Biomedical Engineering Reference
In-Depth Information
Force sensor
Strain-Stress
curve
of the object
Data
acquisition
card
Force
Data
acquisition
card
PID
controller
Shaft position
Linear actuator
D/A
converter
Shaft position control
Figure 11.1
Block diagram of the tactile display
sensor was used to show the feasibility of the approach and this idea can be expanded to
a matrix of pins utilizing miniaturization techniques.
A force sensor is used to measure the force exerted by a finger to the tip of the
shaft. The measured force is registered by a data acquisition card and interpreted by
processing software. The position of the shaft is also measured and transmitted to this
same processing software through a data acquisition card. The mechanical properties of
the object to be simulated are, in this case, the force - displacement data analogous to the
stress - strain relations for that object, which have already been determined and stored.
Knowing the force applied to the linear actuator by the fingertip, and the dimensions of
the cap over which the force is applied, the stress can be calculated and the desired strain
can be found by using the characteristic curve of the simulated object. The desired strain
is then converted into a displacement and is used as the input for a proportional integral
derivative (PID) controller, which is a generic control loop feedback device widely used
in industrial control systems.
Having obtained the actual and desired position of the shaft, the PID prepares the
control commands for the actuator and transmits them to the driver circuit. Figure 11.1
shows the block diagram of the system. A photograph of the tactile display is shown in
Figure 11.2.
11.2 Tactile Actuator
The tactile display, shown in Figure 11.3, consists of a linear actuator, a Plexiglas cap
(glued to the tip of shaft), a force sensor, a shaft position sensor and current driver
electronics. The shaft of the actuator is able to move up to 2 cm in the vertical direction
and can apply forces of up to 30 N. A force sensor, placed on the cap, measured the force
applied by a finger to the shaft and a built-in position sensor was used to measure the
