Background Information

All machines, including robots, regardless of their complexity, are nothing more than an assembly of interacting simple machines. The major changes in machine development during the past century have, surprisingly, not been in the physics of their design.

The development and improvement of robots have been due entirely to advances in the science of materials, (stronger, lighter, heat-resistant, etc.), and the miniaturization of electronics combined with the development of computer technology.

The extremely high cost (per kilogram) of launching materials into space has been a fundamental motivation behind the huge improvements in materials science. Some materials developed for space applications have found their way into everyday applications that we usually take for granted, ranging from automobiles to zippers.

In addition to improved materials, miniaturization has lead to the development of "smart" robotic machines designed to undertake hazardous tasks as an aid to astronauts performing space-based operations.

Inevitably many of these space applications find their way into numerous earth-based, or terrestrial, applications.

Consider for example a robotic arm which must emulate a human arm. Robotically controlled arms have been used for a wide variety of Earth-based applications such as the handling of explosives, toxic materials, and radioactive isotopes for medical treatments.

Robotic arms can also be found collecting rock samples deep in underground mines, and assisting in high-tech surgeries in brightly lit hospital operating theatres. Robotic arms are even found in Hollywood movie studios where life-like dinosaurs and monsters are brought to life using the robotic technology originally developed for space applications.

	Simulating the up and down motion and the flexibility of the human arm invokes some interesting applications of simple levers. As shown in the photo, the arm (excluding the shoulder and wrist) is composed to two major segments.
The upper arm is attached to the shoulder. The forearm is attached to the upperarm at the elbow. With the upper arm in the vertical postion, the forearm can move in two directions, upwards or downwards.

	Mechanically speaking, the upper-arm and the forearm can be thought of as two rigid levers, mutually joined at the elbow at a pivot point which acts as a fulcrum for both parts of the arm.
Muscles attached to these levers provide the force required to articulate their motion. Since muscles can only provide force by contraction, they must always work in pairs. As the contracting muscle, (red), undergoes tension, (tightening), it applies a force to the arm. At the same time, the opposing muscle, (black), relaxes and stretches, thus allowing the arm to move

	The arm mechanism is really nothing more than a pair of simple levers (as can be seen in the diagram). The pivot point at the elbow acts as the fulcrum, (indicated by the triangle), for the levers.
When the pairs of muscles are relaxed, that is, neither stretched nor contracted, they will remain at rest between the two extended extremes of completely straight or completely folded.

	Extending and folding the arm is a matter of contracting and stretching the appropriate pair of muscles. In a real human arm there are multiple sets of muscles which act as coordinated pairs. These numerous coordinated pairs work together to produce an extremely large range of possible motions, yet the physics of each motion is exactly the same as shown here.
Notice in the diagram to the left that as one muscle contracts, the other member of the coordinated pair must stretch.

Activity: Build a Model Robotic "Elbow" and "Arm"

Materials

A very simple, yet instructive model of an arm-and-elbow mechanism can be built using two pieces of wood, several eyescrews, some elastic bands, and a bolt.

Procedure

The dimensions are not critical; use the diagram to the left as a guide. Note the alignment of the eyebolts and their placement with respect to the pivot (bolt). Hint: A nylon lock-nut is very useful at the end of the bolt since constant articulation of the model tends to loosen an ordinary nut.

	A complete arm-and-elbow model is shown to the left. Elastic bands are used to simulate the muscles which would cause the arm to move. Several elastic bands can be placed on each side of the arm so that equilibrium can be achieved in a way that allows the arm to seek a right angle position when it is "at rest".
Caution: Avoid using too many elastic bands, as this could lead to a serious injury if the model, or part of the model should suddenly fail. An eyescrew can become a dangerous projectile if accidentally "launched" by a tightly stretched elastic. Students should be instructed to wear protective eye wear.

	There are many sets of muscles in the human arm. For example, in addition to the large muscles in the upper arm, there are also sets of muscles in the forearm which facilitate the arm's articulation as well as increase its strength.
One may, for example, add these to the model, (following the method used to simulate the upper-arm muscles). Of course you could build an ultra-model arm to include a wrist, a hand, and ending in five fingers. Every joint in your ultra-model would need at least one pair of coordinated muscles to allow it to move in a controlled manner.

If you consider the complexity of building this ultra-model you will quickly begin to realize the difficulty in building a robotic arm that actually emulates not only the motion, but the capabilites, of the human arm. Your ultra-model, for example, cannot rotate as a real arm can, nor can it exercise any range of strength; and it certainly cannot exercise reactive control.

Through explorations using the model arms they have built, students should come to appreciate the design challenges of Canadarm and Canadarm2. How do you build a jointed arm that:

• can work continually in the harsh environment of space;

• can capture a payload in free flight;

• can controllably and safely handle payloads as large as a school bus;

• has a mass of less than 500 kilograms?

Canadians are world leaders in robotic technology. Canadarm and Canadarm2 have demonstrated their reliability, usefulness and versatility performing many and varied tasks such as deploying satellites into their proper orbit; retrieving malfunctioning satellites for repair; acting as a mobile work platform; moving equipment and supplies around the International Space Station, supporting astronauts working in space, and servicing instruments and other payloads attached to the space station.

Investigations

1. Here on Earth, robotic arms are frequently used in manufacturing. They perform tasks that are difficult, dangerous or boring to human beings. Industrial robots never need breaks and can work 24 hours a day doing exactly the same thing over and over again without complaint. For instance robotic arms can tighten bolts on car, always with exactly the same amount of torque, and never get tired. Make a list of other jobs robotic arms could perform on Earth. Include in your list why you think a robot could do (or is doing) the job rather than a human being.

2. Investigate 1st, 2nd and 3rd class levers.

3. What class of levers are used in the simple elbow model?

4. Obtain a photo of astronauts asleep in space. What is the angle between their upper-arm, (at their elbow), and their fore-arm when they are completely relaxed? Can you explain this?

5. Count the number of joints between your shoulder and the tip of your index finger. Use an Anatomy book to check your result.

6. What is the main difference between a real elbow joint and the model elbow joint that you have built?

7. Robotic arms use electric motors in the place of the coordinated muscle pairs of the human arm. (Motors can run both forwards and backwards.) How many electric motors would be needed to operate one robotic arm from the shoulder to the tip of the index finger? (Count one motor at the shoulder and one for each joint.)

8. There are three basic types of joints (illustrated below). Make a chart listing the advantages and disadvantages of each type.

   

9. The human elbow uses a "ball and socket" joint as shown below. This joint can move up and down, from side to side, as well as in a rotational motion.

   

   How many separate "off-set joints", (like the ones in your model elbow above), would be needed to produce the same amount of motion as the ball-and-socket joint? Build a simple model using soda straws and pins to demonstrate this.