High-Power-Density Drives Run Mobile Robots in Smart Warehouse

Machine requirements

The sorting centers need to handle up to 9000 parcels per hour. The solution was to place the inventory in pods that are transported around the facility by mobile robots. The robots roll underneath the pods and rise vertically, lifting the pods above the floor and transporting them to packaging stations.

The robots need a low profile to fit under the pods, and they must be very compact to avoid collisions. At the same time, they must be able to lift heavy loads up to 300kg, and have the longest possible lifetime per charge, to minimize downtime in charging stations.

Motion control solution included:

• Gold Solo Guitar servo drives for the traction wheels
• Gold Solo Whistle servo drive for the rotating lift

High Power-Density

Given the space limitations, the robots required drives with the highest possible power density. The two traction wheels, which propel the robot and also have a vertical lift capability, are powered by 750 W brushless servo motors. Elmo’s compact Gold Solo Guitar drives provide the motors up to 4.8 kW of continuous power or 5.4 kW of peak power.

The rotating lift is powered by a 100 W brush motor. An Elmo Gold Solo Whistle drive delivers 2 kW of continuous power as well as peak power, receiving pulse/direction signals directly from the controller.

High Efficiency

The per-charge operating lifetime of a mobile robot depends heavily on the efficiency of its electrical components. Because the Gold Solo Guitar drives are over 97% efficient and the Gold Solo Whistle drives are over 99% efficient, they are ideal for handling the limited power supply and increasing the overall duty cycle of the robot.

Minimizing Jitter and Latency

To steer the robots, the traction axes must be precisely synchronized. The design is based on a master-slave architecture with the primary axis receiving its commands from a controller via an analog +/-10V command. The engineering team initially planned to use analog current following for synchronization, but the Elmo drives provided a more effective alternative. They use high-speed PWM synchronization, enabling axis following with minimal jitter or latency delays.

The outcome

The Elmo drives’ performance and overall functionality met requirements. In the next generation of robots, communications will be routed over an EtherCAT signal rather than an analog +/-10V signal. With the addition of Elmo’s full EtherCAT support, flexibility in drive programming, and a wide voltage range, the retailer locked in on the ideal servo drive solution for its growing fleet of warehouse robots.

The initiative is moving into production mode, with an expected 1000 robots to be produced over three to four years for 10 unmanned sorting centers.

Today’s Solution:

Elmo has introduced new solutions and options since this mobile robot was designed. Today:

• The optimal solution for this application would be a Platinum Solo Quartet, a 4-axis miniature “ready to use” motion control system – 4 x 5000W servo drive and 4-axis motion controller
• An alternative option would be to use 4 Gold Twitter servo drives