Jul 27, 2016

Mobile Robots: An Energy Consumption Example

Mobile robots are used in many applications. The consumer-oriented vacuum robot cleans our rooms, the service robot aids us at a great age and the Kepler spacecraft discovers Earth-size planets in our universe.

Every mobile robot on and outer Earth shares the problem of its limitation in energy. Every sensor and every actor that the robot uses consumes energy. Energy can be provided for a limited time when it’s using the battery. Each architect and engineer who wants to build a mobile robot has to take care of this design consideration. Every non-moving is a useless robot.

An unmanned aerial vehicle (UAV) for instance has several motors (actors) that keeps the vehicle in the air and steers it. The control unit controls the UAV. If you also want to make the vehicle autonomous you will add some sensors on the vehicle. Let’s take a concrete hexacopter scenario:

Six brushless motors will lift the UAV off and transport the UAV to its destination. Each motor approximately consumes 2.6 A (50% throttle) up to 12 A (100% throttle).
The battery provides a capacity of 5000mAh, so that the UAV’s actors and sensors can theoretically consume 5A for a complete hour. In the real world you don’t want to exceed the remaining capacity of 20% (=1000mAh). With this calculation the UAV would consume 5A for 48 minutes instead of a complete hour.
The control unit could be a Raspberry Pi 3 Model B or an Arduino-based control unit. The decision depends on the goal. A fully autonomous UAV needs computing power to solve mapping and navigation algorithms. This approach demands the Raspberry Pi solution. A remote controlled mobile robot is perfect for an Arduino-based solution. The difference in the controller’s power consumption is the factor 10. The Raspberry Pi 3 Model B consumes approximately 0,6 A when stressed (see reference), the Arduino-based solution approximately 0,05 A (see reference). In the overall solution the controllers’ power consumption is negligible since the motors are the major consuming parts of the battery’s capacity.

Arduino on the left, Raspberry Pi on the right

The robot will make use of sensors to orient himself in his environment. In our scenario the robot will use four sonar senors to scan the distance to the ceiling (above the UAV) and four sonar sensors to scan the distance to the floor (below the UAV). In order to “see” the obstacles that are in front, behind or next to him, we provide the UAV with a 360° LIDAR system. Altogether these sensors require 1,016 A of energy.

Accumulated the six motors (at 50% throttle), the Raspberry (under stress) and the sensors require a accumulated current of 17,216 A. With the provided batteries, the UAV would fly for approximately 14 minutes. That is not very much.