"移动机器人定位研究手册：路标和信标等方法"

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This manual, "Where am I? Sensors and Methods for Mobile Robot Positioning," is a comprehensive guide to the early research on localization in mobile robots. It covers various methods such as landmarks, beacons, and other techniques used for positioning. The authors, J. Borenstein, H. R. Everett, and L. Feng, along with contributing authors S. W. Lee and R. H. Byrne, provide detailed information on how sensors and algorithms can be utilized to accurately determine the location of a mobile robot. This manual, prepared by the University of Michigan for the Oak Ridge National Lab (ORNL), serves as a valuable resource for researchers and engineers working in the field of robotics. Overall, it offers a thorough overview of the strategies and technologies available for mobile robot localization, making it an essential reference for anyone interested in this area of study.

Collimating

lens

Multi-track

encoder

Detector

array

Beamsource

expander

LED

Cylindrical

lens

disk

16 Part I Sensors for Mobile Robot Positioning

Figure 1.2:

A line source of light passing through a coded pattern of opaque and

transparent segments on the rotating encoder disk results in a parallel output that

uniquely specifies the absolute angular position of the shaft. (Adapted from [Agent,

1991].)

1.1.2 Absolute Optical Encoders

Absolute encoders are typically used for slower rotational applications that require positional

information when potential loss of reference from power interruption cannot be tolerated. Discrete

detector elements in a photovoltaic array are individually aligned in break-beam fashion with

concentric encoder tracks as shown in Figure 1.2, creating in effect a non-contact implementation

of a commutating brush encoder. The assignment of a dedicated track for each bit of resolution

results in larger size disks (relative to incremental designs), with a corresponding decrease in shock

and vibration tolerance. A general rule of thumb is that each additional encoder track doubles the

resolution but quadruples the cost [Agent, 1991].

Instead of the serial bit streams of incremental designs, absolute optical encoders provide a

parallel word output with a unique code pattern for each quantized shaft position. The most common

coding schemes are Gray code, natural binary, and binary-coded decimal [Avolio, 1993]. The Gray

code (for inventor Frank Gray of Bell Labs) is characterized by the fact that only one bit changes

at a time, a decided advantage in eliminating asynchronous ambiguities caused by electronic and

mechanical component tolerances (see Figure 1.3a). Binary code, on the other hand, routinely

involves multiple bit changes when incrementing or decrementing the count by one. For example,

when going from position 255 to position 0 in Figure 1.3b, eight bits toggle from 1s to 0s. Since there

is no guarantee all threshold detectors monitoring the detector elements tracking each bit will toggle

at the same precise instant, considerable ambiguity can exist during state transition with a coding

scheme of this form. Some type of handshake line signaling valid data available would be required

if more than one bit were allowed to change between consecutive encoder positions.

Absolute encoders are best suited for slow and/or infrequent rotations such as steering angle

encoding, as opposed to measuring high-speed continuous (i.e., drive wheel) rotations as would be

required for calculating displacement along the path of travel. Although not quite as robust as

resolvers for high-temperature, high-shock applications, absolute encoders can operate at

temperatures over 125

(

C, and medium-resolution (1000 counts per revolution) metal or Mylar disk

designs can compete favorably with resolvers in terms of shock resistance [Manolis, 1993].

A potential disadvantage of absolute encoders is their parallel data output, which requires a more

complex interface due to the large number of electrical leads. A 13-bit absolute encoder using

a. b.

Chapter 1: Sensors for Dead Reckoning 17

Figure 1.3:

Rotating an 8-bit absolute Gray code disk.

a. Counterclockwise rotation by one position increment will cause

only one bit to change.

b. The same rotation of a binary-coded disk will cause all bits to

change in the particular case (255 to 0) illustrated by the

reference line at 12 o’clock.

[Everett, 1995].

complimentary output signals for noise immunity would require a 28-conductor cable (13 signal pairs

plus power and ground), versus only six for a resolver or incremental encoder [Avolio, 1993].

