Arduino Stack Exchange is a question and answer site for developers of open-source hardware and software that is compatible with Arduino. It only takes a minute to sign up.
Sign up to join this community
I currently have a standing desk at work, and I thought a fun way to learn Arduino would be to automate it, as it currently uses a hand crank.
I would love some help determining what I would need for this task.
What kind of motor should I use? -Stepper motor? -High/low rpm? -High/low torque?
What sort of power would I need?
Looking forward to hearing people's suggestions!
I think this might end up being more of a fun way to electrocute yourself, but let's find out...
Energy required to lift an object with mass "m" an upward distance of "h" in the Earth's gravitational field, on the surface... E = mgh = m(9.8)h
So, to lift a 50kg desk one meter, we will need 50 x 9.8 x 1 = 490 Joules
How fast do you want to lift it? Let's say we want it to take 10 seconds - that will give us the power level we need. Power is in watts (Joules per second) - so if we want to lift the desk in 10 seconds we need 49 watts - let's call it 50.
Now, to produce 50 watts with a 12-volt battery, that would be 4.2 Amps. This is do-able, and it's not the high power level I expected for lifting a desk. Even if we throw in some efficiency losses, we're well within the safety zone and won't have to get out the high voltage equipment. So, I'm glad I did the math - my initial impression of this task was the joke at the beginning of the post.
So, you need a 50 watt motor... that's easy to find. Hell, why not go way over that and get a 100-watt motor... google that...
http://www.electricscooterparts.com/motors24volt.html - bang! Scooter motors! Nice!
So, that's an example of how you might figure out what parts to get - it all starts with basic physics and ends with Google (or Bing if you're one of those people). I simplified a lot, but you get the idea.
The other part of this task is to figure out the control system. I would probably not use Arduino to do this. I would have three switches - one "directional" switch to run the motor, and two sensor switches to disconnect the directions of the motor at each end of the track. I think that's how automatic power windows in cars used to work, which explains why if you didn't hold the button down long enough, it would immediately instead of continuing to the other end - it was still sitting on the sensor.
Sticking an Arduino in the mix and having it read the sensors would just waste power all day long, but you could do it that way. It's the same idea - just mount sensors on the track, which get tripped when the desk rolls up to that spot. Coding wouldn't be that tricky - when the user activates the "change button" or whatever, check which sensor is currently active, and move the desk the other way until the other sensor activates.
You're going to need a good power supply for that motor - a 24-volt 200-watt power supply if you like to over-engineer like I do. You'll need a relay to control the motor - it needs to handle 24 volts/200 watts. Then you'll need some kind of activation button, and two sensors. You want some kind of sensor that is definitely going to be activated when the desk is in position, or you risk over-running your target, binding the mechanics, and burning the motor, and possibly the office as well. So, I would choose some kind of mechanical sensor, not a light sensor or something like that.
This should get you started on finding those parts... https://www.sparkfun.com/categories/145 - these are all kinds of buttons and switches - you should be able to find something that you can mount to the track which will activate when the desk runs into it. Search on this site for the other parts - they have the relays and stuff too.
Final warning though - this may be too complicated and dangerous for a first project. At 24 volts and 100 watts, we are getting into dangerous power levels. But, these are common power levels in consumer equipment. So, if you know what you're doing you should be safe. If you don't, I strongly suggest getting an experienced electrical engineer to look at your designs. Also, you probably can't install this device in an office - there is an insurance reason that it hand-cranks, your company isn't just being cheap.
What happens if someone bumps the desk and it becomes active when you're not around? What happens when the power goes out? What happens when the power comes back on? Do we have enough fuses in the right places to shut this system down before it catches fire? You need to consider all that.
For initial playing, and possibly for a final solution, battery powered electric drills with external powering are an excellent source of reasonable wattage well geared down motors. Drills are often discarded when the batteries die and may be available free or at lo cost.
For relatively low duty cycle use 9V drills operate OK on a small 12V battery and drills rated at up to 18V usually operate well enough on 12V.
Use of a 12V lead acid battery charged by a trickle charger makes high current peaks easy to accommodate. Almot any condition battery will do - one which is too dead for eg motorcyle use probably works well enough here. Floating 12V lead acid batteries at 13.7 V allows them to be permanenetly connected to the "charger". A power pack can be used without a battery if enough current is provided.
Battery powered drills typically have a two stage epicyclic gearbox giving typically hundreds of RPM output speed. If desired you can use the supplied switching which has a DPDT mechanical reversing switch. If you remove the mechanical reversing switch you can feed voltage directly to the motor. Changing polarity changes direction.
Dismantling the gearbox can be educational but may be a one way journey. Beware a flood of ball bearings.
