next topic »

[on:]

Hi
Please feel free to point me to a previous post if this is a redundent question.

I'm prototyping a simple drive using a cam follower. The drive requires a high torque, low speed motor (less than 50, probably 30 rpm).
My problem is identifying the correct type of motor and controller necessary.
I need a potentiometer to control speed. The motor will always turn in one direction.
High speed is not a requirement and is actually a drawback.
The cam follower would be lifting (leveraging/pushing, actually) about 20-30 pounds using the follower as leverage.
Being a total and complete noob, is this (I think) a dc brushless motor application?
What is the easiest, most direct means to accomplish this? Do I even need a motor controller?


Thanks in advance
john_s

[Reply #1 on:]

I would like to help you but you really haven't give me much to work with. i know you have explain your set up but you must atleast give me numbers that i could use as a baseline.
for instant yes you did say high torque, for some people 2N-m is consider as high but for some 10N-m is high Torque. so give me numbers.
low speed yes that value help but which one is better, 30 rpm or the 50 rpm?
20-30 pound of leverage the info help but not enough. i mean if you talk about leverage there will be a fulcrum length from the motor and the fulcrum length to load. so again not enough info.
A dc brushless may not suitable for this application, due to the fact that most DC brushless motor have very high speed and low torque.
What i would suggest to you is that maybe you could use a permanent magnet DC Motor with lots and lots of gearing. or maybe a gear head attach would be great too. if you use a DC motor then you need a Mosfet that you could control with your arduino(I assume you would use an arduino since this is the arduino forum) Use the one that control a relay

you could also use a Stepper motor, Its have a very high torque and very low speed, if the torque is not enough like always use gearing or even belt could work. However for the driver you need to Use 2 Set of H-Bridge if your using Bi polar motor or a Unipolar motor with the common not connected.

But I think for most part your i think that a Permanent Magnet DC Motor is suitable for your project and the control is so much easier since the requirement is to drive the motor one way.

[Reply #2 on:]

Do you need an accurately controlled speed, and/or control over start/stop positions? Both will be difficult to achieve since the cam will apply a large and varying torque to the shaft. Your best bet will be to use a worm drive, but if you need to control speed or position I suspect you will still need to provide some form of feedback.

[Reply #3 on:]

Hi, thanks for the replies.
I'll try to clear up your questions.

The operation of the cam looks like this rough drawing:
<img src="http://img13.imageshack.us/img13/8932/circularcamdriverandlev.jpg" alt="cam drawing" border="0"></img>



The item on the right is on a fulcrum itself the the power to "push it" is very low, probably in ounces even though the total weight is 25-35 llbs of the object itself.
I understand that the stepper motor offers great control and higher torque, but is usually spec on devices requiring bi-directional control. so I'm thinking that the additional flexibility it offers isn't really required here.
Additionally it requires a reset to a zero, doesn't it?

In this case, the motor turns (always) to the right and the cam lever controls position., so only motor speed (pot) control, low cost are real requirements.
Could someone explain the difference between brushless dc motor and permanent magnet DC, or are they one and the same?
If they are different, per your note below, I assume the brushless (with higher speed) would require some step down.
I would really like this to be driver either direct or direct via a bevel gear with no reduction.

Thanks for your comments in advance.
John S

[Reply #4 on:]

I understand that the stepper motor offers great control and higher torque, but is usually spec on devices requiring bi-directional control. so I'm thinking that the additional flexibility it offers isn't really required here.
Additionally it requires a reset to a zero, doesn't it?

In this case, the motor turns (always) to the right and the cam lever controls position., so only motor speed (pot) control, low cost are real requirements.
Could someone explain the difference between brushless dc motor and permanent magnet DC, or are they one and the same?
If they are different, per your note below, I assume the brushless (with higher speed) would require some step down.
I would really like this to be driver either direct or direct via a bevel gear with no reduction.

yes you have given me a good drawing but still lacking dimension. and yes for your application, your best bet is permanent DC Motor. follow the diagram in the web link http://bildr.org/2012/03/rfp30n06le-arduino/

[Reply #5 on:]

Could someone explain the difference between brushless dc motor and permanent magnet DC, or are they one and the same?

Not the same.  A brushless DC motor is actually an AC synchronous motor.  It's called a brushless "DC" motor because it's characteristics from the controls side come out to be the same as traditional brushed DC motor.  Brushless DC motors can do all kinds of fancy things like regenerate batteries when braking.  Segways operator with a Brushless DC motor. 

Brushed DC motors are going to be cheaper.  They have brushes, however, which will wear out.  DC motors like to run at high RPM, so you need some gearing to get down to where you want to be.  I recently got a Crouzet geared motor (82869013) from Allied Elec and I've been happy with it.  Not particularly cheap, but seems hard to find power without spending money.  Worm Drive will give you a big reduction so maybe you can use a higher RPM DC motor (check out QTCgears).  Stepper might work, but more fancy to control.

Seems what you need is to do is figure out what forces are at play in your setup and work backwards to find out what you need.