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Topic: Linear actuator setup (Read 3687 times) previous topic - next topic

Marcoco

Hi,

For quite a big project we have to make a lot of linear actuators.
The budget for each has to be around 25 dollars max.

I have made a sketch of a setup im trying now.
But the stepper i am using (mercury motor SM-42BYG011-25) doesnt have enough power around high speeds. It just stalls.
Also using steppers is quite inneficient because of using all seperate drivers.
But i have never worked with dc motors. The position the cart is in has to be pretty precise. Is there a way to control that with dc motors.
The speed it has to turn is pretty high too but maybe there are threaded rods wich have fewer threads per cm. The fewest i have found is 8 per cm.
The movement from top to bottom is around 50 cm. So that would be 400 turns for going from the top to bot position.
Ideally we would want that to happen within less than a second, so that would mean 24000 rpm.
I don't think that is very realistic.

The reason we chose this kind of solution is because there is going to be a elastic material pushing down or pulling up the sled depending on the sled position.

And controlling around 150 dc motors, is that really difficult or doable when you build custom hardware?
We have acces to a fablab and we can outsource some things if it isn't too expensive.

So to sum up:
- What motors would be the best choice?
- Are dc motor positions controllable without expensive equipment.
- Are there threaded rods with fewer threads per cm than 8. 

Boffin1

I made similar prototype a few years back, and I used a DC motor, and had black and white stripes on the pulley wheel to control the position. ( counting how may turns of the threaded rod )

Mine was an analog system - before Arduino was heard of - but it should be easy nowadays.

I dont know enough about servos and micros, but it could be the way to go, I think I saw an Arduino project with PID to avoid overshoot etc.

terryking228

Quote
But the stepper i am using (mercury motor SM-42BYG011-25) doesnt have enough power around high speeds. It just stalls.


Are you starting the motor at the high speeds, or accelerating through low to high speeds...???  Steppers will not start immediately at high speeds..

See: http://www.open.com.au/mikem/arduino/AccelStepper/
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PeterH

With that many motors, I would have thought that conventional DC motors would be easier to get working than stepper motors. For position sensing, you can either count rotations of some part of your drive mechanism, or use a linear pot. The screw thread+nut design can be made extremely strong (and is cheap) but very inefficient which will push up your power requirements. Is power consumption a concern?

What sort of positioning accuracy are you aiming for?

What sort of magnitude of forces will the actuators need to apply/resist?

Do you have any packaging constraints for these actuators i.e. any restriction on how big they can be or where the mounting points need to be?
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dc42

With 150 motors, I think your biggest problem is going to be the wiring. I suggest you use either one microcontroller to control each motor, or perhaps one to control a small group of motors. You could probably control a single motor with an ATtiny85 and communicate with it over SPI or I2C.

As for motors, here are some pros and cons:

1. DC motor:

+ High torque
+ Probably cheaper than a stepper
+ Needs only one H-bridge to control it
- Brushes wear out eventually
- Need a sensor to measure position or count rotations
- Probably not safe to drive it against the end stop because of the high stall current

2. Bipolar stepper:

+ Safe to drive it against the end stop to establish the initial position
+ No position sensor needed
+ No brushes
- Lower torque than DC motor
- Needs 2 H-bridges (but you can get dual H-bridge chips)

3. Unipolar stepper:

+ Safe to drive it against the end stop to establish the initial position
+ No position sensor needed
+ No brushes
+ Needs 4 mosfets or transistors to drive it
- Lower torque than DC motor

See also Terry's advice on accelerating steppers to get them to work at higher speeds.
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Marcoco

#5
Nov 16, 2012, 06:08 pm Last Edit: Nov 16, 2012, 06:47 pm by Marcoco Reason: 1
Thank you for all your replies.
We probably have to decide against using the threaded rods(ballscrews or acme screws as i found out they were called).
The cost of them is just too high. Around 30 dollars for a 1 meter ACME screw without the bolt is too much.
So we probably have to look at some other cheaper solution that still uses 150 motors but with less power on them.

Am busy with testing l293D chips atm. Got the dc motor doing exactly what i want but trying to find a way to see how many revolutions the axle has made.  Was looking at rotary encoders but the guy from the hardware store said thats the wrong way to go.
He said to me what i should do. I had no idea what he was talking about. He noticed that and then tried to sell me an 100$ learn electronics kit and didn't want to explain further.
Hall sensors might be something i could try but i dont know how precise that is and if it will work at all on a small axle.

