Motion control design of the Bearcat II mobile robot

Ming Cao, Xiaoqun Liao and Ernest HallCenter for Robotics ResearchUniversity of CincinnatiCincinnati, OH 4522 1-0072ABSTRACTMotion control is one ofthe most critical factors in the design ofa robot. The purpose ofthis paper is todescribe the research for applying motion control principles for a mobile robot systems design, which is ongoing at the University of Cincinnati Robotics Center. The mobile robot was constructed during the 1998-1999 academic year, and called BEARCAT II. Its design has inherited many features of its predecessor,BEARCAT I, such as vision guidance, sonar detection and digital control. In addition, BEARCAT IIachieved many innovative motion control features as rotating sonar, zero turning radius (ZTR), currentcontrol loop, and multi-level controller design (conventional control and fuzzy logic control). This paperwill focus on the motion control design, development and programming for the vehicle steering control androtating sonar systems. The systems have been constructed and tested at the 1 999 International GroundRobotics Competition with the Bearcat II running an obstacle course for 153 .5 feet and finishing fourth inthe competition. The significance ofthis work is in the increased understanding ofrobot control and thepotential application of autonomous guided vehicle technology for industry, defense and medicine.Keywords: Motion control, motion behavior, compensation, mobile robots1. INTRODUCTIONMotion control is one ofthe technological foundations of industrial automation. Whether the motion of aproduct, the path of a cutting tool, the motion of an industrial robot arm conducting seam welding, themotion of a parcel being moved from a storage bin to a loading dock by a shipping cart, or anotherapplication, the control ofmotion is a fundamental concern. Putting an object in the correct place with theright amount of force and torque at the right time is essential for efficient manufacturing operations. To beable to control a motion process, the precise position of objects needs to be measurable. Feedbackcomparison ofthe target and actual positions is then a natural step in implementing a motion controlsystem. This comparison generates an error signal that may be used to correct the system, thus yieldingrepeatable and accurate results.However, the use of feedback can lead to an unstable system whose output may oscillate or even go toinfmity with a small input signal. Stability determination is therefore an important design consideration.One specification for absolute stability requires that the poles ofthe transfer function must be in the lefthalfofthe s-plane. Absolute stability, often specified in the frequency domain, is essential and necessarybut not sufficient. Frequency domain specifications relating to relative system stability may also be given.For relative stability, a certain phase margin and gain margin may be specified to ensure that the systemwill remain stable although some parameters change due to temperature variations, aging or otherenvironmental factors. If a system is stable, then other performance criteria specified in either the time orfrequency domain may be considered to meet the performance requirements. Short-term, or transient,response specifications such as rise-time or percent overshoot to unit step function input may be given.Long term, or steady state, response such as zero steady state error may also be specified.Design Objectives:The desire for a super intelligent robot vehicle that can beat all other teams during the contest forces us tobuild a successful control mechanism that can drive the mobile robot along a changing contest course,such as steep ramp, sand, or over wet ground. This requires the robot have the ability to "see" its course.Another consideration of this design is that the robot must sense the obstacles in front of it. This indicatesthe design of separate systems that may generate different control signals, which may be integrated withcertain method to drive the conventional motion system. The specific goals can be listed as follows: