How Industrial Robot Controllers Work with Different Types of External Axes

External axes are a great way to enhance the flexibility, reach, and efficiency of industrial robot systems. Although they are not part of a robot’s native kinematic chain (i.e. the robot’s inherent joints and motions), external axes provide additional degrees of freedom. In this article, we explore how different types of external axes are managed and integrated with industrial robot controllers.

Terminology

In robotics, different manufacturers may use slightly different terms. Let’s start by clarifying the vocabulary commonly used in this area.

External Axes / Auxiliary Axes – they are the same. Both refer to mechanical axes external to the robot arm, such as tracks or turntables, which are generally controlled by the robot controller.
Sychronised vs Coordinated – used somewhat exchangeably and refer to how external axes move in time with the robot. Synchronised usually means real-time, joint-level coordination, while coordinated can also include asynchronous but orderly motions – continue reading to find out!
Coupling and Decoupling – coupling means linking the external axis to the robot’s kinematic group, enabling it to be considered in the robot’s path planning. Decoupling separates it from the robot’s motion planning, allowing independent operations but still timed to move with the robot.

Types of External Axes

External axes can be grouped into three main categories.

1. Robot Positioner

These axes move the robot itself, expanding its reachable workspace.

Linear Track – the most common robot positioner. The robot base is mounted on and moved with a linear rail, extending its horizontal reach.
Cartesian Gantry – a multi-axis overhead system (typically 2 or 3 axes), where the robot is suspended upside down and moved along gantry rails.
Robot Base Riser – a vertical lift that raises or lowers the robot to access different levels, often used in palletising or tiered operations.

2. Part Positioner

These move the workpiece, enabling optimal access and orientation for the robot.

Turntable – a platform that repositions around 1 or 2 linear axes. It reorients parts into position, ideal for welding, coating, or machining.
Servo-Controlled Fixture – a programmable clamping device with motorised axes that repositions workpieces during operation.
Conveyor – a moving belt or panel on which parts travel. It can be used with encoders and sensors to track moving parts.

3. Tool Positioner

These work as drive units to move or reorient tools attached to the robot.

Tool Changer – a mechanism that enables automatic switching or reorientation of tools.
Servo-Driven Tools – such as spindles or grippers with their own motion axes, driven by servo motors and controlled through the robot.

Motion Types

External axes can be moved synchronously or asynchronously to the robot axes. How a robot controller manages external axes depends on their motion, type and application.

Synchronous Motion

In synchronous operation, the robot and its external axes start and finish a motion simultaneously. All axes coordinate together to maintain time alignment of axis movements in every position. Each axis follows its own path, but timed to reach the target position in synchronisation. Synchronous motion can be mathematically coupled or decoupled.

1. Coupled Synchronous

The robot and external axes are kinematically linked, moving as one system. Motion planning includes external axes as part of the robot’s joint chain. The easiest way to understand it is to think of external axes as additional robot joints. Example: a robot mounted on a linear track with all 7 axes moving together to work across a long piece.

2. Decoupled Synchronous

The robot plans its motion paths without taking external axes positions into consideration. The movements of the robot and external axes still start and stop simultaneously, but their paths are calculated independently. Example: a robot welds a circular seam while a turntable rotates slowly, both stopping at the same time.

Asynchronous Motion

In asynchronous mode, the external axes operate independently of the robot, though they may be coordinated logically with robot movements, or triggered during the robot’s program.

1. Coordinated Asynchronous

The external axes are controlled and managed by the robot controller, but run on their own timeline. Asynchronous external axes are typically triggered by program logic or I/O signals. Example: a conveyor moving a part into position before the robot picks it up.

2. Uncoordinated Asynchronous

In this scenario, the external axes are controlled entirely outside the robot system, such as by a PLC or manual input. Example: an operator manually repositions a part on a turntable between robot cycles.

Conclusion

External axes are essential for enabling advanced robotic applications, especially when handling large, complex, or multi-stage tasks. Whether it’s repositioning the robot itself, moving a part, or adjusting the tool, auxiliary axes offer great flexibility that would be impossible with the robot alone. Understanding the distinctions between synchronous, coordinated, and asynchronous motions, along with the types of external axes available, is key to designing an efficient, safe, and scalable robotic cell.