Which motor do i use - neophyte question

craigcurtin

Senior Member
Guys,

I have a small needle valve that controls the flow of oil to an external boiler that i have. I have not done any work with motor control before, i would like to be able to control this both forward and backwards (multiple revolutions) to enable full control - what sort of motor do i use - presumably DC, but is it a Stepper or a Servo ?

Sorry for the basic question - i am assuming a Stepper ?

Can anyone point to sources in Australia - this is a very light touch valve so i do not believe a lot of torque would be required.

Craig
 

Michael 2727

Senior Member
Use a good quality RC Servo motor with a
rotary encoder or even a geared pot for
feedback.
You can hack an RC servo to rotate forever.
 

MiB1986

Member
Hello,

personally id use a stepper as there easy to use, no hacking involved and can be found in printers, floppy drives etc... so very easily obtainable from junk maybe in your house.

Obviously stepper motors can be stepped clockwise or anticlockwise continuously or a single rotate, you will never be able to get the accuracy using a standard 2 pole motor,

Hope this helps, servos are good but can sometimes be a little tricky

Kind Regards
 

BrendanP

Senior Member
Does the needle valve go in and out in a linear fashion? Someone on the forum made a homemade linear actuator.
If you use a linear actuator dont use a H bridge to reverse the motor. More trouble than theyre worth. Use a double pole double throw relay and a transistor. The transistor will give you speed controll of the motor via PWM and the relay will give you direction.

 
 

Wrenow

Senior Member
The problem with a stepper motor is that there is no position feedback, so you have a new system to design. With a hobby servo, you can just replace the feedback pot with an external pot hooked up in your drivetrain (and you can use a multiturn pot if appropriate). All the logic and PID for moving to the position you want is built in. Interface is incredibly simple.

Just my .02, of course.

Wreno

Edited by - Wrenow on 11/06/2007 14:02:25
 

craigcurtin

Senior Member
Hi guys, thanks for the answers. Yes the valve is on a screw thread so it does move in and out - however the thread is very fine so the actual amount of movement in and out is minimal.

If i was to use a servo - even though it does not give me position feedback, could i not just position it past the "off" point by rotating it clockwise (e.g. 20 steps) which i would know is much more than i have undone it, and then turn it back anticlockwise however many steps i want (say 10) i..e i would never need position feedback if at the start of each session i was to turn if past off first ?

The Bits about hacking a RC servo lost me as i am no electronics genius

Craig
 

hippy

Senior Member
As long as you have some 'slipping clutch' or not enough torque to do any damage when trying to turn past off, you'll be okay with that method ( presuming turning it off doesn't cause any problems in itself elsewhere ).

If you can design your controller so it only needs to 'home' to off after it has lost its positioning data ( power fail, reset, download ), you can minimise the number of 'homings' you have to do.

You could always write the current step position/count to Eeprom so you know where it was after a power fail or reset. Or use a battery backup to keep running through a power fail.

A separate motor driving PICAXE and a separate controller PICAXE would allow the motor driver to keep knowing where it was no matter what messing about you were doing with the main controller. That would simply tell the motor driver what it wanted, and the motro driver would deliver it.
 

craigcurtin

Senior Member
Good one thanks Hippy. Good idea re seperating the control and the brains. I am currently using a 40x1 as the brains, so would be quite feasible to do this.

So stepper it is to start with - now to work out how to control it !!

Craig
 

craigcurtin

Senior Member
Will do Demonic. This is in info gathering stage at the moment - setting up the new oil burner etc now, once this is tested and running will start playing with the Picaxe control - so about 1 month away

Craig
 
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