2 switches on a linear actuator?

[suthy]

I'm looking to hook up 2 pneumatic switches to a 6v mini linear actuator. The switches are pressure and vacuum. I want to push air into one switch to make the actuator extend and suck into the other switch to make the actuator retract. Does anyone know how to reverse polarity with the 2 switches rather than the standard dpdt momentary toggle switch? Btw- the actuator is connected to a claw. Appreciate any help here.

[Soeren]

Hi,

I'm looking to hook up 2 pneumatic switches to a 6v mini linear actuator. The switches are pressure and vacuum. I want to push air into one switch to make the actuator extend and suck into the other switch to make the actuator retract.

How are the switches on these two units configured and what currents can they handle?
How are the liner actuator energized (changing polarity or using separate terminals)?

Why not use only one sensor - whether suck or blow?
Either let the reverse condition (i.e. not sucking/blowing) cause it to reverse direction, or alternatively, first suck(/blow) energizes in one direction, and next one reverses?
The purpose of it all might reveal further ways of doing it and will make easier to help.

Does anyone know how to reverse polarity with the 2 switches rather than the standard dpdt momentary toggle switch?

Without knowing the switches involved, it could either be with just the switches on the sensors, or it may take a bit of circuitry.

The more you tell, the better help we can provide.

[suthy]

The switches are very low pressure pneumatic switches- they are the 501 series in the link below. They say 20 milliamps of current, 5-24 VDC.
 http://www.pressureswitch.com/ecommerce/index.php/part/viewAll/
They have a high pressure side port and a low pressure side port. If you push air(pressure) into the high side port, contact is made between the 2  terminals. If you suck air(vacuum) on the low side port, contact is made between terminals. 6volt Nimh battery- 2200 maH.
 Imagine a piece of tubing in user's mouth connected to a barbed tee, where 1 tube goes to the high port on 1 switch, and 1 tube goes to low port on other switch.
The linear actuator is energized by reversing polarity. 2 wires coming out of actuator- reverse polarity on those 2 wires to change direction.
http://www.firgelli.com/pdf/L12_datasheet.pdf
I make reacher for quadriplegics, quadtools.com, reason for 2 switches is so a user can suck on tube to close jaws until they have picked something up. I don't want them to have to keep sucking to keep jaws closed. Perfect scenario would be: suck till you squeeze jaws on something, stop sucking, lift item, hover item over lap, blow on tube to open jaws, item drops onto lap. This all needs to be very lightweight and as small as possible.I really appreciate your expertise on this, you are helping paralyzed people gain independence.

[jkerns]

I assume you will have some sort of microcontroller operating the actuator. (strictly switch / pneumatic solenoid could be made to work, but...) A pressure / vacuum sensor into an A/D would give more flexibility than a switch (you can change the "switch" point). And,  you could potentially look at the magnitude vacuum / pressure to control how fast / hard the actuator is moving.

http://www.mouser.com/catalog/catalogUSD/645/2056.pdf

"Perfect scenario would be: suck till you squeeze jaws on something, stop sucking, lift item, hover item over lap, blow on tube to open jaws, item drops onto lap. "

Doing that part of the software doesn't sound difficult at all.  Switches and relays get more complicated, expensive, and bulky.

[Soeren]

Hi,

They have a high pressure side port and a low pressure side port. If you push air(pressure) into the high side port, contact is made between the 2  terminals. If you suck air(vacuum) on the low side port, contact is made between terminals. 6volt Nimh battery- 2200 maH.
 Imagine a piece of tubing in user's mouth connected to a barbed tee, where 1 tube goes to the high port on 1 switch, and 1 tube goes to low port on other switch.

I a very familiar with the concept and I had an idea that it was the purpose, but I think you misinterpret what this sensor will do. The reason for two ports is just that it's a relative pressure switch and blowing into one port has the same effect as a suction at the other end.

The two ports goes into a chamber that's separated by a membrane. blowing into one port will make it deflect away from this pressured port and suction at the other port will make it deflect in the same direction. So, either blowing into one port at the time, or blow and suck the same port, will get the effect you're after - unfortunately this sensor cannot do what you want though as it's unidirectional in its output.

For your purpose, any sensor that have different output for positive and negative pressure in the needed range will work.

The Freescale MPXV7002 is a +/-2kPa sensor with around 2.5V output at zero pressure (lower voltage with negative pressure and v.v.) This would work for a single blow/suck tube and you'd need a bit of electronics between sensor and actuator anyway

I have no idea of how hard your target user can suck and blow, but the 2kPa (~0.3 PSI)
is just the max range and a simple comparator can make it work with eg. a tenth of that if needed.

The linear actuator is energized by reversing polarity. 2 wires coming out of actuator- reverse polarity on those 2 wires to change direction.

OK, You'll need to measure the current it takes, to stop it when a suitable pressure is reached to hold whatever it needs - or you may burn out the windings.

A conventional (rotating) servo would probably be cheaper.

Including a spring may help in holding without crushing.

[...] you are helping paralyzed people gain independence.

No, I'm helping you make money

I really dislike when people try to con me into something with the "for the kids/sick/whatever" Please don't, I've been making stuff for disabled people since I was a pre-teen anyway - dishonesty is just putting me off, if anything.