Journal of Intelligent Material Systems and Structures http://jim.sagepub.com
Development of ER Brake and its Application to Passive Force Display Junji Furusho, Masamichi Sakaguchi, Naoyuki Takesue and Ken’ichi Koyanagi Journal of Intelligent Material Systems and Structures 2002; 13; 425 DOI: 10.1106/104538902030340 The online version of this article can be found at: http://jim.sagepub.com/cgi/content/abstract/13/7-8/425
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Development of ER Brake and Its Application to Passive Force Display JUNJI FURUSHO,* MASAMICHI SAKAGUCHI, NAOYUKI TAKESUE
AND
KEN’ICHI KOYANAGI
Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan ABSTRACT: Force display systems are important in virtual reality and other applications. While conventional force displays are active systems with actuators and as such may become inherently dangerous, passive force displays are effective methods for assuring safety. In this paper, we developed a brake using electrorheological (ER) fluid, and showed a passive force display system using ER brakes with two degrees of freedom. We also discuss basic control experiments of the system. Key Words: passive force display, brake, ER fluid, haptic interface, mechatronics
INTRODUCTION ORCE information is often required for tele-operation and virtual reality systems. Many types of force display systems are now available, but most of them are active systems using servomotors or other actuators. While these active systems can provide operators with a wide variety of force senses, they inherently involve potential hazards in that they could move without direction and endanger their operators if something went wrong. On the other hand, passive force displays using brakes or clutches to present resistance to an operator’s force or movement are quite safe (Davis and Book, 1997). The authors have developed an ER brake using an electrorheological (ER) fluid as its functional material. A passive force display using such ER brakes has also been developed. In this paper, the system is outlined and basic experiments with it discussed.
F
Figure 2 shows the basic structure of an ER brake consisting of a fixed cylinder and a rotating cylinder with ER fluid between them. The two cylinders also serve as a pair of electrodes. The rotating cylinder is fitted with an output shaft and is driven by an external force through the shaft. When voltage is applied between the cylinders or electrodes, an electric field is produced in the ER fluid, increasing the viscosity of the fluid. Thus, the rotating cylinder is forced to slow down, or brake, providing the rotating output shaft with resistance (braking torque). The specifications of the ER brake are shown in Table 1. As the maximum braking torque depends on the characteristics of the ER fluid used, the characteristic of the ER fluid used in this paper, ARP–06, which was provided for research purposes by Asahi Chemical Industry Co. Ltd, is shown in Figure 1. The basic characteristics of the ER brake were measured. Figure 3 shows the relationship between the
ER BRAKE 1400
Electrorheological fluid is a type of functional fluid, and has recently attracted much attention (Furusho, 1995; Nakano and Koyama, 1998). The shear stress of ER fluid is almost independent of the shear rate, and changes according to the magnitude of the applied electric field (Figure 1). Making use of the unique characteristics of particle-type ER fluid, the authors developed a clutch-type actuator (Furusho and Sakaguchi, 1996; Sakaguchi et al., 1998, 2000) and studied its applications (Furusho and Sakaguchi, 1999; Sakaguchi et al., 1999).
1200
DC 3kV/mm
Shear Stress (Pa)
1000 800 DC 2kV/mm
600 400
DC 1kV/mm 200 DC 0kV/mm 0 0
*Author to whom correspondence should be addressed. E-mail:
[email protected]
JOURNAL
OF INTELLIGENT
MATERIAL SYSTEMS
50 100 150 –1 Shear Rate (sec )
Figure 1. Characteristics of the ER fluid used in this paper.
AND
STRUCTURES, Vol. 13—July/August 2002
1045-389X/02/7/8 0425–5 $10.00/0 DOI: 10.1106/104538902030340 ß 2002 Sage Publications Downloaded from http://jim.sagepub.com at PENNSYLVANIA STATE UNIV on April 17, 2008 © 2002 SAGE Publications. All rights reserved. Not for commercial use or unauthorized distribution.
425
426
J. FURUSHO
an accurately controlled dynamic braking torque, and the braking torque of the ER brake can rise faster than that of a powder clutch.
Rotational Cylinder
Output Axis
ER Fluid
Electric Field
PASSIVE FORCE DISPLAY USING ER BRAKES
Fixed Cylinder
Figure 2. Basic structure of ER brake.
Table 1. Specifications of the ER brake. Diameter Height Weight Electrode-to-electrode distance Brake Torque Shear stress of ER fluid: 1 (kPa) Shear stress of ER fluid: 5 (kPa)
156 (mm) 135 (mm) 2.6 (kg) 1.0 (mm)
A 2 DOF passive force display using ER brakes has been developed. Figure 5 shows the front view and top view of the system. Figure 6 shows the force display system that uses two ER brakes connected through a belt-pulley mechanism to a parallel-link mechanism, with a computer display. Each ER brake controls the movement of the first or second link independently, and an operator holds the handle at the tip of the parallellink mechanism. The reduction ratio of the belt-pulley mechanism is 1 : 4, and the first and second links of the
1.3 (Nm) 6.3 (Nm)
3.0 (kV/mm)
1 Torque (Nm)
ET AL.
