Development of a Proximity and Tactile Sensor Array Using Self-Capacitance Measurement for Robot Hand

We proposed a proximity and tactile sensor using self-capacitance measurement for robot hand. The proposed sensor consists of the two electrodes (E 1 and E 2 ), GND and elastic body. The capacitance between E 1 or E 2 and GND is measured by switching between E 1 and E 2 . The capacitance between E 1 and GND is used to detect the object before contact (proximity range) and to discriminate the material on contact. In addition, the capacitance between E 2 and GND is used to detect the indentation on contact. In this paper, we propose the array system of this proposed sensor. In the experiment, detection of the object before and after contact, and identification of materials on contact were demonstrated. In addition, the prototype sensor array measured the shape of the object. The proposed sensor array may be useful as a tactile and proximity sensor on robot hand.


Introduction
Nowadays, human-collaborative robots are rapidly emerging in our society.The robot should provide tactile sensor so that it carries out various tasks in collaboration with humans.So, many kinds of tactile sensor have already been developed (1)- (3) .Most of these sensors detect contact condition only.On the other hand, proximity sensors have been developed to detect the object before contact (4) .In addition, multiple tactile and proximity sensors have been proposed to obtain various information.Most of these sensors consist of some single sensing system.However, the wiring of the sensor may become complicated when covering a wide area.Previously, we proposed a new single unit tactile and proximity sensing method which simultaneously uses optical and electrical measurement to become the simple wiring (5) .However the sensor structure may become complicated because the proposed sensor consists of photodiode, LED and electrodes.
The goal of our research is to establish a simplified system of the tactile and proximity sensor for robot.Fig. 1 shows the goal image that the sensor detects the object both before and after contact on the robot hand.When the robot hand approaches the object (Fig. 1(a)), the sensor detects the object and its position before contact (Fig. 1(b)).When the robot hand touches the object (Fig. 1(c)), the sensor detects the pressure and its position.Therefore, the sensor will assure the safety and workability of the robot because the robot will avoid unnecessary contact with objects including humans.We proposed a single unit tactile and proximity sensing method using self-capacitance measurement (6) .In this paper, we propose a proximity and tactile sensor array using self-capacitance measurement for robot hand.The array of the proposed sensor have been detected the object before and after contact.Therefore the structure becomes simple, and the sensor would improve the safety and workability of the robot.

Principles
In the proposed method, the self-capacitance measurement is used to detect the object both before and after contact.Fig. 2 shows the schematic diagram of the proposed sensor.The proposed sensor consists of the two electrodes (E 1 and E 2 ), GND and elastic body.The capacitance between E 1 or E 2 and GND is measured by switching between E 1 and E 2 .Fig. 3 shows the image for each measurement.As shown in Fig. 3(a), the capacitance (C 2 ) between E 2 and GND is measured to set the switch on A (Connection A).When the object approaches the sensor, C 2 does not change because GND are set between E 2 and object.When the object touches the sensor, C 2 changes according to the indentation which is equal to the distance between E 2 and GND.Here, the variation (ΔC 2 ) in C 2 is written by Here, C 20 is steady-state value of C 2 .Therefore, the sensor can detect the indentation (pressure) using ΔC 2 .
As shown in Fig. 3(b), the capacitance (C 1 ) between E 1 and GND is measured to set the switch on B (Connection B).When the object approaches the sensor, C 1 changes according to both the permittivity of the object and distance between the sensor and object.Here, ΔC 1 is the variation in C 1 .Therefore, the sensor can detect the object before contact when ΔC 1 is changed at non-contact.In addition, the sensor can distinguish the material in the object from ΔC 1 upon contact.Thus, the sensor can detect object both before and after contact using the self-capacitance measurement (6) .

The Prototype Sensor and System
Fig. 4 shows the structure of the prototype single sensor.The separation between the electrodes needs to be reduced to realize high spatial resolution.However, a large electrode is required to increase the sensitivity of electrical measurements.Thus, we propose a structure such that E 1 are large to increase the sensitivity at proximity range, and E 2 are small to realize a high spatial resolution on contact.Thus, in our design, one unit of the sensor consists of 2 × 2 electrodes for E 2 , and one electrode for E 1 .GND (G) electrode is set between E 1 and E 2 .The urethane gel (thickness: 2 mm, hardness: 5 (ASKER C), UG in Fig. 4) is set between E 1 and E 2 .A silicone sheet (thickness: 0.1 mm, Si in Fig. 4) is placed on top of the sensor that is isolated from the object.In addition, A silicone sheet (thickness: 0.5 mm, Si in Fig. 4) is set on top of the sensor to increase the sensitivity of contact measurements.The capacitances are measured by capacitance measurement IC (Analog Devices, AD7148) which are capacitance-to-digital converter.AD7148 is attached to the back of the sensor to reduce electrical noise by wiring (6) .Sensor array Microcomputer are serial data line (SDA), serial clock line (SCL), power line (V dd ) and ground line (GND).Each slave devices is connected to a master device by each serial data line and common lines which are serial clock, power and ground line.Thus, this array system needs the 15 wires which are twelve serial data lines, one serial clock line, one power line and one ground line.Each serial data lines are switched by the multiplexer continuously to send measurement data.Furthermore, the sensor can be increased on same I 2 C line.Thus, the wiring can be reduced.In addition, each AD7148 measure the capacitance of the sensor continuously when the other AD7148 send the data to microcomputer to reduce the response time.