Anyone got any other suggestions?

zoomkat

You could attempt to DIY your linear actuators using inexpensive threaded rods and nuts/couplings from your local home improvement store. If much force is involved, then thrust bearings would probably be required. A ~$10 6v Black and Decker cordless screwdriver from walmart might be a start for the motor part. Below is a small DIY actuator that might provide some ideas.

http://www.youtube.com/watch?v=1fUwPSn32T8
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PeterH


Anyone got any other suggestions?


You haven't explained what sort of travel, positional accuracy or speed you need to achieve, or whether you have any packaging or power constraints. With a hundred and fifty of these things to make and operate, it's worth putting the thought in up front to get the cheapest and most effective solution available.
I only provide help via the forum - please do not contact me for private consultancy.

Marcoco



Anyone got any other suggestions?


You haven't explained what sort of travel, positional accuracy or speed you need to achieve, or whether you have any packaging or power constraints. With a hundred and fifty of these things to make and operate, it's worth putting the thought in up front to get the cheapest and most effective solution available.


The travel is around 50 cm. It has to be able to do that in about half a second.
The power that will be pushing down or pulling up will be around 3kg.
So we will need specialized threaded rods which are quite expensive. Even when ordering from china.
The accuracy needs to be quite precise. We need to know the position of the bolt at all time.

PeterH


The accuracy needs to be quite precise. We need to know the position of the bolt at all time.


Does that mean you need the position to the nearest inch, or the nearest thousandth of an inch?

Is that 3Kg per actuator, or shared across the whole set?
I only provide help via the forum - please do not contact me for private consultancy.

Marcoco



The accuracy needs to be quite precise. We need to know the position of the bolt at all time.


Does that mean you need the position to the nearest inch, or the nearest thousandth of an inch?

Is that 3Kg per actuator, or shared across the whole set?


Its 3kg per actuator.
And the installation is going to run for a few weeks. The accuracy doesnt have to be spot on so an half an inch difference would be ok but it can't become larger than that and not grow over time.

PeterH

Lifting 3 kilos 50 cm in 0.5 seconds requires a lot of power. Just starting and stopping the mechanism within half a second will be a challenge because the motor will either be big and heavy, or moving extremely quickly. I'm not at all confident that you will be able to build/buy them for under $25 each. I suspect your best bet will be to get a car window lifter assembly and see if it gives you the speed, force and travel you need. If you're buying in bulk you may be able to negotiate a price from a vehicle dismantler that brings them within your budget, but obviously you'd only want a single one for prototyping.

Window lifters don't usually come with any position sensing, so you'd need to add a sensor. A simple potentiometer should do the job fine at the resolution you're talking about, either set up as a string pot or using a pushrod/crank to get the travel. I don't see any problem getting the sensing done - it's the actuator side that will be difficult.

150 of these is going to take a lot of current at 12V, I hope you're preparing to install a substantial power supply - as a ballpark figure, I suspect you'll need in excess of 10 KW.
I only provide help via the forum - please do not contact me for private consultancy.

Riva


Lifting 3 kilos 50 cm in 0.5 seconds requires a lot of power.

What about using a funicular mechanism to reduce the weight. Then the major problem will be inertia.

PeterH


What about using a funicular mechanism to reduce the weight. Then the major problem will be inertia.


I may be wrong, but I have got the impression that the 3Kgs could be in either direction and wasn't simply a static load. I agree, if it's a static load then a simple counterbalance system could be used to reduce the power requirements (at the expense of increased inertia).
I only provide help via the forum - please do not contact me for private consultancy.

Riva



What about using a funicular mechanism to reduce the weight. Then the major problem will be inertia.


I may be wrong, but I have got the impression that the 3Kgs could be in either direction and wasn't simply a static load. I agree, if it's a static load then a simple counterbalance system could be used to reduce the power requirements (at the expense of increased inertia).

But reducing the net weight form 3Kgs to just about zero means smaller motor/power needed to raise/lower camera. Just noticed the OP had written...
Quote
The reason we chose this kind of solution is because there is going to be a elastic material pushing down or pulling up the sled depending on the sled position.

So probably not suitable anyway as elastic is not linear.

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