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## The Robotics Primer: Chapter 6

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By Adam Kroeker July 7, 2022 - 8:00pm

### DOF in the human hand

In my answers to the Food for Thought questions from Chapter 4, I calculated that there are eighteen degrees of freedom in the human hand. After reading Chapter 6 and learning about joints, I realized that I may have miscalculated the number of DOF in each finger. In my answers to the Chapter 4 questions I calculated that each finger has three DOF, since each finger can open and close, move forwards and backwards, and slightly move side-to-side. However, If I consider each joint separately, I calculate that there are four DOF in each finger except for the thumb. The joints closest to the palm of the hand allow the fingers to bend forward and back and move side-to-side a little bit, so these first joints each contribute two DOF to their fingers. The next joints up the fingers away from the palms only open and close, so they each contribute one DOF to their fingers. Four fingers have a third joint that also just opens and closes, so they also contribute one DOF to each of those four fingers. Finally, there are three DOF in the wrist, one each for the up-down, side-to-side, and rotation movements. Adding up the three DOF in the thumb, four in each finger, and three in the wrist, we get twenty-two DOF in the human hand. However, not all of these can be controlled independently. The two joints furthest up the fingers away from the palm on each finger except the thumb can only be opened and closed simultaneously. Also, it is very difficult to keep only the ring finger up when all the others are bent, but I can sort of do it if I use my thumb to hold down the other fingers.

### Joints in biological bodies

Rotational joints are much more common in biological bodies than prismatic joints. Knuckles, shoulders, hips, knees, elbows, wrists, and ankles are all examples of rotational joints because they facilitate rotational movement rather than the linear movement that is allowed by prismatic joints. One example of a prismatic joint in nature might be a bee’s stinger since the stinger needs to be extended linearly out of its body to sting someone. Another example could be a cat’s claws, since a cat can extend and retract its claws from its paw. Perhaps tongues have prismatic joints since we can extend and retract our tongues into our mouths. Maybe hair and fingernails have prismatic joints since they extend in a linear direction very slowly. However, the growth of hair and fingernails is different than the motion caused by a joint because a joint can move an effector without changing its size, whereas the linear motion caused by the growth of hair and fingernails requires the hair or nail to continuously grow longer, so the biological mechanism responsible for the growth of hair and fingernails is not a joint.

### Astronaut suits

Astronaut suits do not provide additional strength or teleoperation abilities to their wearers, but according to the NASA website, the suits include a tool called SAFER that has small propulsion devices that can be controlled by the astronaut to move around in space. Since astronaut suits can be worn and controlled by humans, they are a kind of exoskeleton.

### Backhoes

A lever-controlled backhoe is controlled by humans, and it does provide additional strength for digging and moving dirt, but it is not an exoskeleton since it is not worn by humans. Perhaps it could be an exoskeleton if instead of levers it were controlled by a device worn on the operator’s arm. If there were a sleeve worn by a human that sent signals to the backhoe, and the backhoe moved depending on the movements of the human’s arm, then it could be an exoskeleton.

### Robot exoskeletons

Robot exoskeletons that can be controlled by humans but can still make and act on their own decisions might be useful if the exoskeletons can move in ways that are not like the ways in which humans can move their bodies. For example, maybe there could be an exoskeleton that has a pair of legs with knees that bend backwards like a dog’s leg, so a human’s leg motions would not exactly match the motions that the exoskeleton’s dog-like leg could perform. If this exoskeleton could make and act on its own decisions, it could mimic the direction and speed with which the human operator is walking while making its own decisions about the exact trajectories of its legs and how its knees bend. Robot exoskeletons may also be useful for avoiding damage. An exoskeleton that could detect obstacles and hazards could simply stop moving if the operator were to accidentally steer it into a wall or off a cliff.

### Robot dogs

The two front legs of soccer-playing robot dogs are their endeffectors, since they are the parts that directly contact the ball. The dogs are not purely mobile robots but also manipulative robots because they can manipulate the ball with their front legs as well as move themselves around with their hind legs.