06.11.23

Recently, I was contacted to specify a servo-based motion control system for a large, high axis-count machine. This customer provides machine solutions for the forging, material shaping, and industrial manufacturing industries. The specific machine handles, sorts, bundles, and prepares steel rebar reinforcement used in concrete structures. Their particular processing centre needed 66 servo axes working in precise coordination to provide smooth and accurate rebar positioning.

Specifying a large system like this, we approach it from a top-down focus, starting with the overall outcome the machine builder wants to achieve. Usually, this objective is a combination of increasing throughput, relating to the machine’s speed, matched with repeatable quality of the process, based on the precision the machine can achieve.

Combining these general requirements informs an approximate design of the machine’s architecture, including the overall control needs, spanning from the user interface, down to the specification of the servo motors and drives.

Motion coordination

While large motion coordination projects involving tens of axes are commonplace, a lot of the time, requirements are for machines with two to six axes. Though fewer axes may seem like a simpler design, the motion coordination complexity, as well as the requirements on precision and accuracy, can be just as demanding. Machines based on three-dimensional gantry-based systems fall into this category and include applications such as pick-and-place, 3D printers, CNC, as well as cutting machines, like plasma or water jet.

Whatever the scale, the motion controller is the central hardware for a machine that depends on speed, precision, and coordination. A starting point in specification is taking into account the number of servo axes the controller can coordinate, combined with its motion performance capabilities.

Often, the machine’s mechanical constraints mean that the precision offered by a servo system will be far greater, at an individual axis level, than the machine can benefit from. However, the advantage of a controller optimised for advanced motion becomes evident when the high speed coordination of multiple axes are required.

Dedicated motion controllers with features such as path speed and integrity optimisation, as well as high execution benchmark and maths precision, achieve the optimum level of motion coordination. Speed and coordination also depend on the performance of the control signals. This is why real time networks like EtherCAT, offering deterministic, high-speed communication, are frequently relied on for performance motion applications.

Servo sizing

In order to actuate the motion controller’s commands, we turn to the servo axes themselves. To specify the motors and drives, the OEM design engineer team typically provides the machine’s overarching mechanical requirements per axis, like whether it needs rotary or linear motion, and if special kinematics, such as SCARA robot integration, are involved. In the most basic terms, this step includes working out the requirements to move a certain mass over a set distance in a specific time. Servo sizing, where we calculate the necessary speed and torque to achieve the general motion requirements, is therefore fundamental.

Often, especially with applications like a three-axis gantry, one axis may support the load of another, so it’s not always possible to size each axis on an individual, one-by-one basis. The type of load, as well as the style of linkage, will add inertia and must be accounted for. As a result, the servo motor should be specified with a speed and torque safety margin, balanced against the need to ensure a compact, efficient design, contributing to a cost-effective machine build.

A typical characteristic of a servo axis is high dynamic performance for rapid acceleration, with precise control. To achieve this, the servo motor should have low inertia with a high peak torque capability, allowing it to rapidly accelerate and reach set velocity in the shortest time. Ensuring control over position and speed, a crucial component of a high dynamic servo motor is the encoder. The encoder provides position feedback, ensuring that the motor precisely follows the controller’s signal. The signal from the controller to the motor is amplified via the servo drive. While the drive must be rated to match the current demand on the servo motor, it needs the performance to convey the control signals with sufficiently high-speed cycle rates.

Complex kinematic profiles

Now we return our focus to the motion controller, because for any dynamic servo system, it’s this ‘brain’ that’s responsible for management of the machine’s motion profile, including its trajectory and position, as well as velocity and acceleration. Even for our three-axis example, which could apply to a CNC machine or a 3D printer, control can involve complex motion, such as contouring to follow an intricate cutting pattern, as well as synchronisation of all three axes to achieve three-dimensional motion.

To create complex motion, a development environment where exacting commands can be quickly created is also vital. OEM engineers might need to integrate commands developed in a different environment, such as a G-code format, or they may want to author their own profiles. IEC61131-3 languages provide familiarity to some engineers – and most motion control environments can accommodate them – but dedicated motion languages that are fast and simple to learn, thanks to their reliance on English text commands, can make complex programming tasks much easier.

Machine management

Most motion control developers can also develop bespoke applications for machine builders on request. Whichever approach, the capability and scale of the ‘motion engine’ is crucial. This includes the development environment, complete with an extensive library of motion commands for the most complex kinematic profiles

This development environment should also be able to manage the integration of the machine’s sensors and safety devices. Today, modern motion controllers can ably fulfil the role of complete machine management, thanks to their built-in I/O ports or integrated I/O systems. For this reason, specifying a controller optimised for servo motion coordination not only maximises machine productivity and performance, but it can also enhance the efficiency and cost of machine design

ENDS