Joints and degrees of freedom
A rigid object in space has six degrees of freedom: three for position and three for orientation. The arm has six rotary joints in a chain, so it can reach a point and point the tool in almost any direction there. J1 to J3 (turret, lower arm, upper arm) mostly place the wrist. J4 to J6 meet at one point, the wrist centre, and mostly set the tool's orientation.
Servo motor and gearbox
Each joint is driven by a servo motor with an encoder on its back that reports the shaft angle many thousands of times a second. The motor is fast but weak, so a compact reduction gearbox (often 50:1 to 160:1, strain-wave or cycloidal types) trades speed for torque and stiffness. The controller cabinet holds one servo amplifier per joint and closes a position loop for each one.
Forward and inverse kinematics
Forward kinematics goes from joint angles to the tool position: multiply the transforms of each link in turn. That always has one answer. Inverse kinematics goes the other way: given where the tool must be, find the six angles. Because the wrist axes meet at one point, it splits into two easier problems: J1 to J3 place the wrist centre (a two-link arm in a plane), then J4 to J6 turn the tool. There are usually several answers (elbow up or down, wrist flipped), and the controller picks the one closest to where the arm is now.
Joint moves and linear moves
In a joint move (MOVJ) every joint turns from its start angle to its end angle, all finishing together. It is the fastest and safest way between two points, but the tool path is a curve you do not choose. In a linear move (MOVL) the controller solves inverse kinematics many times along the way so the tool travels in a straight line. That is what you want when inserting, welding or approaching a part, but it can run into a joint limit or a singularity partway along.
Teach points and programs
Most industrial robots are still programmed by teaching: an operator jogs the arm with the pendant, records points, and writes a program as a list of moves between them, plus I/O commands such as opening a gripper. The pendant has an emergency stop and a three-position enabling switch on the back; the robot only moves while it is held in the middle position.
Singularities
A singularity is a pose where the arm loses a direction of motion. The common one is the wrist singularity: when J5 is at 0°, the J4 and J6 axes line up, so turning one is the same as turning the other, and a small tool move near that pose can demand a huge, fast spin of both. The arm is also singular when it is fully stretched, and when the wrist centre sits right above the J1 axis. Controllers slow down or stop near singularities, which is why a linear move can fail where a joint move works.
