February 2020

52 n ROBOTICS AND AUTOMATED MANUFACTURING February 2020 www.drivesncontrols.com Choosing the best motors for robot arms T he rise of automation is fuelling a demand for robotic arms – typically consisting of a series of joints controlled by servomotors. The cost of developing and producing robots can be extremely high, with some arms costing around £30,000. But these arms are often capable of functions that may not be needed for every application. In some projects, stepper motors may be a better choice for driving robot arms than servomotors. In the right circumstances, a stepper motor can be more accurate than a servo, but the main advantage is cost. There is a reason why the servos are known as “the king of motors”. If you want a positioning motor with a high torque capability, the servomotor is the way to go, but most people actually need anywhere near all of the functions of a servo. They either want the high torque, high speed, or they want the positioning accuracy. With many robotic applications, the high torque and speed of a servomotor are unnecessary. Robot arms do not usually have to swing around at 100 mph. Engineers want them for their positional accuracy and flexibility. Using a high-end closed-loop stepper motor makes sense in such scenarios. A stepper motor can cost up to two thirds less than a servo. In addition, lead times are usually shorter and, in terms of set up and programming, steppers are easier to use than servos. When designing an arm, you need to consider what you want to move, how fast you want it to move, and in what positions it will start and end. You can then assess how many axes you will need to achieve that level of performance. Why design a £30,000 eight-axis robot when the task may only need three axes of movement? Or why struggle with four, when the angle warrants a fifth axis? One you have your basic specifications, motor suppliers can then consider the sizes, weights, dimensions and speeds required, and do the calculations to determine the most suitable motor for the weight and role of the robotic arm and to deliver the torque required to overcome levels of inertia. By taking into account all of the physical aspects of moving a load early on in the design process, you can avoid the risk of running into difficulties and wasting money on over-sized motors at the development stage. Communications systems also need to be factored in to ensure that the motors will integrate with control systems. If your preferred protocol is not supported, third- party products can provide a link between protocols. As the adoption of Industry 4.0 increases, communications has become a key factor when choosing motors. Effective motor sizing and selection at the development stage can accelerate a project significantly and simple systems, such as conveyors, can be operational in just two to three weeks. In an example of a real-use case, Oriental Motor in Japan produced a demo kit for a robotic arm based on its AZ series motors with harmonic gearheads, to demonstrate how a robot arm can optimise accuracy without the cost penalty of a servomotor. A network-controlled multi-axis drive co- ordinates the arm’s axes. The design of such arms can be adapted to suit their application and payload capacity, from a complex seven-axis arms, handling up to 2kg, to sturdier three-axis systems that can handle 40kg. Flexibility increases with the number of closed-loop stepper motors used. n When designing robot arms, stepper motors can offer a cost-effective alternative to servodrives. Careful analysis of the application can allow the use of steppers that cost a third as much as servos, as Paul Jepson, new business analyst at Oriental Motor, explains. A 3D CAD design (top) that Oriental Motor used to test the concept for a seven-axis, table-mounted robot arm before it built a demo kit (above). The arm is driven by closed-loop stepper motors, equipped with harmonic gearheads.