Science Buddies: "Ask an Expert"

For the project I Like to Move It: Motorizing a Robot Hand after I've programmed the servos how do I control them?

I'm not exactly clear on what you mean by "after I've programmed the servos". This is what I'd do if I were taking that project and motorizing it.

1) Start by building the hand/grabber/whatever out of the straws. Attach the rubber washers and manipulate it. See how far you have to pull each of the strings to make it do what you want it to do. Maybe experiment with some actions and make a table of how far you'd pull each of the strings to accomplish the goal. You clearly wouldn't just pull all of them as far as you can. Think of a few actions for your device to accomplish and plan out what the motors will have to do with the strings.

2) Will you be planning to control each finger individually? You might have found that two of them will always be doing the same thing so maybe you can tie those two strings together. Or just have a separate motor for each. That's up to you.

3) Attach the device to something big enough to mount all the servos and come up with mounts. I like to glue small wooden blocks next to the ears and screw the ears down. That makes the servos easy to remove.

4) Look at the servo horns and decide if they're long enough to provide the amount of string pull you need, or will you need to augment them with a longer section.

5) How will you plan to power the servos? What computer will you be using? An Arduino UNO can control up to 12 of them and you probably only have 4 to 6. A Raspberry Pi iso usually used with an add-on board that supports up to 16. Start by deciding what you prefer for a computer. The next problem is power. A Raspberry Pi can power *1* servo. An Arduino can manage two. Beyond that you have to provide 5 or so volts from somewhere else. AA batteries work fine.

6) To connect the servos to the computer, you need one output pin with PWM for each servo. As I said, the Arduino can manage all 5 or 6 of them. On the Raspberry Pi, the preferred solution these days is the Adafruit PWM controller (http://www.adafruit.com/products/815) which works on either the Arduino or the Pi. I'd suggest you consider that because it also solves your power connection issues. Adafruit's website has guides for connecting this board to each of the two computers. It will also work with anything else that handles I2C if you're so inclined.

7) Tie the strings from the hand to the servos and write some code. You had already figured out what kinds of string movement would make the hand do what you want, so start writing out code to make that happen.

Please write back if you have any other questions.

Howard

I meant after i've written the code how do i control the hand so that a particular moves.

The code you're writing will have to get commands from something. What did you want you hand to do? Grab someone's wrist when they reach into a cookie jar? You can put an ultrasonic sensor (https://www.sparkfun.com/products/639) nearby to detect the prank victim's hand as they stick it in the jar. Do you want it to mimic the movements of your own hand? Put a collection of bend sensors (http://www.adafruit.com/products/1070) on your fingers. Once your computer knows what your hand is doing it can have the robot hand do the same [an old vintage PowerGlove would be a cool controller!]. A simple controller (https://www.sparkfun.com/products/10330) would let you push buttons and control the fingers.

One student said they tried to make their hand do ASL. Another set up two of them to play a piano. What you do with yours and how you control it is probably the most fun part and possibly the most challenging. How did you want to show it off? Maybe if you can explain that we can help you come up with some ideas for sensors.

Howard