# HELP--what type of motor?

Hello,

this is my first post here. Should've got involved years ago, but via Raspberry Pi ended up here eventually.

I need help selecting a motor/drive. I want to control a sliding door that controls how much fertilizer flows out of the hopper bin above. The metal slide is probably 5 x 10cm, and moves with little effort.

My idea is to control the opening with 3 buttons (1. fully closed 2. open for light spreading 3. open more for heavy spreading).

I have already written a little code and it works in conjunction with some buttons on a breadboard all of which is hooked to a little servo. The problem with the servo is that the motion is circular, but the metal slide on the fertilizer needs linear movement.

My first thought was to use someone's 3d print in conjunction with a continuous servo. However I've had trouble controlling the continuous servo. It needs to rotate a certain amount in a certain direction and then stop.

I've also seen someone use a stepper motor, but their setup only indirectly opens the metal slide on the fertilizer. (it pulls the manual lever)

So finally I though I'd just buy a linear actuator. But now I'm confused. Do I need any linear actuator? Do I need one with a potentiometer for position feedback? Or do I need a linear servo? Is there even a difference? :o

Remember that a crank can convert rotary motion to linear motion.

Paul

Yes, I had thought of that also. Like a steam locomotive drive wheel and piston. However the whole reason I started this project was to introduce some precision to the amount the metal slide opened. If I try and fabricate something it'll be a lot of work and probably less precision than if I just buy something from Amazon.

The metal slide is probably 5 x 10cm, and moves with little effort.

Does it slide 5cm, or does it slide 10cm? Or some other amount?

Define "little effort" in units of force. Measure this if you have to.

always_innovating:
Yes, I had thought of that also. Like a steam locomotive drive wheel and piston. However the whole reason I started this project was to introduce some precision to the amount the metal slide opened. If I try and fabricate something it'll be a lot of work and probably less precision than if I just buy something from Amazon.

I cannot understand your last comment. No matter what or how you intend to open and close the gate you are going to have fabrication to do for mounting the mechanism and ensure it can freely move to open and close the gate. Then you will have to fabricate some way to hold the Arduino and associated controller and power supply.

Paul

1. The size of the metal piece that slides to cover the opening is 5x10cm. It slides along the axis that is 5cm long, and it probably needs to have 5cm or so of movement.

2. Little force- i would guess a 50-100g weight would pull it open if the direction were vertical rather than horizontal. I can easily move it in and out with my smallest finger, although with fertilizer loaded there might be more friction.

3. Fabrication. I like doing the fab, but I dont want this project to be how to fab a rotary motor into linear, I want it to be about making an arduino controlled fertilizer spreader.

"Little" and "guess" are not measurements. You need to know the figures to choose the correct machinery.

I can easily move it in and out with my smallest finger, although with fertilizer loaded there might be more friction.

Obviously there will be more friction, we must know how much before we can start... If lucky its a simple servo+linkage job, but with larger forces a different approach might be needed. To decide the
numbers are needed.

always_innovating:
My first thought was to use someone's 3d print in conjunction with a continuous servo. However I've had trouble controlling the continuous servo. It needs to rotate a certain amount in a certain direction and then stop

I suspect you should NOT be using a continuous servo. A regular servo can be instructed to move to a particular position.

...R

A "continous rotation servo" is not a servo, its a motor + driver only, no feedback.

Measured by a spring scale it looks like 1-2N. I think if I plan for not more than 4N I'll be ok.

Lets say 2N, and a servo arm moving 120 degrees has to move 5cm. That's about 3cm radius, so 0.06Nm
torque needed. Small hobby servos tend to have about 0.15Nm stall torque, so simple, hobby servo with 3cm arm ought to do it.

You can buy bell cranks , ball joints and bits and pieces from a good model aeroplane/boat model shop , or from eBay

Do I need any linear actuator? Do I need one with a potentiometer for position feedback? Or do I need a linear servo? Is there even a difference?

linear actuator is powered by a DC motor. you send it voltage, probably through an H bridge

linear servo is powered by a DC motor, but controlled by PWM. you arrange power directly to it, and control it via the Arduino

if you smoke the H bridge in a linear servo you replace everything. you can't substitute a newer bigger better. I prefer the extra flexibility of discrete components. I would go linear actuator + H bridge of my choice, rather than linear servo