Fingertip
Haptic Display (FHD)
Abstract
Fingertip
Haptic Display (FHD) is a 2 degree-of-freedom haptic device whose
mechanical design is optimized for the workspace of the human
finger. This device is being used to study human perception of
curved surfaces and surface discontinuities.
Specifications
.
| Active
Degrees-of-Freedom |
2 (translational) |
| Workspace |
Index
Finger |
| Maximum
Force |
gf |
| Position
Resolution |
mm |
| Backdrive
Friction |
<
X gf |
| Inertia
- apparent at handgrip |
<
X gr |
FHD
Mechanical Design and Fabrication
Objective:
Detailed development/fabrication of FHD system. This includes
design details such as shop drawings, coil specifications, etc.
Actuator driver and position sensor electronics are also part
of this objective.
Work Status/Results:
The mechanical design was completed by the end of CY'97. Unique
features of this design include embedded analog optical encoders
for the A1 and B1 joints, as well as coils which are embedded
in the A1 and B1 links. The latter feature saves space over traditional
flat-coil actuator design and also aids in the removal of heat
from the coil, which one of the primary performance constraints
for this type of actuator.
A solid modeling package called
Solidworks was used to develop the design, perform interference
checking between parts, and produce production drawings. 3-D views
of some the FHD sub-assemblies are shown in the table below.
Some Design Facts and Features:
- Most
of the manufactured FHD components are made of 2024-T6 aluminum.
However, the magnet backing irons are made of a high-permeability
steel, and the magnets were custom-made Neodymium-Iron-Boron
magnets from
Crumax Magnetics
- A total
of 14 part drawings were used to capture the custom- manufactured
parts of the design. The fully assembled FHD includes more
than 100 components, including many off-the-shelf bearings,
spacers, shafts, etc.
- Custom-wound
actuator coils are wound on machined coil bobbins. The wire
(#28AWG) is pre-coated with an epoxy bond coat by the magnet
wire manufacturer so that the entire wound bobbin assembly
may be baked to bond the wire into a self-supporting coil.
- The
coils are bonded into the actuator armatures (A1 and B1 links)
using a ceramic-like
magnesium phosphate cement (Omegabond 600) that has very
high thermal conductivity to help carry the heat out of the
actuator coils.
FHD
Development/Runtime Environment
Objective:
The development of both the FHD force display simulation environment
as well as the concomitant visual simulation environment. This
includes the ability to represent and display complex curved surfaces.
Work Status/Results:
To date, most of the haptic simulation work in the BRL lab has
been using a 16-bit DOS development/runtime environment on Pentium-class
Intel architecture machines. While having the advantages of giving
direct access to the hardware (and thus allowing predictable 1kHz
update rates for haptic servo loops), other aspects of this environment
have been less than desirable:
- Large-memory
programs have difficulties and inefficiencies due to the real-mode
operation of the Intel chips.
- Program
debugging is difficult: if your real-mode program crashes,
it often crashes the entire computer, making it difficult
to track down what went wrong.
- Support
of add-on shrink-wrapped hardware such as accelerated graphics
cards or network cards is difficult.
- Multiprocessor
programming is very difficult to implement.
One approach
used by one of our lab members was to use a 32-bit protected-mode
extender for DOS that is part of the freely distributed DJGPP
package. However, while this helps to address the first problem
listed above, it still leaves much of the difficulty of the remaining
problems unchanged.
The desire to increase the available computing power for haptic
simulation (using multiprocessor systems) as well as the desire
for a more stable and supported development environment motivated
the search a new way to do haptic simulation.
One new approach that seems to be workable is to use
Sun Microsystems
Solaris-2.6-x86 operating system on an Quad-processor PentiumPro
system (200MHz chips with 0.5MB Cache per chip). This system contains
adequate (0.5GB) RAM for any of large dynamic model that we might
want to have in the future.
A Solaris device driver has been written that allows access to
an ISA card (a
Tech80 5641 quad IP carrier board) that supports the addition
of various industry-standard IP daughter modules. A 6-channel
24-bit digital encoder module and an 8-channel 12-bit DAC module
are currently in use with two remaining empty IP slots available
for future expansion..
The driver activates a programmable timer on the card at configuration
time. This timer generates a high-priority interrupt to the driver
at every timeout (typically 1ms intervals), responding with about
12-20 microseconds to sample the encoder positions and update
the DAC outputs. These encoder positions and DAC outputs are readable/writable
respectively via shared memory by the user program which opened
the Tech80 device driver. Using the POSIX real-time scheduler
priority capability provided by Solaris, user processes (or threads)
may be bound to one or more of the 4 processors and perform the
haptic simulation with input/output via shared memory. Other lower
priority threads can be used to perform user I/O and associated
graphic simulation output.
Research
Projects
Fingertip
Haptic Display - Reachable Workspace Model
Fingertip Haptic Display - Kinematic Design
Detection
Thresholds and Performance Gains for Small Haptic Effects
Publications
(*)
(*)
Note: Most of the BRL
publications
are available on-line in a PDF format. You may used the publication's
reference number as a link to the individual manuscript.
[132]
S.C. Venema, B. Hannaford,
'Experiments in Fingertip Perception of Surface Discontinuities,'
Intl. Journal of Robotics Research, vol. 19, pp. 684-696,
July 2000.
[140]
S.C. Venema, E. Matthes, B. Hannaford,
'Flat Coil Actuator having Coil Embedded in Linkage,' U.S.
Patent Pending, 2000.
[143]
S. Venema, B. Hannaford,
'A Probabilistic Representation of Human Workspace for Use in
the Design of Human Interface Mechanisms,' IEEE Trans. Mechatronics,
vol. 6, pp. 286-294, 2001.
[Th021]
S.C. Venema,
'Experiments in Surface Perception Using a Haptic Display,'
Ph.D. Thesis, University of Washington, Department of Electrical
Engineering, April, 1999.
S.C.
Venema,
System Design Issues for Virtual Reality SImulations with Haptic
Displays, April, 1998- Slides [PDF
805K]
|
