Integrating motor control electronics into robot joints

Robot joints with integrated motor control electronics provide great advantages in terms of ease of use, reducing wiring costs and EMI. However, this environment is also the most demanding for electronics as they have to withstand high temperatures, vibrations and even dust or heavy magnetic fields.

Finding the right motor controller is not always easy

Most off-the-shelf controllers available in the market were designed for industrial applications. The purpose of this type of servo drives is to cover as much feedback types and communication busses as possible so that they can cover most applications but this has drawbacks especially for integrated robot joints.

• General purpose architectures making use of DSP and FPGAs increases the heat generated by the electronics.
• Servo drives designed for industrial machines where trajectories are planned in advance do not prioritise communications latency.
• A small industrial servo drive is still way bigger than what is required when integrating it inside a robotic joint.
• These drives have a heavy weight compared to servo drives for motor integration.

Servo drives for integrated robot joints

Ingenia Servo Drives for integrated motors are designed and manufactured with the latest semiconductor technology in order to achieve the lowest standby power dissipation as well as optimised DSP technology for high positioning accuracy. This is the solution shown on Figure 1 where the off-the-shelf Everest servo drive has been integrated inside the robotic joint.

Enabling safe robot joints

Most robots implement safety functions based on torque feedback from the robot joint. This torque measurement can be done both with strain gauges or with angle sensors. By monitoring the torque applied by the robotic joint, the force that the robot arm could apply to a patient or user around the machine is controlled so that it may not harm anyone. The most common implementation consists of using a motor controller that can read torque sensors and feed the information back to the master controller through the real-time, deterministic EtherCAT bus. On figure 2 a series of safety mechanisms where implemented using an Everest NET plug-in servo drive and a custom built interface board:

• Motor controller torque input is fed back to the master controller over the EtherCAT bus.
• The system includes dual BiSS-C encoders and digital halls for redundancy. If there is a mismatch between the encoders the servo drive notifies the master controller.
• The motor controller includes Safe Torque Off (STO) functionality, which disables the power stage when activated.

Exceeding the limits of robotics integration

When designing robot axis joints, the main limiting factors that mechanical and electronics engineers find include the complexity of finding components with enough power density, the right form factor and with low heat dissipation.

Power Density

Ingenia provides both off-the-shelf and custom solutions for robotic axis joints with the highest power densities in the market. The use of new, non-silicon based transistors altogether with state-of-the-art gate driver technology allows us to provide power densities up to 0.21 W/mm3 which is 175% higher than our closest competitor and above 700% from other servo drive suppliers. This high power density enables the integration of motor controllers like where it was not possible before like on end of arm effectors for surgical robots or humanoid robot wrists and fingers.

Size and Heat Dissipation

For more than a decade Ingenia has been developing its technology focusing on robotics applications where space is at a premium. Our solutions are the smallest in the world and are used in surgical robots, optronics, robotic joints or humanoid robots to name a few. But size alone is not enough. In order to integrate a motor controller inside a robotic joint, heat must be properly managed. Ingenia flexible architecture allows both for the highest performance and the lowest heat dissipation possible with standby power consumption down to 0.5W.

Learn more about Ingenia servo drives range and custom servo drive solutions.

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