IEEE Robotics & Automation Magazine - December 2022 - 129

contact with an object, which will result in the displacement
of permanent magnetic particles in the magnetic layer. The
changed magnetic field values can be collected by five Hall
effect sensor arrays. We extracted the tactile features by analyzing
the collected data.
Fabrication
In the fabrication process, liquid silicone rubber and permanent
magnetic particles are mixed at a weight ratio of 1:1 to
form a magnetic layer, during which time the materials are
not magnetic. Then, we pull it into a 75 × 75 × 2-mm mold,
and the mold and liquid silicone rubber with the magnetic
particles are put into a vacuum machine for curing. After it
solidifies, we magnetize it by putting it in a magnetizing
machine. After magnetization, permanent magnet materials
in the magnetic layer can retain magnetism to provide magnetic
field information. Meanwhile, we pull liquid silicone
into the sensor pedestal to form a silicone layer. After the silicone
layer is solidified, we coat the bottom of the magnetic
layer with the same liquid silicone and then press it against
the silicone layer. When the liquid silicone between the two
layers solidifies, the magnetic layer, silicone layer, and pedestal
are bonded to form a complete elastomer. Finally, we
mount the sensor array at the bottom of the sensor pedestal
to complete the overall assembly of the magnetic tactile
sensor. The overall size of the magnetic sensor is 80 × 80 ×
7 mm; the thickness of the magnetic layer, silicone layer,
and Hall effect sensor is 2 mm; and the thickness of the
pedestal is 1 mm.
For the Hall effect sensor array, we
cascade multiple Hall effect sensors to
form a bus acquisition network. By
using an interintegrated circuit bus,
the acquisition frequency of the Hall
effect sensor is better than 80 Hz to
ensure that the sensor array responds
quickly to the change in the magnetic
field. The sensor array can acquire the
magnetic field intensity of 20 points,
shown in Figure 1.
Magnetic Field for the Sensor
For the proposed magnetic tactile sensor,
the distribution of the surface
magnetic field will affect its performance.
As long as the magnetic layers
are magnetized in different postures,
they can generate different magnetic
fields. To improve the performance of
the proposed sensor, we consider
changing the posture of the magnetic
layer when it is magnetized to obtain
the effect of multipole magnetization.
We compare the performance of the
multipole magnetization and the commonly
used magnetization method.
Magnetization
The residual flux strength of a soft magnetic layer is usually
not as good as that of a hard magnet under the same magnetization
conditions. However, to ensure that the sensor can
receive a stable magnetic field signal, it is necessary to make
the residual flux strength of the magnetic layer sufficiently
large. In the process of magnetization, the magnetic direction
and the shape of the magnetic layer affect the distribution of
the remaining magnetic field. It is expected that, by changing
the magnetization method, the characteristics of the magnetic
layer can improve.
Halbach magnetization methods and folding magnetization
methods have shown unique advantages in improving
magnetic tactile sensor performance [14], [24]. The folding
magnetization method allows for a higher magnetic flux at a
particular location. To improve the performance of the magnetic
tactile sensor, we enhance the magnetic flux above the
Hall effect sensor array to acquire a higher quality magnetic
signal. Therefore, we used the multipole magnetization
method to magnetize the magnetic layer, and it is able to
form a multipole magnetic field in its surface. For the magnetic
field formed by using this magnetization method, we
carried out simulation analysis and actual measurement. The
result is shown in Figure 2.
Test
First, we measure the magnetic field distribution of the magnetic
layer. As shown in Figure 2, the actual magnetic field
strength distribution for the multipole magnetic layer is
1
2
3
4
5
(a)
28.07
Serial
Port
(b)
A2 A3
A1
1
2
3
80 mm
(c)
Figure 1. The sensor structure. (a) The designed pattern. (b) The 3D Hall effect sensor
array and its communication protocol. (c) A prototype of the magnetic elastomer part
and the distribution of the Hall effect sensor. PCB: printed circuit board.
DECEMBER 2022 * IEEE ROBOTICS & AUTOMATION MAGAZINE *
129
A4 A5
Distribution of
3D Hall Sensor
Magnetic Layer
Silicone Layer
Pedestal
3D Hall Sensor
PCB
7 mm
7mm
15.11
IEEE Robotics & Automation Magazine - December 2022

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