The Dexterous Hand Master (DHM) system is designed to control dexterous robot hands such as the UTAH/MIT and Stanford/JPL hands. It is the first commercially available device which makes it possible to accurately and confortably track the complex motion of the human finger joints. The DHM is adaptable to a wide variety of human hand sizes and shapes, throughout their full range of motion

Churchill, Philip J.

Little, Arthur D.

Marcus, Beth A.

English

NASA Technical Reports Server

e -  
N89 - 19883 
SENSING HUMAN HAND MOTIONS 
FOR CONTROLLING DEXTEROUS ROBOTS 
Beth A. Marcus Ph.D. 
Phil ip J. Churchil l 
Arthur D. Little 
Center for Product Development 
20 Acorn Park 
Cambridge MA, 02140 
commercially available device which makes it 
possible to accurately and comfortably track the 
complex motion of the human finger joints. The 
DHM is adaptable to a wide variety of human hand 
sizes and shapes, throughout their full range of 
mot ion. 
ABSTRACT: 
The Dexterous Hand MasterTM (DHM) system is 
designed to control dexterous robot hands such as 
the UTAH/MIT and Stanford/JPL hands. It is the first 
Figure 1- Dexterous Hand Master (DHM) system 
BACKGROUND: 
Laboratories around the country have begun to 
use dexterous robot hands such as the UTAH/MIT 
and the Stanford/JPL hands for robotic research 
applications. These hands are currently controlled 
through computer software, with little capability for 
teleoperation. As robotics technology has 
progressed, the dexterity of end-effectors has 
improved greatly, yet to make these dexterous 
hands widely and commercially applicable, an easy 
to use method for controlling them is required. 
In January, we began a program to develop a 
sensing device capable of measuring human hand 
joint angles and translating them into position 
control commands. ADL selected an exoskeleton 
design approach based on initial development 
work performed by Center for Engineering Design 
(CED) at the University of Utah in Salt Lake City, in 
conjunction with their dexterous robot hand 
program. Although their basic exoskeleton 
concept was viable for this application, several 
improvements were required to make this 
approach into a commercially available product. 
With this starting point we developed a system 
which collects joint angle data on 16 finger joints, 
in real time, through the use of Hall Effect sensors 
and an AT compatible microcomputer. The result 
is a system which is accurate, light weight, 
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comfortable to use, and easily adjusted to fit a wide 
range of human hand sizes.(figure 1). 
PROGRAM OBJECTIVE: 
The goals of this program were to develop a 
system which could accurately transduce human 
joint motions and provide them as control signals 
to robots. In order to achieve this objective we set 
the following design goals to improve on the work 
of CED : 
Reduced Bulk, 
Reduced Weight, 
Improved Comfort, 
Improved Reliability, 
Improved Fit, 
Improved Attachment, 
Improved Resolution, and 
Addition of Calibration to Accommodate 
Hand Sizes. 
SYSTEM DEVELOPMENT: 
In order to achieve these goals we began with the 
selection of the sensor. The CED device, which 
was created primarily for testing the UTAH/MIT 
hand during development, used off the shelf 
potentiometers. Through experimentation with a 
variety of conventional, high precision, and 
conductive plastic potentiometers, we found that 
these sensors had the following disadvantages: 
Unacceptable levels of resistance to 
rotat ion (friction), 
Large package volume, and 
Signal output levels overly sensitive to 
noise, wear, and other environmental 
factors. 
Therefore, we began considering other sensing 
methods and finally settled on Hall Effect sensors 
which had been successfully used for 
measurement of motion in other robotics 
applications (i.e. the UTAWMIT hand, also 
developed at CED). A Hall Effect sensor is a small 
semiconductor device which changes its signal 
output voltage in proportion to the magnetic field it 
is experiencing. Its advantages include: 
non-contact measurements (no friction), 
small package, 
signal output not subject to many 
environmental influences, and 
available with amplification on the chip. 
Using a magnet assembly designed by CED for the 
UTAH/MIT hand, we tested various Hall Effect 
sensors and selected a Honeywell product which 
featured built-in amplification and excellent 
temperature stability. We then experimentally 
determined the geometric relationship between 
the magnet and sensor which provided us with the 
desired response curve (figure 2). The important 
dimensions derived from these tests were the air- 
gap between the sensor and magnet faces, and 
the radial displacement of the magnet and sensor 
centerlines. In the proper configuration, the 
sensor output voltage will vary sinusoidally with the 
angular position of the magnet. 
Once the sensor geometry was fixed, we moved 
on to the simultaneous development of the 
linkage mechanism and the electronics package. 
Through study of anthropomorphic data and 
experimental observations, we selected optimum 
linkage lengths for each joint of each finger. We 
built a single finger prototype using a planar 
linkage, with rigid attachment blocks, held to the 
fingers with Velcrom straps. Extensive work with 
this prototype revealed three key problems: 
ORIGINAL PAGE IS 
OF POOR QUALlTy 
