Robot Hand

In this article we will be creating a robotic hand out of household objects and learning about how we can optimise grippers. 

Firstly we should understand the concept of degrees of freedom(DOF). In robotics a robot's DOF represents the number of independent  joints that can be rotated within the robot. For example if we use our finger as an example, each joint has one DOF and in total there are three DOF in our finger. Robots that have equal DOF and actuators are called a fully actuated. Cases where there are more DOF than actuators can be categorised as underactuated. 

The good thing about underactuated robots is that they cost less to produce since you use less actuators. In the case of the robot we are making today, each finger has 3 DOF and we have 5 fingers. If we multiply that, you would need 15 motors, but with an underacutated system we are able to do it with 5. Due to the nature of the design it also allows for the gripper to be adaptive or compliant. 

Compliance is important in robotics when grasping objects with uneven or varying shapes. The design in this article is based on a human hand and reacts in the same way as its biological counterpart. The finger design allows for the other joints to compensate if one joint is restricted. This allows for each finger to wrap around the contours of an object. We can see below that the finger design is adaptive and can conform to grip many different objects. 

Now we know a little about the background lets get onto making it!

Materials 

• EVA foam mat:
  • EVA foam mats are usually sold as foam tile flooring for children’s rooms.
  • Make sure you check the sturdiness of the mat, sometimes the mats are less dense and more squishy. The mat has to be able to keep its shape when the tension increases on the finger strings. 

• 5 x Bubble tea straws:
  • Bubble tea straws are recommended since they are stronger than normal straws and will hold their shape when compressed by the tension of the string(also it's an excuse to drink bubble tea)
• 5 x MG90S servo:
  • If you are going to use other servos take the following into consideration:  
    • Servo must be able to rotate 180degrees
    • Dimensions must match 1.2cm * 2.2cm to fit the template 

• Printed hand template (Download it here) 

  • Make sure you print it out with the "fit to page" setting.

  • Check with a ruler to confirm the template is to scale.

  • Since the foam stretches you have a bit of leeway with the measurements, so if it’s a few mm off don’t worry.

• Hot glue gun and hot glue
• Permanent marker 
• Fishing wire
• Scissors 

Kitsi Blocks :

• 2 x MotorDriver Blocks
• 1 x Wifi Block
• 1 x PowerStation Block

Make fingers:

1. Line up the the straw with your index finger and mark out where each of the joints are. Also add a mark 3cm away from the last joint of the finger. Cut the straw at the 3cm mark. 
   
2. Try to flatten out the straw slightly and draw triangles at each of the joint marks.
   
3. Squish the straw at each of the triangle marks and cut out the triangles. 
   
4. Cut a length of fishing line twice the length of the Straw
5. Loop the fishing line through the tip of the finger twice and tie a knot. 
   
6. Thread the rest of the fishing line through the finger. 

Now if you pull the string your finger will bend

Repeat this process 4 more times to get a whole hand worth of fingers. I also changed the length of my finger segments for my thumb straw to match my thumb. 

Assemble hand

1. Cut the template out. Make sure to cut out each of the holes for the servos and thumb hole.
   
2. Trace the template onto the EVA foam and cut out the outline. 
   
3. When cutting out the holes for the servo motors try not to go out of the lines. You want a snug fit so that you don't have to use glue. Same goes for the thumb hole. Try twisting a sharp pair of scissors into the foam to create the thumb hole. 
4. If you cut out the servo holes correctly you should be able to squeeze them into each of their holes. Follow the image below to orient the servo motors. The first four, controlling the fingers should be facing the front. The thumb servo (under the thumb hole) should face the back. 
   

   Front

   Back

5. For each of the four fingers glue the sides of the straw to the foam channels. Make sure to put the fishing line for each of the fingers on the front side of the foam when gluing the straws in. For the thumb orient it so that it  bends towards the middle finger and glue it in with hot glue. Also make sure that the fishing line for the thumb goes through the thumb hole to the back of the hand.  
   

Logic Maker & configuring your hand

1. Connect up your Wifi Block, PowerStation Block and two MotorDriver Blocks. 
   
2. In your kitsi creator, configure each of the motors in the motor driver block properties to be servo motors. 
   
3. Once your MotorDriver Blocks are configured use the logic maker to send the initial zero angle to the all the servos. Select the setting "Rotate to angle" on servos 0-3 on the first block and servo 0 on the second block. When you select the drop down menu on the action block you will see a separator telling you which block's servo you are connecting to (highlighted in the second photo). Send angle 0 to all blocks
   
   NOTE: If you don't want to use two MotorDriver Blocks you could make a hand with 3 fingers and a thumb instead. The functionality will still be the same. 
4. Once all the servos have been set to their zero position put their servo horn on pointing up towards the fingers. Most of the time your servo will come with a few different types of servo horns. Use the half servo horn pictured below. This is because if you use a full one, it will interfere with the rotation of the other servos as they are all in close proximity. 
        
                 No Servo Horn                  With Servo Horn in upright position 
5. Once your servo horn placed on it the zero position tie each of the finger's fishing line to their corresponding servo horn. Make sure to tie it to the top hole of the servo horn and the fishing line is taut.  
6. With our hand now configured correctly we can create logic to open and close the hand. Use the same logic as the zero position but change the value to 180 instead. By doing this it will pull the string and close the hand. 
   

By clicking between the two play gates we can open and close the hand!

Improvements 

-The hand has difficulty picking up objects on flat surfaces. I think using some sharp straws will act as nails to get under to object and allow you grasp easier. 

-Since the straws themselves are quite slippery they are not very good at gripping onto things. Adding some grip on the contact surfaces, such as a hot glue texture or fabric will help hold things.