Movement

robots
guide

Industrial robots are classified by the International Standards Organization as:

Automatically controlled, reprogrammable, multipurpose manipulator programmable in three or more axes.

However 6-axis anthropomorphic robot arms are considerably more flexible, adaptable and standardised than others.

As the name suggests anthropomorphic arms mimic the movements of a human arm.

As the diagram below shows, axis 1 and 2 are effectively a shoulder, axis 3 and 4 elbow and forearm and axis 5 and 6 are the wrist of the robot.

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The 6 axis system allows the robot to have an expansive work envelope and allows the tool on the end of the arm to be manipulated in almost anyway within that envelope.

Other terms for these axis include degrees of freedom or DOF, joints or axels. Most manufacturers number them 1 to 6 starting at the bottom of the arm. However some manufacturers use letters for different axis. Motoman term them S, L, U, R, B and T for axis 1 to 6 respectively. Fanuc called them J1 to J6.

Some robots have greater degrees of movement than others, especially on axis 2 and 3 which are often limited by the mechanical structure of the arm. The best arms are developed to be well balanced and require the minimum of force from the motors possible. Various types of robots use counterbalance weights, gas springs and mechanical springs to help the achieve this balance.

All robot axis are braked on almost every robot type. This means that even if the power is switched off the robot will retain its position. Also if an emergency stop is engaged in any part of the system these brakes will immediately engage and prevent movement. 

The robot computer knows the position of the arm from feedback from each of the axis in the robot arm. The computer uses this information to control the movement of the robot.

The next diagram describes the different types of robot movements that are commonly available with a 6 axis machine. The teach pendant (see below) will often have a button that selects the movement type.

 

Axis or joint movement is the simplest to understand and fixed co-ordinate is again quite straightforward, it simply moves the robot in relation to the base of the robot in X, Y and Z co-ordinates. 

More complex are the wrist and tool oriented systems, these use the centre of the faceplate at the end of the wrist or a specified point on the tool (the Tool Centre Point or TCP) as their reference. This is typically used repositioning the angle of the tool, for example changing the angle of attack on a grinding wheel.

Overall this gives the programmer excellent and hopefully intuitive control over the robots positioning.

The computer calculates not only which motors to turn on but also calculates the acceleration and deceleration of the arms movement. This means that the robot uses information about the mass of the arm and indeed the mass of its load to move in a smooth and accurate path to the required position. Path or movement accuracy has been increased over the years as computing power has increased. Almost any robot manufactured in the past 15 years will have excellent positional accuracy and most of the main manufacturers robots have excellent path or movement accuracy but this can be at the expense of speed.