The work is conducted on designing absolutely soft sensors applicable for soft robots, the basic structure of the soft sensor is coating soft electrodes on a sheet of Eco-Flex membrane, when subject to external tension, compression or shear load, the sensor body deforms, which will result in capacitance change. By detecting the resultant capacitance change, the information of external stimuli can be achieved. Classified by external stimuli, 3 types of soft sensors are designed: stretch sensor, compressive sensor and multi-axis sensor. These 3 types of sensors are all designed based on the capacitance change phenomenon, while differ from each other by body structure and electrode pattern. Besides, a method to fabricate soft electrode based on carbon grease is proposed by our group.

The fabrication process of these 3 types of sensors is shown in Fig. 1, and the details are described as follows:

1. mixing Eco-Flex 30 A and B components with mass ratio of 1:1;

2. vacuumizing the mixture for 10 minutes;

3. casting the mixture into 3D printed moulds with corresponding patterns;

4. curing the mixture and moulds in an oven at 650 for 30 minutes;

5. smearing carbon grease electrodes with certain pattern on surface of the cured sensor;

6. coating a thin layer of uncured Eco-Flex mixture over electrodes;

7. curing the sensor body in oven at 650 for 10 minutes.


Fig. 1. fabrication process of soft sensor

1.Stretch sensor

The aim of the stretch sensor is to design a soft sensor that can detect tension load while bearing 100% or even larger strain. The structure of the sensor is a sheet of Eco-Flex membrane coated with two layers of carbon grease electrodes. The experimental relative capacitance change – applied tension load curves of the sensor is shown in Fig. 2, which shows good stability of the sensor, and applicable to detect tension load. Besides, the mechanical property of stretch sensor is dominated by material, which can be predicted by Mooney-Rivlin hyperelastic material model: , where C1 and C2 are material coefficients, in the experiment conducted by our lab, the coefficients are: C1=-2746,C2=1.843E4. And the electrical property of the sensor is ∆C=λ-1

 

Fig. 2. stretch sensor experimental result

2.Compressive sensor

The aim of the compressive sensor is to design which is sensitive to external compressive load, by comparing structure without and with embedded air chamber, the sensor with embedded air chamber shows higher sensitivity, stability and linearity. The experimental fabricated compressive sensor is shown in Fig. 3. A theoretical model to analyze the mechanical – electrical property of the sensor is proposed, for the sensor with embedded air chamber can be view as 3 layers with different boundary conditions: top and bottom Eco-Flex membrane layer, inner air chamber layer. By separately analysis mechanical – electrical property of each layer, the relative capacitance change – compressive load curve is achieved, besides the impact of embedded chamber dimension on sensitivity is also investigated in the paper. The simulation result of the compressive sensor with and without embedded air chamber is shown in. The experimental result of compressive sensor with different dimensions are shown in.

Fig. 3. compressive sensor

3. Multi-axis sensor

The project is aiming to design a structure that can sensing both compressive and shear load, the structure of this sensor is similar to compressive sensor. To improve the directivity of the sensor, the electrode pattern is changed from rectangular in compressive sensor to triangular type. And the theoretical analysis of the electrical – mechanical property of the sensor is shown in.