Acceleration Sensors Shape
groundwork has included development and improvement of tactile sensors compatible
with our telemanipulation system. Our first concern in integrating tactile
sensing with our telemanipulation system was to develop robust fingertip contact
sensors. Contact location is necessary for regulating internal grasp forces
during rolling operations. Our design utilizes an array of contact switches,
fabricated as part of a flexible printed circuit. The conductive traces of
the flex circuit, which make up one half of each contact switch, are distributed
around the circumference of the robot fingertip. The switches work much the
same way as keyboard membrane switches, except in our case thin strips of
dielectric tape seperate the conductive traces of the flex circuit from a
copper grounding foil. The design is simple, low profile and reconfigurable
for use in concert with other tactile sensors envisioned for our system. These
sensors detect a line of contact on the circumference of the fingertip, allowing
us to calculate a contact angle which is sufficient for control of our planar
robot hand. Future designs may include the ability to distinguish where the
contact was made along the depth of the finger. See below for more details
of the construction of our contact sensors.
Skin Acceleration Sensors
also recognize the importance we place on dynamic cues during routine
manipulation tasks. Much of our ability to perceive object features or
roughness relies on our ability to sense dynamic events using cutaneous
mechanoreceptors. For this reason we have investigated methods for measuring
such events. Our design is based on the "skin acceleration sensor" presented
by Howe and
Cutkosky (1989). See below for more details of the construction
of our skin acceleration sensors.
We are also investigating a sensor capable of measuring local contact
geometry, which we call a "shape sensor". Local object curvature (along
with contact location) is essential for planning finger motions during
a given manipulation task. The sensor employs an array of strain gages
with local signal conditioning deposited on a flexible printed circuit
to measure local curvature directly (see figure below).
Contact Sensor Construction
flexible printed circuit is the heart of our contact sensor design. The conductive
traces of the flex circuit form lines around the circumference when wrapped
over the base of the robot fingertip. Each trace is seperated from a copper
grounding plain by thin strips of dielectric tape, making up an individual
switch on the circumference. The contact switch array works much the same
way as keyboard membrane switches. The sensor is covered with a layer of 3M
These sensors detect 16 lines of contact on the circumference of the fingertip,
giving us a a contact angle resolution of approxmately 6 degrees. Future designs
may include the ability to distinguish where the contact was made along the
depth of the finger.
We have designed simple interface electronics to multiplex digital outputs
of two "16-bit" contact sensors into a standard 8-bit digital port
on our robot's control card.
Robot Fingertip Base
The fingertip base is machined from ABS and covered in a layer of
dense closed-cell packaging foam. Robot fingertips are designed to fit
interchangably onto a dovetail interface on our robot hand. This picture
shows small (4-48) ball plunger detents, purchsed from Carr-Lane,
projecting out of the circular cutout in the fingertip. When the fingertips
are placed on the robot hand, the detents rest in mating grooves of
the dovetail interface, providing interface preload and positive positioning
of the fingertip.
Robot Fingertip with Copper Grounding
Mylar-backed copper foil* (Arlon
3 mil 1oz. laminate) is laminated onto the foam using Avery
3032 SL) 2 mil. acrylic film adhesive from their
Specialty Tape Division.
* Initially we used copper foil without mylar backing, but had problems
with changing sensitivity of the contact sensors during use. We attributed
these changes to small indentations in the foil over the strips dielectric
tape. These indentations reduce the gap between the contact traces and
the grounding foil making the sensors hypersensitive, registering false
contacts. For more detail see below.
Contact Sensor flex circuit
The flex circuit shown here is complete with 20-pin, 0.050" pitch
grounding wire (shown here as white) and the strips of dielectric tape
(black vertical strips). These dielectric stips seperate the contact
traces (the interlaced horizontal copper features to the left) from
the copper grounding foil (shown above).
The flex circuit was photoimaged using dipcoated copper-clad Mylar
3 mil 1oz. laminate). The copper-clad mylar was dipcoated with (PC
179) positive photoresist at Injectorall.
The connector is held on with small barbs which protrude through holes
in the flex circuit into a small piece of 0.031 inch thick FR4. This
sandwiches flex circuit between the connector and the FR4. The connector
mates to a SAMTEC
(.050" pitch) ribbon cable using a FTSH-110-01-L-D
Robot Fingertip (shown with flex circuit & skin pealed back)
Thin dielectric strips (shown in black) seperate the flex circuit
from the copper grounding plane.
Completed Robot Fingertip
The completed fingertip is approximately 25mm in diameter.
The contact sensor is covered with microtexture 3M
skin. The assembly is clampled in place using 2 small aluminum plates
and 2-56 machine screws. A few coils of a steel torsion spring were
used for the grounding wire clip.
Contact Sensor Issues and Tuning
Robot Fingertip (show with flex circuit & skin pealed back)
Initially we used copper foil without mylar backing for the sensor
grounding plane, but we expereienced problems with changing sensitivity
of the contact sensors over time. We attributed these changes to small
indentations in the foil over the area of the strips dielectric tape
(shown on the left). These indentations reduce the gap between the contact
traces and the grounding foil making the sensors hypersensitive, resulting
in registered false contacts.
In addition to the indentations under the dielectric strips, there
are indentations which run across the width of the finger (perpendicular
to the former) (shown on the left). We believe these are the result
of continued contact pressure on the fingertips. However, regardless
of the cause these indentations are equally detrimental. These problems
were corrected by using mylar backed copper foil.
Skin Acceleration Sensor
|The skin acceleration sensors are compatible with our contact
location sensors and are built into the robot fingertip. Small accelerometers
(Analog Devices ADXL202E)
protrude through a hole in the contact sensor flex circuit and foam foundation.
The accelerometer is soldered onto a mylar flex circuit. The flex circuit
provides a convenient interface to signal conditioning electronics located
near the robot fingertip.