Stepper motor question

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chris-s

New Member
This is more of a 'mechanical' question about the torque available in a stepper motor, I hope someone can help a little.

A while ago I created a 'dosing' pump using a timeswitch as a cam driver for a syringe with an arrangement of one-way valves so that on one half of the rotation it filled the syringes and on the other half it emptied them.

The photo shows a 'twin' model I played with, but there was not enough torque in the drive mechanism, so the final version just used a single syringe.



It worked fine for a couple of months but eventually the torque was too much for the plastic gearing.

I had previously thought of using a geared ac/dc motor with some sort of cam/switch/sensor to count the revolutions but that seemed a bit messy.

It's just occured to me that maybe I could use a stepper motor instead, driven from a picaxe, but I have no idea of the torque or what to look for in a stepper motor so any suggestions would be useful.

Cheers

Chris
 
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eclectic

Moderator
Measuring the force.

Chris.
I have no engineering expertise, but could I suggest an initial test?

Use a spring balance and scales to measure the

"push" force to press the plunger and the
"pull" force to pull the plunger back.

e.
 
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chris-s

New Member
The snag is that the torque required varies during the rotation, it is highest with the cam pin in the 3'o clock and 9' o clock positions.

I think ultimately I will need to just get some motors and try it, but I imagine differing specs of motor will have different torque abilities, but don't know where to start.

Chris
 
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SD2100

New Member
One option would be to use a wind screen wiper motor/gear box, these are a worm drive and extremely strong, they also have an internal switch which
could be used to indicate when it has completed one cycle.
 
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papaof2

Senior Member
Measure the force required to push and to pull the plunger as eclectic suggested.

Then measure the length of the arm that will move the plunger.

You now have a rough measure of the force needed in ounce-inches or gram-centimeters, which is how small motors are rated. Find a gear motor or stepper or servo with that rating (plus a 50%-100% safety factor).

Another possibility would be a linear motion device, such as a motor turning threaded rod with an arm attached to a nut on the rod. As the motor turns the rod clockwise or couneter-clockwise, the nut moves forward or back. You can adjust the length of travel with microswitches (or photo sensors or hall-effect devices) at appropriate points.

Another possibilty for a linear drive would be a stepper motor with a threaded or spiral shaft, such as this one: http://www.mpja.com/prodinfo.asp?number=16513+MS You can control the length of the stroke by the number of steps the motor moves.

John
 
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Mycroft2152

Senior Member
If youu didn't recognoze it the MPJA stepper motor is from an old 5.25" floppy drive, where it positions the head.

Another possibility is to attach a screw rod to the shaft of the motor and have a blind nut move along the shaft. this is a pretty standard trick for home built CNC machines. This avoid the rotational weaknesses.

Myc
 
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chris-s

New Member
Thanks guys, plenty of food for thought, just got to make sure I can get a Picaxe in there somewhere ;)

Cheers

Chris
 
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ljg

New Member
The wind screen suggestion is a good one. I took apart a rear windscreen motor from a minivan and converted it to continuous rotation, but as the motors come, they are intended for about 90º rotation and have limit switches built in to do that. The thing only consumed about 1/2 amp, too (at 12 volts).
 
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SD2100

New Member
Larry said:
they are intended for about 90º rotation.
Click to expand...
On a lot of these wiper motors the output shaft rotates continuously through360 degrees, don't get your finger jammed in the mechanism as they can cause some serious injury. :eek:
 
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elf1564

New Member
A neat little setup Chris.

I work with commercial style laundry equipment that uses a controlled dosing meter for the soap. These are for the most part just a geared motor turning a wheel or cam with 2 to 4 high points with small rollers on the end of the high point to pinch a silicon tube to draw the liquid (think milking a cow)from a bucket sending up and into the washing machine. Motor speed is geared downed so that cam rotates 50-60 Rpm. Most units typically pumps a few ounces per minute.

Might I suggest looking at the following page toward the bottom to get an idea for something you may be able to build on the workbench perhaps using a
small windshield wiper motor for a dosing meter.
http://www.cannonwater.com/Products/metering_pumps.html?gclid=COqW3t3uk5ECFQiaPAodylehQg

A 8m pic could handle the chores of timing the motor run time. For example, 1 push runs the motor for 1 minute. If you want to get fancy I suppose you could mount a hall effect sort of sensor to the meter cam and count the rotations.

Good luck,
Rusty Bates
 
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BrendanP

Senior Member
You can buy a linear actuator off the shelf that would do the job preety cheaply. Around $100 good for 20K strokes at 150lbs!. That will push those syringes in!

http://www.firgelliauto.com.au/product_info.php?cPath=62_79&products_id=31

Use a DPDT relay with a transistor or mosfet for speed and direction control.

It would make a good home suicide machine actually, death with dignity etc etc.

BTW, What they say about the wiper motors is true, Ive jamed my hand in them, it damn hurts!
 
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Wrenow

Senior Member
You can also get a linear converter for a regular hobby servo. http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXCJG6&P=7 They are limited to about 3/4" of travel, but are proportional and, best I recall, give about 3lbs of force or more with a standard servo.

