Motorizing a Robotic Hand
Howard Eglowstein, Science Buddies
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AbstractImagine how cool it would be to have your own motorized robot hand. Well, stop imagining and turn that daydream into reality! Start out by designing and building a robot hand. The Science Buddies project Grasping With Straws: Make a Robot Hand Using Drinking Straws shows you a simple way to make a robot hand with drinking straws, or you can design a robot hand from any other materials you think are suitable. Your hand design will need sewing threads, or some other mechanism, for motors to act on and move, causing the fingers to flex. See Figure 1 for an overview of the key materials for this engineering project.
Figure 1. For this robotics engineering science project, you will need a robot hand (similar to the straw robot hand shown here or any other design you fancy), a microprocessor board like the ArduinoTM shown here, and several servo motors, such as the one shown removed from its box.
Once you have your robot hand, you will need to add small servo motors. Servo motors are capable of doing all sorts of useful work, as they do every day in toys and household appliances. For instance, your DVD player ejects discs using servos; toys employ servos to steer (cars) and walk (robotic pets). The list is almost endless. You will need one servo motor for each finger and movable joint on the robot hand. Servos come in many shapes and sizes, so you will have to think about what might work best for your hand design. A good starter servo is the standard-size Futaba® S3003 servo sold for radio-controlled cars (available from Amazon.com). You may also get some ideas and suggestions at a hobby store. You can read more about servo motors in the Science Buddies Introduction to Servo Motors.
Make sure your robot hand is attached to something that can support all the necessary servo motors. We'll refer to that as the "robot arm." When you are ready to mount the servos to the robot arm, look at the round plate or bar that comes attached to the servo motor shaft. This is the servo horn. Consider how this will rotate when the servo moves, and mount the servos so that one of the holes in the servo horn will get closer to or farther from the hand as the shaft rotates. Tip: It is best if you can arrange things so that the threads (or other control mechanism) from the robot hand do not cross. Servos usually mount with screws into a servo mount, but you may find it easier to glue wooden blocks to the robot arm and screw the servos to those. We do not recommend gluing the servos directly to your robot arm assembly.
To control the servos, you will need a hobbyist microprocessor board. Boards of this type include all of the circuitry you need to make simple computer projects and program them from your personal computer. An ArduinoTM is a popular board that uses Atmel® processors, is easy to program, and has a lot of widely available help resources (see Bibliography). The ArduinoTM UNO is one of the most popular boards for getting started. You can learn more about Arduinos and how to use them on the Science Buddies How to Use an Arduino page. You can also purchase the Science Buddies Electronics Kit for Arduino. Another good alternative is a board based on the Microchip PIC®. One good PIC® board to consider is the SBC44B from Modtronix.com. As you get more comfortable programming microprocessors you may find the PIC to be a more capable processor than the Arduino. Whichever board you choose, you may find it easiest to connect the servos to your board, write a short test program to activate the servos, and see how they move before you attach the robot hand threads (or other control mechanism) to the servos. Free Microsoft Windows® software for programming an ArduinoTM is available on the web and Microchip offers free PIC® programming software on the at Microchip website. Tip: When attaching the threads, you want to have little or no slack but not have them pulled so tight that the fingers of the robot hand are curled.
Now you are ready to write the code to make the robot hand move the way you want. You may have to experiment with different amounts of rotation so that you don't pull the threads too hard. Go slowly and try different values until things work. Learning a little about pulse width modulation, or PWM, may be helpful. A logic analyzer and/or oscilloscope may also be helpful tools for troubleshooting if you have access to them. What surprisingly human-like actions can your hand perform? Can it play the piano? Pick up and sort small items? In the end, what fun and useful things will your robot hand be able to do?
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