Proximity Measurement on Single Sensor
We evaluated the proximity measurement using a sensor on the array system.The distance (d from 0 to 50 mm) between the sensor and object was changed by the robot arm.The object and sensor were set up in the following conditions shown Fig. 6: (A) The object covers the center on the sensor (Fig. 6(b)).(B) The object covers a point (P 1 ) on the sensor (Fig. 6(c)).The touch condition between the sensor and the object is detected from force gauge.The objects (The tip is a semicircle and the diameter is 20 mm) are grounded conductors (GND) which are human models and acrylic.Fig. 7 shows the variation (ΔC 1 ) which was measured by E 1 .ΔC 1 changes according to the distance between the sensor and object.Here, when a priori information regarding the electrical property of the object is lacking, the sensor may not detect the distance between the sensor and object from ΔC 1 because ΔC 1 changes with both distance and the permittivity of the object.However, the sensor detected the approach of object and vice versa using ΔC 1 before contact.In Fig. 7, the standard deviation (SD) of ΔC 1 was 8.8 digits when the object was far from the sensor.Thus, the sensor can detect the object when ΔC 1 is changed more than ±3SD (26 digits) from a steady state value of ΔC 1 .In the case of the grounded conductor, the sensor detected the object within approximately 15 mm, and in the case of the acrylic, the sensor detected the object within approximately 7 mm.Fig. 8 shows the variation (ΔC 2 ) which was measured by E 2 on condition A of GND.In Fig. 8, ΔC 2 does not change as the distance varies between 1 mm and 50 mm (proximity range).In addition, ΔC 2 changes after contact.Thus, the sensor can detect the contact using ΔC 2 (6) .In Fig. 8, the SD of ΔC 2 was 2.5 digits when the object was far from the sensor.Thus, the sensor can detect the contact when ΔC 2 is changed more than ±3SD (7.5 digits) from a steady state value of ΔC 2 .

Contact Measurement on Single Sensor
We evaluated the contact measurement using a sensor on array system.The pressure (force) between the sensor and object is changed by robot arm.Force is measured by the force gauge.Fig. 9 shows the variation (ΔC 1 ) which was measured by E 1 .In Fig. 9, ΔC 1 changes with the permittivity of the object.The values of grounded conductor are high, and the values of acrylic are low.Thus, the sensor can discriminate between the grounded conductor and other.Here, the conductance of human is high and a part of the human is grounded.Thus, the values of the finger are similarly changed as the grounded conductor.Therefore, the sensor can recognize whether the object is a human (grounded conductor) or not (6) .Fig. 10 shows ΔC 2 for each experimental condition (A and B) on P 1 .Fig. 11(a) shows ΔC 2 on condition A of the grounded conductor, and Fig. 11(b) shows ΔC 2 on condition B of the grounded conductor.In Figs. 10 and 11, ΔC 2 changes according to the force and position.In addition, ΔC 2 does not change with the permittivity of the object.Therefore, the sensor can detect the pressure for each position (P 1 -P 4 ).

The Array System of the Proposed Sensor
We evaluated the measurement using all sensor of the Object Sensor array   array system before and after contact.Fig. 12 shows the object position on the array system.Fig. 13 shows ΔC 1 and ΔC 2 when the object is a grounded conductor (GND, 20 × 20 mm), and Fig. 14 shows ΔC 1 and ΔC 2 when the object is an acrylic object (20 × 20 mm).Figs.13(a) and 14(a) show ΔC 1 , and Figs.13(b) and 14(b) show ΔC 2 .Here, ΔC 1 and ΔC 2 became the white color in Figs. 13 and 14 when the values are less than ±3SD lower than each value of air.In Figs.13(a) and 14(a), ΔC 1 change according to the distance and the permittivity of the object at non-contact.Thus, the sensor can detect the object and position before contact (proximity range).In addition, the sensor can discriminate between a grounded conductor and another after on contact.
In Figs.13(b) and 14(b), ΔC 2 changes according to the pressure and the precise position of the object when the object is pressed.In addition, ΔC 2 does not change without contact.Thus, the sensor can detect the pressure and the precise position of the object.Therefore, the sensor would improve the safety and workability of the robot.It takes approximately 64 ms to obtain one complete cycle of measurements (12 sensors) with the data transmitting time from a microcomputer to PC.This data transmitting time from a microcomputer to PC is approximately 46 ms.Thus, the main time-consuming of this system are the data transmitting time.Therefore, we think that the response time of this system can be reduced to reduce the data transmitting time.

Conclusions
In this study, we propose a proximity and tactile sensor array using self-capacitance measurement for robot hand.The proposed sensor consists of the two electrodes (E 1 and E 2 ), GND and elastic body.In the experiment, detection of the object before and after contact, and identification of materials on contact were demonstrated.In addition, the prototype sensor array measured the shape of the object.The proposed sensor array may be useful as a tactile and proximity sensor on robot hand.
. Relationship between ΔC 2 and force on contact.(a) Condition A. (b) Condition B. Data are represented in mean ± SD (n=15).