1.2 Doppler Sensors

The rotational displacement sensors discussed above derive navigation parameters directly from

wheel rotation, and are thus subject to problems arising from slippage, tread wear, and/or improper

tire inflation. In certain applications, Doppler and inertial navigation techniques are sometimes

employed to reduce the effects of such error sources.

Doppler navigation systems are routinely employed in maritime and aeronautical applications to

yield velocity measurements with respect to the earth itself, thus eliminating dead-reckoning errors

introduced by unknown ocean or air currents. The principle of operation is based on the Doppler

shift in frequency observed when radiated energy reflects off a surface that is moving with respect

to the emitter. Maritime systems employ acoustical energy reflected from the ocean floor, while

airborne systems sense microwave RF energy bounced off the surface of the earth. Both

configurations typically involve an array of four transducers spaced 90 degrees apart in azimuth and

inclined downward at a common angle with respect to the horizontal plane [Dunlap and Shufeldt,

1972].

Due to cost constraints and the reduced likelihood of transverse drift, most robotic implementa-

tions employ but a single forward-looking transducer to measure ground speed in the direction of

travel. Similar configurations are sometimes used in the agricultural industry, where tire slippage in

soft freshly plowed dirt can seriously interfere with the need to release seed or fertilizer at a rate

commensurate with vehicle advance. The M113-based Ground Surveillance Vehicle [Harmon, 1986]

employed an off-the-shelf unit of this type manufactured by John Deere to compensate for track

slippage.

The microwave radar sensor is aimed downward at a prescribed angle (typically 45

(

) to sense

ground movement as shown in Figure 1.4. Actual ground speed V is derived from the measured

velocity V according to the following equation [Schultz, 1993]:

deadre05.ds4, .wmf, 10/19/94

Chapter 1: Sensors for Dead Reckoning 19

Parameter Value Units

Speed range 17.7

0-40

m/s

mph

Speed resolution 1.8

0.7

cm/s

in/s

Accuracy ±1.5%+0.04 mph

Transmit frequency 62.5 kHz

Temperature range -29 to +50

-20 to +120

(

Weight 1.3

Power requirements 12

0.03

VDC

Table 1.1:

Specifications for the

Trak-Star

Ultrasonic

Speed Sensor.

Figure 1.6:

A typical differential-drive mobile robot

(bottom view).

1.2.2 Other Doppler-Effect Systems

A non-radar Doppler-effect device is the

Monitor 1000, a distance and speed monitor

for runners. This device was temporarily

marketed by the sporting goods manufac-

turer [NIKE]. The Monitor 1000 was worn

by the runner like a front-mounted fanny

pack. The small and lightweight device used

ultrasound as the carrier, and was said to

have an accuracy of two to five percent,

depending on the ground characteristics. The

manufacturer of the Monitor 1000 is Ap-

plied Design Laboratories [ADL]. A micro-

wave radar Doppler effect distance sensor

has also been developed by ADL. This radar

sensor is a prototype and is not commercially

available. However, it differs from the Moni-

tor 1000 only in its use of a radar sensor

head as opposed to the ultrasonic sensor head used by the Monitor 1000. The prototype radar sensor

measures 15×10×5 centimeters (6×4×2 in), weighs 250 grams (8.8 oz), and consumes 0.9 W.

1.3 Typical Mobility Configurations

The accuracy of odometry measurements for dead reckoning is to a great extent a direct function

of the kinematic design of a vehicle. Because of this close relation between kinematic design and

positioning accuracy, one must consider the kinematic design closely before attempting to improve

dead-reckoning accuracy. For this reason, we will briefly discuss some of the more popular vehicle

designs in the following sections. In Part II of this report, we will discuss some recently developed

methods for reducing odometry errors (or the feasibility of doing so) for some of these vehicle

designs.

1.3.1 Differential Drive

Figure 1.6 shows a typical differential drive

mobile robot, the LabMate platform, manufac-

tured by [TRC]. In this design incremental

encoders are mounted onto the two drive

motors to count the wheel revolutions. The

robot can perform dead reckoning by using

simple geometric equations to compute the

momentary position of the vehicle relative to

a known starting position.

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