Biomedical Engineering Reference
In-Depth Information
(A)
30
(B)
25
20
15
10
5
0
0.75
0
1
2
Current (A)
Figure 11.5
Torque vs.current of MR Fluid Break and its model.
and applied current. Torque is calculated from the load on the force sensor on the
lever on the output shaft of the brake. The body of t brake is fixed to the turntable,
which then rotates at a constant speed when output torque is measured from input
current to the coil.
Figure. 11.5
s
hows the relationship between applied current and brake torque.
Current is increased in steps of 0.25 A from 0.0 A to 2.0 A, then decreased the
same way to 0.0 A. Turntable rotation is 1.0 rad/s. Black dots in
Fig. 11.5
indicates
measurement results. Although slight hysteresis exists in torque properties against
input current, it is negligible and is ignored in this paper.
To unambiguously determine current corresponding to arbitrary brake torque,
we must make an approximation model of torque properties, here using model
A and Model (B) for 0.75 A or more. Model (A) is given as follows:
9.74
I
2
T
=
+
3.25
I
+
0.26
(
forI
<
0.75
)
(11.5)
where
T
denotes output torque [Nm] of the MR fluid brake and
I
coil current [A].
Model (B) is given as follows:
T
=
15.5
I
−
3.44
(
forI
≥
0.75
)
(11.6)
Current corresponding to command torque is obtained using inverse functions
of these model equations.
11.3.2 Muscle Strength Evaluation and Training using MR Fluid
Brake
Figure 11.6
s
hows the system we developed and
Fig. 11.7 a
diagram of it (Kikuchi,
2009). The system is for isokinetic exercises for the upper arm. Users hold the grip
with the left or right hand and exert maximum force to rotate it. With a belt-pulley
reduction unit (reduction ratio: 1/3.63) between the handle and the brake rotation
