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Autonomous robot

Gallop like a horse

The Pegasus robot, which was created by students of the ETH Zurich, does not have wings like the famous horse from Greek mythology. Instead, it has legs that can gallop almost like those of a horse – with motors by Maxon.

Pegasus robot with four modules and the Maxon DC RE40 motor visible in the upper part of the leg module (Photo: ETH Zurich)

As part of a research project at the ETH Zurich, the autonomous robot Pegasus was created. With its telescopic legs, it is capable of moving across vast distances. The legs are designed for moving in a dynamic trot and can achieve top performance with low energy consumption, due to state-of-the-art drive technology. Pegasus has a modular design and can therefore be used for a range of applications. Each module consists of two robot legs. Hence it can be sent on its journey with two, four, or even six legs. During the time frame of the project, a total of three modules were built.

In the joint project by the ETH Zurich and the TU Delft (Netherlands), ten mechanical engineering students – six students from the ETH Zurich and four students from the TU Delft – developed the autonomous robot during the last two semesters of their bachelor's degree. The specified target was that the robot has to cover a distance of 10 km in less than 10 000 seconds (2 h 47 min, approximately 3,6 km/h) with a single battery charge. The prerequisite was that the robot has to have an extremely energy-efficient design and has to store energy, for example through the use of springs in its legs.

Initially, only one leg was used to extensively test the jumping ability. In each leg, a Maxon DC RE40 motor drives the spindle that is responsible for extending and retracting the leg. Additionally, an EC 4-pole motor controls the rotational degree of freedom in the hip module (connecting part between two leg modules). Thus it is the 200 W motor that makes it possible that the entire leg can turn.

Detailed view of the hip module with EC-4pole motor (Photo: ETH Zurich)
The DC RE40 motor in the leg module takes care of the jump motion (Photo: ETH Zurich)

Thanks to the special winding technology and the 4-pole magnets, the EC 4-pole drives delivering a high power per unit of volume and weight. The motors have no cogging torque, an efficiency of more than 90 %, and control dynamics. The metal housing ensures good heat dissipation and mechanical stability.

Two position controllers

Two different positioning controllers are used to control the Pegasus robot. One of these is the Epos 24/2 controller, which is used to control additional sensor interfaces. This position controller, like all other Maxon motor Epos products, was especially developed for commands and control functions in CANopen networks. Commanding can take place using a CAN master, for example Epos2 P, SPS, Soft SPS, PLC, or using PC via USB or EIA-232. Several drives can be networked and coordinated through the standardized CANopen interface. Additionally integrated gateway functions such as USB to CAN or EIA-232 to CAN help simplify access to the CAN network even more.

The CANopen Epos 24/2 controller (Photo: Maxon Motor)

Additionally this controller has been equipped with motion control functions. With the Interpolated Position Mode, the position controller is able to synchronously move along a path specified by means of set points. Secondly, the Epos 2 70/10 controller is used. It is specially tailored to the brushed DC motors with encoder or the brushless EC motors with Hall sensors and encoders from 80 W to 700 W. In the Pegasus system, the controller on the one hand is responsible for torque control (current mode) of the EC 4-pole motors and for speed control (CANopen profile Velocity Mode) of the employed RE motors. In current mode, a constant torque can be controlled on the motor shaft. In CANopen profile Velocity Mode, the motor axis is moved with a specified set speed.

The original target of the project was not achieved, due to the short timeframe. The functionality of various parts of the robot are not yet perfectly matched to each other, explains Steve Heim, former student and a member of the Project Pegasus team. The project is being further developed in the Autonomous Systems Lab of the ETH Zurich. Interns at the lab are working on an implementation of the project. In the mean time, the two-legged Pegasus has executed its first stable hops. The target of achieving a distance of 10 km in less than 10 000 seconds with a single battery charge is within grasp.

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