Feasibility of using stepper motor for long term, high precision use

Projects General Guidance
1

Hello, and thank you for taking a look here.

I'm trying to wrap my head around getting started on a new project. The problem is that I'm trying to portion 1 oz bits of frozen fruit efficiently and quickly. My idea is that the fruit will funnel into a 1-oz-of-fruit-sized compartment attached to a wheel. Some sort of motor will turn the compartment/wheel, dropping the fruit down a chute. Ideally, the wheel would turn in one direction continuously and move in quarter rotational steps. A button will be pressed and a microcontroller will send the signal to turn the motor and drop a specific amount of portioned fruit down the chute.

A continuous servo is out, there is no precise angle control. A regular servo won't turn continuously and the wheel would have to move forward then back for each portion. I'm considering a stepper motor but I've never worked with one.
Some of the project parameters include:

  • reasonably sized. The prototype will have two motors but the end product will have 6.
  • must operate in cold temperatures. The fruit must remain frozen so everything must operate at low temperatures (-5 degrees F is what I believe the fruit sits at in the freezer).
  • must operate for extended periods of time. Power from the wall is obvious, but there can be no issue with long term use in the range of 12 hours straight.
  • be able to withstand the force of 5 lbs of fruit on top of it and still rotate reliably.
  • reasonably priced. At least for the prototype. I might be given money to finish the project

If a stepper motor seems the best choice, does it fall within the project parameters? Any suggestions on a motor or at least an estimation of required specs? What should I use for power that will also run the arduino? I think I've covered all the necessary information here, but if you think I missed a consideration, please leave a comment. Any ideas on alternatives are welcome, too. Again, thank you for looking and commenting.

2

You may get some useful info in stepper motor basics.

It sounds like you want a rotating disk for handling the fruit. That suggests to me that there is no need for the motor to carry any of the weight of the fruit.

You need to figure out the torque that will be required from the motor. There are some ideas how to do that in the link.

Stepper motors are usually robust so they should have no problem with long term use.

HOWEVER ... stepper motors run very hot - perhaps hot enough to be uncomfortable to touch - so you may need to have some way to prevent that heat from affecting your fruit. It is common to use toothed belts between stepper motors and the things they drive. That could allow the motor to be a distance from the frozen food.

You will need some method for setting the motor at the ZERO position when you start the Arduino.

I suspect the real challenge will be the mechanical design - not the programming.

...R

3

Any motor will work providing that the gearing is correct. You'll need a closed loop control system with position sensors.
Fruit is normally sold by weight, not volume, so you'll need a weighing mechanism instead of a measuring cup.

Building food handling machinery is a non-trivial exercise. See the "How Its Made" series on Youtube for dozens of examples. Debugging electro-mechanical problems in an icing environment is not fun.

Your primary concerns should be operator safety and hygiene.
Food handling equipment is often cleaned by unskilled people using pressure washers and caustic solutions therefore everything must be sealed against moisture ingress.

Equipment of this type is usually made from 316 grade stainless steel and bearings must use food grade lubricants.

4

Have a look at the "How Its Made Frozen Fish Products" video on Youtube

My guess is that almost all the motors, actuators and possibly sensors could be driven (and sensed) using compressed air, thus enabling you to house all the control electronics, valves and pressure sensors at a remote location at room temperature. Certainly not cheap, but easy to clean and maintain.

5

stepper run HOT.

for an exact quarter turn, you could use a solenoid with gear, like an escapement.

a simple DC motor with an encoder wheel is an industrial servo. works similar, but you add points like 4 points to show an exact location. the minor points are for rotation speed as well as step location. pretty easy with an Arduino.

you can use a DC motor that has feedback from sensors, either magnets, or reflective, or even LED's

lots of ways to skin this cat. the stepper is the only one I would not recommend.

6

knaving:
A regular servo won't turn continuously and the wheel would have to move forward then back for each portion.

Why would this be an issue?

You stated earlier that:

knaving:
The problem is that I'm trying to portion 1 oz bits of frozen fruit efficiently and quickly.

While there are any number of possibilities to handle this problem, you're also going to want something reliable over many actuations. If you do use a servo, it likely won't be one of the standard hobby variety, as they won't be able to cope with your environment over a long period of time with many actuations. Instead, you will need to develop your own servo system (such a position feedback system isn't that complex, actually - you don't need precise positioning, either, so don't go down any PID road, whatever you do).

Have you thought about a screw auger system, perhaps with a weight measurement cutoff? Basically, you would have a "dump bin" into which the screw auger would move the bits of fruit. The dump bin would have some kind of sensor to measure the weight/mass of the fruit as the bin fills up; when the weight/mass reaches the cutoff value, the auger is stopped, and the bin is tilted to empty it of the fruit.

Finally - if you need this to be high-speed, and accuracy isn't needed to be super-precise, then you might forgo using any kind of automation to solve the issue, and instead just go for a plain mechanical solution. Do a google search for "mechanical movements" - there are a couple of books available out there that will have a movement that you can apply for your solution, most likely:

507 Mechanical Movements

This book also has a website, with some of the mechanical movements animated to give you a clearer picture of how they work:

Here is another book, with many more movements:

1800 Mechanical Movements

Most likely, you will want something akin to a Geneva drive:

7

Professionals use a "Multihead Weigher" to give the closest target weight.

Essentially it's an unmetered gravity feed into multiple weighing trays then a computer selects the best combination to give the closest target weight.