2.0 (kV/mm)
0.5
1.0 (kV/mm) 0
0.0 (kV/mm)
0
100 Shear Rate (1/s)
200
Figure 3. Shear rate vs. braking torque.
1 Torque (Nm)
2.5 0.5
1.5 : Torque : Electric Field
0
0.5 3
3.05 3.1 Time (s)
Electric Field (kV/mm)
Figure 5. Top and front views of 2 DOF passive force display using ER brakes.
3.15
Figure 4. Step response of ER brake.
output torque and the relative shear rate. Flat curves mean that the output torque is independent of the relative shear rate, and corresponds to just the strength of the applied electric field. Figure 4 shows the output torque response as measured in a step-response experiment. Note that the electric field was instantly increased from 0.5 to 2.5 (kV/mm) at the time 3.0 (s) as the output torque changed in several milliseconds. Unlike a conventional friction brake, the ER brake can produce
Figure 6. Picture of passive force display system with computer display.
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427
Development of ER Brake and Its Application to Passive Force Display (a) 0
1
BASIC EXPERIMENTS OF THE FORCE PRESENTATION
–200
0.5
Force (kgf)
Velocity Velocity (mm/s)
parallel-link mechanism are 20 (cm) long each. Each ER brake is provided with a high-resolution encoder to measure its rotational position and thereby the position of the handle, in which a force sensor is set. The operator manipulates the handle while watching the computer display.
Force –400
Virtual Walls
0
1
2 Time (s)
2.5
3
(b) 0
Force
200
–0.5
Force (kgf)
400 Velocity (mm/s)
The authors set up the system so that it represented virtual walls in a two-dimensional plane, and used it in preliminary experiments. Figure 7 shows a sketch of a top view of the passive force display. In the figure, the coordinates of the handle are (x, y), and the line of the virtual wall is Y ¼ k. When the handle touches a virtual wall, or y is less than or equal to k, an electric field is applied to the ER brakes and the operator feels a reaction force from the wall. Since the ER brakes feature a fast response, the sensation of touching a hard surface was well represented. However, the energized ER brakes provided a resistance force equal to the detaching movement of the handle, and represented a sticky wall. To prevent this phenomenon, the ER brakes were instantly de-energized when a detaching force of the handle was detected, that
1.5
Velocity 0
–1
3.5
4
4.5 Time (s)
5
5.5
(c)
ER brake I
400
0
Wall Y = k
Y
200
–0.5
Velocity 0
–1
3
ER brake II
3.5
X
Figure 7. Top view of passive force display.
Force (kgf)
Handle (x, y)
Velocity (mm/s)
Force
4 Time (s)
4.5
5
Figure 8. Experimental results (sticky and less sticky walls): (a) Touch (sticky wall); (b) Detach (sticky wall); (c) Detach (less sticky wall).
Table 2. Control algorithms of ER brakes.
Sticky wall Less stickly wall
Conditions
State of ERBs
y>k yk y>k yk Fy > 0 yk Fy 0
OFF ON OFF OFF ON
is to say, Fy , the y directional force of handle, was greater than zero. The control algorithm of the ER brakes is shown in Table 2. Figure 8 shows the experimental results for touch and detach of the virtual wall. The horizontal axes show the time, and vertical axes show the y directional velocity and reaction force measured at the handle. Positive velocity means the handle moved in the counter direction to the
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wall. When the handle touched the wall, at both types of walls, the velocity of the handle decreased and the reaction force increased quickly (Figure 8a, showing only a sticky wall). When the handle detached from the sticky wall, a large force was needed, but the handle was detached from the less sticky wall, only a small force was needed (Figures 8b and c).
ET AL.
Handle I
II
I
II
I
Sensation of Tracing Over a Virtual Wall How to represent the sensation of tracing over a virtual wall was also studied. With the passive force display system, an operator could feel a virtual wall because the operator felt a reaction force from the wall when the handle touched the wall. However, unlike a real wall, once the handle touched the virtual wall, the operator could neither move the handle into the wall nor slide the handle over it, because both the ER brakes were operated at the same time to represent a reaction from the wall. It became clear that representing the sensation of sliding over a surface requires clever switching of the two ER brakes such that at some times only one of them is operated. In this system, if only one ER brake was operated, the handle was restricted to move along the arc track, path I or II as shown in Figure 9. The handle can then trace over the virtual wall with a saw-teeth movement in a narrow width area, as shown in Figure 10. Here, the force direction information of the handle is introduced to switch the ER brakes smoothly. Figure 11 shows the relation between the wall and a direction of the operation force, where is the angle between the force direction and the wall, and 1 and 2 are the angles between the wall and tangent lines of each of the arc tracks which are drawn by the handle when only one ER brake is ON. According to Table 3, when the handle touched the wall and the force direction was within 0
