Controlling a Linear Actuator

PParcells

Hi guys, I've been out of the loop here for a while, but I'm starting a new project, and a I need a little help.
I'm making a hand clutch system for my truck, i have my hydraulic throwout bearing, and my master cylinder.
The way i have this set up, is so that a Linear Actuator will depress the plunger in the master cylinder, actualing the
hydraulic throwout bearing.

i want to have this controlled by a trigger on my shifter, so, I need a certain level of precision, and position control.
i want to have the actuator controlled in relation to how far the trigger is pulled, and if possible, with a potentiometer on a
control box to control the amount or speed of the linear actuator extending.

I have my basic stamp, and i know i need a linear actuator that has feedback control, and i know it needs to be controlled with PWM.
what im not sure of is how to actually control it with the stamp. i realize that i will need some sort of motor controller to handle controlling the power
that the actuator requires, since im sure it will take several amps to push the plunger, actuating the clutch.

whats the best way to do this? i was thinking of using a trigger on a potentiometer, and writing a program the moves the actuator based upon the resistance
of the potentiometer on the trigger assy.

anyone have any ideas?

erco

Welcome to the forums, PParcels. Easiest solution is to use a Firgelli linear actuator, which has built-in position feedback and several different control options. Simplest to use operates exactly like a servo, controlled by a steady stream of PULSOUT commands, not PWM. Very simple to use with your BASIC Stamp, or even a $2 Ebay servo tester.

http://www.robotshop.com/firgelli-actuators.html

http://www.ebay.com/itm/New-RC-Helicopter-Plane-CCPM-Servo-ESC-Checker-Tester-/160612842251?pt=US_Radio_Control_Control_Line&hash=item2565457b0b#ht_2955wt_956

http://www.youtube.com/watch?v=_lCjZrLUAP8

Mike Green

PWM is used for speed control and I don't see where you really need speed control in this project. You do need to be able to reverse the motor in the linear actuator and for that you'll need an H-Bridge and there are all sorts of DIY and ready-to-go H-Bridges available for pretty much any size motor. They usually have two digital inputs, one for on/off and the other for forward/reverse. Here's one discussion thread on building your own.

In terms of feedback, it sounds like simple limit switches would work fine. These could be mechanical switches actuated by a metal tab on the actuator shaft or LED/phototransistor similarly actuated by an opaque "flag" on the actuator or a magnet/Hall-effect sensor with the magnet mounted on the actuator shaft. These would be mounted just short of the proper in/out positions so the actuator has enough time to stop once these limits are reached. You'd want the range of the tab or flag to include the actual endpoint so the Stamp can initialize the actuator to a known position (if not at limit, move out until limit switch activates).

If you still want to use speed control, Pololu and Parallax both make DC motor controllers like the HB-25. Which one to use depends partly on the voltage and current requirements of the linear actuator.

PParcells

i do need speed control, i need to be able to adjust this setup to get the right feel for the clutch, being as how its not like a foot setup, where you can "feel" where the clutch is.

but i had been looking at those Firgelli actuators. is it possible to write the code so that the actuator moves with my trigger? i've been out of the programming side for a while now,
and im having trouble remember how one would write it so that the movement of the actuator would directly correlate to my trigger, not in set increments, and still be able to adjust the
ration of movement. (1:1, 2:1, 3:1) ect... and when using those actuators, i know i cant power them from the stamp, but are they controlled directly by it? i guess i really need a wiring
diagram or something to really wrap my head around this, sorry for the trouble articulating my thoughts, and thanks for the help guys, i greatly appreciate it.

Mike Green

If you get an RC Servo compatible Firgelli actuator (like this), it can be controlled exactly like the standard servos described in the "What's a Microcontroller?" and "Robotics with the BoeBot" tutorials. The black lead goes to a common ground (for both motor power and Stamp power). The red lead goes to the (+) motor power and the white lead goes to a Stamp I/O pin. I would suggest putting a 10K resistor in series with the white lead. This will help keep noise from the motor and motor supply out of the Stamp and will help protect the Stamp I/O pin if you accidentally short the white lead to the motor power lead.

erco

PParcells wrote: »

. is it possible to write the code so that the actuator moves with my trigger? .

Yes, exactly as that Youtube video shows with the $2 servo controller with a pot.

Maximum speed may be the limiter for you, how fast the actuator keeps up with your input. You may need a high-force actuator to drive your clutch master cyclinder. That's typically achieved through higher gearing, which slows things down proportionally.

stamptrol

This will be a neat application.

The actuator is pretty well defined in the previous posts.

The trigger is going to need to generate a signal proportional to its position so the actuator will follow it and allow the clutch engagement to be modulated.

If the trigger were a resistive pot, you could use the RCTIME function to measure its position and use that value to give a signal to drive the actuator. Other triggers might generate a varying voltage which could be read by an a/d chip.

Cheers,

PParcells

yeah, sorry, i cant watch the video until i get home. no youtube here at work. i like the ideas im hearing for control and stuff though. now all i need to do is find an actuator thats strong enough, fast enough, and has feedback and control. i've been looking at some ball bearing screw type actuators. its hard to find specs on some of these things though. i guess before i buy anything, i need to figure out if i can build any mechanical advantage into the hydraulic system. thanks for the input guys.

PParcells

so i've purchased a McLeod 13615 hydraulic throwout bearing kit. it comes with all the clutch components i need, and should make this project a whole lot easier. i think i found a figirelli actuator that will suit my project, so i think im good to go, i'll be sure to let you guys know how things go in the weeks to come.