Then there is the high-torque geared servo type solution:http://servocity.com/html/robotzone_servos.html

Or the new linear servos Servo City is carrying - up to 50lbs of thrust, but not cheap at about $200 per - http://servocity.com/html/heavy_duty_servo_actuators_.html

All are noce, proportional, solutions. however it depends on the amount of precision repeatability you need as to whether they are appropriate.

Cheers,

Wreno
 
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eclectic

Moderator
Filling frequency?

Chris.

Looking back to your first post, and the mains timer,
what is the cycle time?

Is it once every 24 hours?

e.
 
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chris-s

New Member
eclectic said:
Chris.

Looking back to your first post, and the mains timer,
what is the cycle time?

Is it once every 24 hours?

e.
Click to expand...
Yes, a dose once a day which is whats required, 5ml per 24 hours. It's a neat solution, if only it had a bit more torque.

elf1564, I'm familiar with peristaltic pumps, got a couple already in use. I've three liquid trace elements that need to be dosed at a rate of 5ml per 25 hours. Three peri pumps or a tripple headed unit works out quite expensive.

Michael 2727, I've got a solenoid dosing pump, picked it up cheap on ebay, but of course thats only good for one feed and I haven't found another one yet.

A linear servo is a good idea, think I've got one somewhere. The short travel may be a problem, but if I found a syringe with a larger diameter then the stroke will be shorter.

BrendanP, a linear actuator is a good idea, I've seen them on ebay pretty cheap. Extending that thought, I've got some small RC model aircraft air rams in the workshop, maybe with a small 12v car tyre compressor.

Hmmm, plenty of ideas, I really appreciate all your time and thoughts guys.

Cheers

Chris
 
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papaof2

Senior Member
Yet another possibility...

The pumps used to move blood with damaging it use plastic tubing in a C shape with a roller that gently squeezes the blood around the circle. Picture: http://en.wikipedia.org/wiki/Image:Coronary_artery_bypass_surgery_Image_657C-PH.jpg

The advantages would appear to be a constant torque requirement, the ability to change the delivered volume by changing size of the tubing (within limits), and simple control - run for enough turns (180 degrees each) to pump the desired amount of liquid.

John
 
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tikeda

Member
You've Googled 'syringe pump'?
I've seen used Cavro, Hamilton, Harvard Apparatus units on Ebay. They're all built with linear actuators. If you have access to a university research lab or know someone there I'd bet you could find a couple units that are just gathering dust in the back of someone's lab.
 
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chris-s

New Member
papaof2, thats basically a peri-pump. I've played with one, but

tikeda, yes I have seen them on ebay, but they are quite large and I would need multiple units to handle several syringes.

Mycroft2152, thats a neat little gadget, not sure about the torque of it tho, well priced and simple to replicate in the workshop.

I found these linear actutors on ebay, they should be more than strong enough to drive several 'syringes' and fit within my budget...

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=230216158858&fromMakeTrack=true&ssPageName=VIP:Watchlink:middle:uk

Thanks once again for the replies

Chris
 
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chris-s

New Member
Mycroft2152 said:
I remembered something that may work from a CNC machine that decorated cookies!

Depending on the viscosity of the fuid, you might be able to modify one of these coordless cookie presses ~ $30

http://www.wilton.com/yearbook/productinfo/cookiemasterplus.cfm

Myc
Click to expand...
Now thats a neat gizmo, if only they were available in the UK. However, I note a couple of points on their website...

5. Never disassemble
8. Do not use appliance for any purpose other than its intended use.

... as if we would even think of that :D

Chris
 
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moxhamj

New Member
Re 'It worked fine for a couple of months but eventually the torque was too much for the plastic gearing.", medical syringes have a small amount of lubricant in them which works great for a short while (they are designed for single use) but they get extremely stiff when the lubricant dries out. That is probably what stripped the gears. Glass syringes don't need lubricant and are far less 'sticky'. There are lots of high torque solutions using gears or nuts on threads or high power servos but the first experiment is to determine the force needed to move an old syringe. Mount it upside down and tie a string to the plunger and add weights to the end of the syringe. It might be a Kg or more for an old 5ml syringe.
 
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sedeap

Senior Member
Other solutions - redefinitions

Please, redefine your needs.
What you require?
A simple timer-pump of 5ml?
You need the dose was applied during 12hs?
(12hs to fill, and 12 to drain)
Or one "shot" after 24hs is the same thing to you?
Is a medical fluid? need to be syringe?
can be a "open" to air solution? or need to be tubing with no air?
( I used to use one aquarius medical treatment of the water, the device drop several drops of pharmacological solution to one little glass jar, and when fill, drop into aquarius)
Other solution can be the Pool treatment gadgets (natatorium) this little devices, keep the water with the right PH and CL and can be override by manual programming.
Anyway, one car-lock (door window) from some car-alarm who rise-up the windows can be enough strong to pump a syringe.

This is a commercial solution, just maybe too expensive to you.

Syringe-pump
 
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