'stay at either end' device is a weight (or spring) on a lever on the shaft of the flap.
Position the flap at the halfway position, and, position the weight at the the highest point. The weight is then at 45deg either side of vertical pulling down when the flap is at either end position.
Adjust the length of lever and or weight to give the optimum settings (just enough force to get the job done - reduced load on the actuator). Also, if the flap is 'heavier' on one limit than the other, the lever can be moved off centre to provide less down force at that end of the travel.

 

[BrendanP]

Small one way spring loaded flap valves are made for use with use with concertina ducting in cooling systems and with exhaust/intake fans.

They have a sort of butterfly valve set up in them.


When the fan comes on the flow of air opens the valve, the springs close the flaps when the fan stops.

I'm not sure if I understood your requirements correctly but I thought I'd mention it.

Electrical suppliers and hydro ganja growing shops have them. People often use them with CO2 injection systems when growing dope under lights.

 

[jglenn]

Is that shutter activated from the airflow, b rendon?

No one has mentioned the ROTARY SOLENOID. Uncommon, but you can get them. Really need a latching one. You only need to move 90 degrees.

Make a simple one with a magnet, and coils at the end points. Put opposing fields on to move it. It will only need a current spike to move, to keep it there a very small current should do it. Or a spring or something. Velcro!

 

[gengis]

Check out some automobile trunk lid or door lock actuators. They are using a small DC motor that turns a worm gear. The travel is only about an inch but the torque is very high and they hold open or closed at the end of the travel without applying power. There is no limit switches on the ones I've got so you'd have to time it or add a limit switch to turn it off.


Two limit switches and you could drive the thing to the limits and let it turn itself off - then you only need one output pin on the axe to run it.


All Electronics here in the US has them surplus. Very well made, should last a long time

 

[sailgene]

Thanks, guys for the plethora of brainstorming ideas. Much to chew on - which is 75% of the fun.

Gene Darby