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The growth of modular robots and the importance of drive train design

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For lightweight robots, modular designs are increasingly popular thanks to the flexible deployment they enable for specialised tasks. This is also enhancing the potential for system integrators and end users to develop their own robots. As part of an in-house robot build, optimising the design of the drive train that powers and controls each robot joint is vital to the robot’s performance. The most effective way of achieving this is by developing the drive train as a complete module, and this approach can also improve the efficiency of robot development overall.    

As the requirements for robots continue to become increasingly specialised to task, the advantages of modular robots are at the fore. The ability to change their form and configuration, thanks to a combination of modules, as opposed to a fixed body, gives greater flexibility. This means that a modular robot can be reconfigured, or customised, as often and as quickly as is required. And should the robot fail, a replacement can be rapidly, and economically, provided. 

In particular, there’s been a marked change towards modular robots in the industrial robot class at the level of lightweight robots and cobots. These smaller robots, typically handling loads between 3kg and 16kg, lend themselves to specialisation and the benefits of modularity. Their small size enables easier configuration, and for this payload range, there are a high volume of tasks that robots are required to fulfil. This is confirmed by reports of the growth in lightweight robots that range between 20% and as much as 40% each year worldwide.

To facilitate the modular robot demand, there’s been a corresponding need for greater flexibility in robot control. Motion controller manufacturers, and PLC manufacturers, are increasingly offering kinematic libraries that enable robot control programming. This is allowing system integrators, as well as end users, ranging from car manufacturers to warehousing and delivery companies, to develop their own robots, instead of relying on dedicated robot manufacturers. The advantage is enhanced robotic control, more specific to their requirements, met with the flexibility to quickly respond to changing needs.

Motor design

Central to the increasing trend towards modular, lightweight robots, designed and built internally, is the need to power their kinematic motion. The motion system, or drive train, is responsible for moving and controlling each robotic joint. Just as a modular robot design must enable the flexibility to fulfil various specialised tasks, so too the capability of the drive train has to match this need.

The motor itself is central to the drive train. To achieve the required high dynamic performance, necessary motor attributes include high torque density and low inertia, enabling rapid acceleration and deceleration. Smooth control of each robot joint is also essential, so the motor must ensure capabilities such as low cogging, which minimises micro ripples and jerks during motor rotation. 

For lightweight robots, a compact motor is also essential, which further emphasises the need for high torque density. A frameless design, like maxon’s EC frameless motor, enhances design integration, and a large, hollow shaft enables through routing of cables. Despite the motor’s small footprint, it also answers the essential criteria of heat and energy efficiency. 

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SCARA (Selective Compliance Assembly Robot Arm) with DT50 joints

The drive train

However, to optimise robot motion performance, the complete drive train must be taken into consideration. The drive train, itself a module of the robot, typically comprises the motor, plus the gearhead, the encoder, which provides continual feedback on position and speed, as well as the motor’s position and speed controller. Like the motor, each of these components should be designed to optimise distinct performance criteria. However, the design of the drive train as a complete module is fundamental to optimising motion performance.  

Ensuring seamless integration of drive train components is vital to achieve this level of performance. This requires correct dimensioning, and crucially, correct sizing, in terms of meeting the required output values, such as torque, speed, acceleration, and position profile. Often, the specialised requirements of modular robots for each individual application also mean that customisation of motion components is necessary. 

As motion components perform as a system, design changes to individual parts can have a subsequent effect, impacting the operation of the motion system as a whole. Therefore, if any customisation is required, treating the drive train as a complete module is a more effective way of achieving the desired outcome, in terms of motion performance and design integration. 

To meet these objectives, the design stage should also include kinematic simulation. Creating a working, virtual model is vital to plan the robot’s motion path, ensuring that the desired kinematic profiles can be achieved, according to factors such as position, acceleration, and torque. Simulation is also crucial to ensure the robot operates within safe limits for the protection of users, as well as the physical environment. This service can be provided by a motion designer like maxon, in conjunction with programming support for each axis of motion.

Optimising design efficiency 

Designing and simulating the drive train as a module also makes the development process significantly more efficient. Firstly, it removes the time required to develop and test the performance, compatibility, and integration of individual drive system components. Secondly, procuring the drive train as a complete module reduces demand on internal expertise. As a result, robots can be brought online more quickly.

The alternative approach of procuring distinct components might be attractive from an initial cost perspective. However, the development time required for in-house drive train engineering can make the process less cost-effective long term. And, while the improved motion performance derived from dedicated motion engineering expertise can increase a robot’s capabilities, a tried and tested approach to drive train development can also enhance reliability, minimising downtime when the robots are deployed in the field.

When engaging a drive train designer, considering the impact that the motion specification has on robot performance and design integration, ideally, the motion engineers should be involved at the earliest opportunity. This approach will speed up development by minimising the amount of iterations required, and quickly help to achieve a finalised design. 

To find out more about maxon’s drive trains and engineering services for lightweight robots, visit Robotics.

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