A robot arm is a machine that moves tools through space using connected links, rotating joints, and electric motors. Industrial arms often have 6 axes, which means they can position and angle an end tool in many useful ways. This matters because the same basic design can weld car frames, sort packages, pick up small parts, or paint smooth surfaces with repeatable motion.
Each joint adds a controlled type of movement, so the whole arm can reach targets that a single motor could not reach alone.
Inside each joint, a motor turns gears or belts that rotate one section of the arm relative to another. Sensors measure the joint angle, and a controller compares the measured angle with the desired angle to correct the motion. The base, shoulder, elbow, and wrist joints work together to control both position and orientation.
Engineers describe this using degrees of freedom, torque, speed, and precision so the robot can move safely and accurately.
Key Facts
- A 6-axis robot arm usually has base, shoulder, elbow, wrist 1, wrist 2, and wrist 3 or end-effector rotation axes.
- Degree of freedom means one independent way a machine can move, such as rotation around a joint axis.
- Torque measures turning effect: τ = rF, where r is lever arm distance and F is force.
- Angular speed measures how fast a joint rotates: ω = Δθ / Δt.
- Motor power depends on torque and angular speed: P = τω.
- Robot position is controlled by combining joint angles, often written as q1, q2, q3, q4, q5, q6.
Vocabulary
- Joint
- A joint is a connection between robot arm sections that allows rotation or another controlled motion.
- Actuator
- An actuator is a device, such as an electric motor, that creates motion in a robot.
- End effector
- An end effector is the tool at the end of a robot arm, such as a gripper, welder, suction cup, or camera.
- Degree of freedom
- A degree of freedom is one independent direction or rotation in which a robot can move.
- Encoder
- An encoder is a sensor that measures a motor shaft or joint angle so the controller knows the robot's position.
Common Mistakes to Avoid
- Counting every visible part as a degree of freedom is wrong because degrees of freedom count independent motions, not links, bolts, or coverings.
- Assuming a bigger motor always makes a better robot is wrong because motors must match the needed torque, speed, mass, accuracy, and safety limits.
- Ignoring the length of the arm when calculating torque is wrong because the same weight creates more torque when it is held farther from the joint.
- Thinking the gripper position is enough is wrong because many tasks also require the tool to have the correct angle and orientation.
Practice Questions
- 1 A robot shoulder joint holds a 4 kg tool 0.50 m from the joint. Using g = 9.8 m/s^2, what torque is needed just to hold the tool level?
- 2 A wrist joint rotates 90 degrees in 0.50 s. What is its average angular speed in degrees per second?
- 3 A 6-axis robot can reach a point on a table, but its welding tip is angled the wrong way. Explain which type of joints are most likely needed to fix the tool orientation and why position alone is not enough.