SparkFun Forums

[BringontheDaemon]

Hey Sparkfunners! I'm an Arduino newbie looking for help choosing the best components for my project and some general guidance.

I'm building a public art piece for my local library. It will be an interactive fiction terminal the player reveals by pressing a glowing button embedded within a book displayed on the library shelves. The button will open a terminal and keyboard bay below by rotating them 90° and revealing the hardware underneath. Players will then be met with a prompt and custom game which introduces them to IF and text adventures.





I have the module bays built, balanced, hardware installed, and an Arduino handy. I'm having difficulty selecting the exact components so I can get my hands dirty coding and move forward. I've been looking to meet these requirements throughout the design process:

1. Can the motor supply the torque to rotate the module? (I don't have a good way of measuring this, but it's definitely more than average hobby motors)
2. Is the motor a modest size? (can fit within a 6"x6"x6" space)
3. Is the motor less than or around $30?
4. Is the motor not crazy loud? (it's in a library after all. It will still be insulated by books, which should help.)
5. Is it easy to control? (I don't have the experience necessary here, but get the impression stepper motors are more complex over DC motors to control, for example)
6. Is it compatible with an Adafruit Motor Shield v2? (This isn't absolutely necessary, but there's quite a bit of documentation and existing examples for using this shield. It would go a LONG way helping me learn to write the code for it if there's great documentation and examples).
7. Can both motors be powered and run off one shield? (I'm looking to control the motors to rotate both module bays at the same time).


I'm currently looking to use a Sparkfun Standard Gearmotor 6RPM (3-12V). It has some excellent torque (Stall Torque: 613 oz-in), is within my price range, and DC motors seem like a decent fit. It seems like it would take approximately 2.5 seconds to rotate the 90 degrees, which I am more than happy with. I'm still lost in a few areas:

1. Is this motor a realistic fit?
2. How can I tell if the Adafruit motor shield can power two of them at once?
3. How do I calculate the type of wall wart to actually power them? (I intend to power them from an outlet nearby, since batteries would require constant monitoring.)
4. If the torque is adequate, can I connect the drive shaft directly to the pivot rod on the module? Is this wise? What is the name of the component I would use exactly?

Any advice would be greatly appreciated.

[HardwareHaquer]

1. Is this motor a realistic fit?

The motor is going to make some noise. You can minimize this by enclosing it and putting a rubber gasket between the motor and the plate you are mounting it to. Also you can do something similar at the points where you mount the plate to the rest of the enclosure. I don't think you will be powering it frequently enough to worry about heat dissipation.

We don't know the dimensions and weight of what you are making so it is hard to know if it will have enough torque.


2. How can I tell if the Adafruit motor shield can power two of them at once?

The stall current listed on the motor spec and the current rating for each bridge on the driver are what let you know if you will be able to use the motors with the driver. In your case the motors have a stall current of 0.5A. It looks like each of the 4 bridges can handle 1.2A continuously and a spike up to 3A. (you will only be using two bridges one for each motor) The adafruit driver should be able to handle the load of both motors without issue.

3. How do I calculate the type of wall wart to actually power them? (I intend to power them from an outlet nearby, since batteries would require constant monitoring.)

The stall current of each motor is 0.5A so the maximum draw would be around 1A. I would suggest you use at least a 12V 2A power supply just to have plenty of overhead. There is little additional cost.

4. If the torque is adequate, can I connect the drive shaft directly to the pivot rod on the module? Is this wise? What is the name of the component I would use exactly?

The drive shaft is a D style meaning that it has a flatted side allowing you to tighten a set screw. You need a shaft coupling like this one: http://www.mcmaster.com/#5395t111/=14n2dx4 This will give you the ability to attach another shaft that could then be secured to the object to be rotated in any manner of ways.

This said how much weight will be moved by the motor? How big is what you are moving? Half a keyboard length or 6'? If there is much weight at all I would suggest finding some small rollers or a lubricated bearing surface(sparkfun sells some ball casters that could work) to support and balance the weight.

Also you will need to have mechanical stops (switches) to stop the motor when it gets to the end of its run.

[BringontheDaemon]

Wow, thank you for such an excellent response!

I took all of your advice and finally picked out some components. I intended to use some limit switches to stop the motors on open and close. I'm thinking this setup will have adequate torque, but I'll find out if I need to add gears or balancers after I try a direct rotation with the initial setup.

I'll let you know how it goes! Thanks again.

[Valen]

How much force do you need to exert on the left light blue or yellow leg to get that side to lift up vertically? You should be able to find force gauges in hardware stores, measuring newtons (lb (pound-force) or oz (ounce-force) if in the US; oz. is 1/16th of lb). It contains a spring which extends based on the force and indicates it on a scale. Then measuring the horizontal distance from the hinge to the point of lifting you get the arm needed to calculate the torque. Force x arm = torque.

[Valen]

Wait a minute, is the pivot point a vertical axis, or horizontal one? Maybe I misunderstood the animation.