Figure 2- Hall Effect Sensor Response For Various Spacings 
Human fingers do not bend in a single 
plane (many people have noticeably 
crooked fingers), 
The relative orientation of the finger axes 
change continuously throughout the 
range of joint motion, and 
Finger segments grow and shrink 
substantially during bending, 
The planar linkage therefore, was not acceptable 
because of its discomfort and tendency to shift at 
the attachment points. This led to pinching and 
measurement inaccuracy. Additionally, the solid 
attachment blocks would either flop about on the 
finger or restrict the range of motion of the finger 
depending on the angle of deflection (usually 
both). 
We subsequently developed two features (figure 
3) which eliminated these problems; the spring 
clip attachment, and the passive pivots. The 
spring clip allows a rigid strap (i.e. VelcroTM) to 
change size as the finger bends, while keeping 
the attachment block firmly located on the finger 
segment. The passive pivot allows the finger to 
bend freely, while maintaining the integrity of the 
sensor measurement. 
Once the finger mechanisms were completed, we 
began solving the more difficult problem of 
measuring the thumb angles. The design of the 
distal joint linkage for the thumb was copied from 
the finger designs. The main problem we 
encountered was the measurement of the two 
proximal joints of the thumb. We concluded was 
that it would not be possible to comfortably and 
repeatably secure an exoskeletal mechanism to 
the proximal link due to the large mass of soft 
tissue which makes up the base of the thumb and 
the unusual geometry of the joint itself. The 
solution we devised was to place two sensors on a 
linkage whose geometric relationship to the thumb 
joint motions was known. This solution requires a 
software algorithm which calculates both joint 
angles in real-time as a function of the two sensor 
inputs. In addition, we added another passive 
pivot to the thumb base to decouple several off - 
axis rotations which are not applicable to a robotic 
thumb command. 
483 
Spring Clip Mount ' 
Figure 3- Linkage Design Features 
The functional block diagram of the electronics is 
shown in figure 4. The electronics effort focussed 
on the development of an analog circuit interface 
board to drive the sensors, condition their output 
for the off-the-shelf N D  board, and perform certain 
additional tasks required during assembly of the 
devices. In addition, we developed software to 
collect the data from the ND board, convert it into 
human joint angles, display the results, and 
transmit the joint angles through a serial port with 
an update rate of 100 Hz. The key element of the 
software is the hand calibration routine, which 
takes input from the user, combines it with fixed 
values from the sensor calibration and linkage 
geometries, and produces a set of look-up tables 
which map the sensor readings directly into human 
finger angles. 
This combination of features allows an individual, 
regardless of hand size or shape, to place the 
device on their hand, perform the calibration, and 
get actual finger angles as a direct output. The 
calibration tables may be stored to allow the same 
individual to use the device again with a minimum 
amount of reconfiguration. 
. 
Figure 4- Electronics Block Diagram 
SYSTEM DESCRIPTION: 
The resulting system illustrated in figure 1 
incorporates the following components: 
Exoskeleton - the mechanical structure 
which attaches the sensors to the human 
hand, 
Hall Effect sensors - 16 sensors, 4 per 
digit, 3 measure bending, 1 measures 
medial / lateral motion, 
Connection board - supplies power to the 
sensors, and returns their signals to the 
signal conditioning board, 
Signal conditioning board - conditions the 
signals for input to the A/D board, 
12 bit, 16 channel N D  converter - creates 
digital representation of the analog sensor 
outputs, 
AT compatible micro-computer - provides 
data storage, date transmission, and 
calibration capabilities, 
RS422 serial port -transmits data at 
19,200 baud for input to other computer 
systems, and 
484 
Proprietary software package - provides 
calibration, display, and communications 
functions. 
CONCLUSIONS: 
The Dexterous Hand Master system will allow users 
of dexterous robots to fully and rapidly understand 
which types of tasks can usefully be performed by 
these robots. It will also allow detailed study of the 
kinematics of the human hand, and the level of 
dexterity required to perform different tasks, which 
can enhance robotic applications. To date, four 
systems have been ordered, with three delivered. 
In the future we look anticipate developing more 
advanced versions of this technology featuring 
faster calibration routines, measurement of wrist 
joint angles and force reflection. 
REFERENCES: 
1. Jacobsen, S.C., Iversen, E.K., Knutti, D.F., 
Johnson, R.T., & Biggers, K.B., "Design of the 
UTAH/MIT Dexterous Hand," proceed inas of the 
F F F  1986 I m i o n a l  Conference on R i  obot G 
and AutomatioQ, San Francisco, California. 
2. Jacobsen, S.C., Wood J.E., Knutti, D.F., & 
Biggers, K.B., "The UTAH/MIT Dexterous Hand: 
Work in Progress," m n a l  i Jou rnal of 
Robotics Research, Vol. 3, No. 4, Winter 1984, 
~ ~ 2 1 - 5 0 .  
485 


Sensing human hand motions for controlling dexterous robots

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