Internal Position Error Correction (IPEC)

by Johann Borenstein, The University of Michigan


This document presents Internal Position Error Correction (IPEC) -- a new method for accurate and reliable dead-reckoning with mobile robots. The IPEC method has been implemented on our recently developed Multi-Degree-of-Freedom (MDOF) mobile platform, a vehicle in which two differential-drive mobile robots (called "trucks") are physically connected through a compliant linkage. In addition to its four wheel encoders, the MDOF platform has one linear and two rotary internal encoders, which allow measurement of the relative distance and bearing between the two trucks. During operation, both trucks perform conventional dead-reckoning with their wheel encoders. But, in addition, the IPEC method uses information from the internal encoders to detect and correct dead-reckoning errors as soon as they occur.

Our system, called Compliant Linkage Autonomous Platform with Position Error Recovery (CLAPPER), requires neither external references (such as navigation beacons, artificial landmarks, known floorplans, or satellite signals), nor inertial navigation aids (such as accelerometers or gyros). Nonetheless, the experimental results included in this paper show one to two orders of magnitude better positioning accuracy than systems based on conventional dead-reckoning. The CLAPPER corrects not only systematic errors, such as different wheel diameters, but also non-systematic errors, such as those caused by floor roughness, bumps, or cracks in the floor.

These features are made possible by exploiting the new Growth-Rate Concept for dead-reckoning errors that was introduced in papers 48/49 for the first time. The Growth-Rate Concept distinguishes between certain dead-reckoning errors that develop slowly while other dead-reckoning errors develop quickly. Based on this concept, truck A frequently measures a property with slow-growing error characteristics on reference truck B (thus admitting a small error) to detect a fast-growing error on truck A (thus correcting a large error), and vice versa.

Some Experimental Results

Figure 1 shows the return position errors for the Rectangular Path Experiment under different test conditions. The CLAPPER ran through the path for 10 runs in cw, and 10 runs in ccw direction. In each of these runs the CLAPPER had to traverse 10 bumps. In one half of the runs bumps were located under the right-side wheels of both trucks, and in the other half of the runs under the left-side. The return position errors of these runs are marked by small squares (see Legend in Fig. 1). None of the 20 runs produced a position error of more than 5 cm. Also shown in Fig. 1 are the results of five cw and five ccw runs with IPEC but without bumps (marked by small circles). Note that the results with bumps are almost indistinguishable from the results without bumps. In a further experiment the vehicle ran through the path with bumps, while the IPEC function was disabled (i.e., using normal dead-reckoning like conventional mobile robots). The results of these runs are marked by stars in Fig. 1.

Project History

The MDOF vehicle project started in 10/91 and will end (after a 1-year no-cost extension) in 9/95. Johann Borenstein is the PI in this project, which was originally funded by an NSF grant over $229,500. Later, the Department of Energy added partial support to the project by funding additional research work. The project has been rather successful: During all stages of the project work progress was ahead of schedule. The final prototype was successfully demonstrated one year ahead of schedule. In the remaining time the project went far beyond the original workscope: We developed and implemented a unique and innovative method for correcting dead-reckoning errors in mobile robots (called "IPEC"). In the course of the project we also studied errors in mobile robot dead-reckoning and developed methods for measuring and reducing such errors (see work on the UMBmark procedure). The many results of this project were disseminated in 14 publications (3 Journal papers, 9 conference papers, 2 video proceedings (IEEE R&A). Two patents are pending.
For more details on the MDOF vehicle see papers 34, 35, 36, 47, video 01

For more details on the CLAPPER see papers 48,49, video 02

For more details on the "Smart Encoder Trailer" -- a system derived from the CLAPPER see papers 53, 57

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This file last updated on 7/5/96 by Johann Borenstein