# Design and Experimental Evaluation of an Electrorheological
## 2 Design and Characterization of the Haptic Module
### Working Principle and Implementation of the Bending Sensor  
To demonstrate the haptic actuator as a more robust haptic interface that communicates digital information between the haptic device and a computer or microcontroller, the actuation loop must be closed. To control the output signal (resistive force and vibration frequency) of the haptic actuator based on the input (speed and position of the actuator's indentation), a sensor must be introduced into the actuator's design. To this end, we place a bending sensor on top of the membrane design, as shown in Figure 3. The sensor operates as a bend-sensitive resistor coupled to the membrane's deformation to track its indentation. The sensor's working principle is based on its strain-sensitive resistance. As shown in Figure 3b, the sensor is made from a flexible substrate doped with conductive particles. The sensor consists of multiple thin layers. The polyimide (PI) shielding layer protects the sensor from external noise from the nearby high voltage. The polyethylene terephthalate (PET) base frame provides structural support for the sensor to bend elastically. The flexible electrode, which is coated with a polymer-based ink dispersed in a mixture of metal particles and carbon black powder, is used to pass voltage through the conductive layer. To make the flexible electrode work as a bend-sensitive resistor, the fabricated sensor was bent excessively on a curved surface to create initial nanoscale cracks between the metal particles in the electrode [62; 63]. The nanoscale cracks between metal particles formed on flexible electrodes enable variable conductivity between adjacent crack junctions depending on the degree of external bending. Here, the carbon black power forms electrically weak interconnections between the cracks in the metal powders, preventing complete electrical separation during bending. Figure 2: Experimental results: (**a**) kinesthetic testing (DC excitation) and (**b**) vibrotactile testing (AC excitation, shown: 5 Hz sine wave). Reproduced with permission [56]. Copyright 2020, IOP. Figure 3c shows the working principle of the sensor at macroscale. When the sensor is not bent, the particles are packed closely together giving rise to greater conductivity and lower resistance. When the sensor is bent, the conductive particles are spread farther apart, thereby increasing the resistance. By placing the sensor in a voltage divider circuit (see Figure 3c) and probing the output voltage with a microcontroller, the indentation depth may be tracked by a computer in real-time and fed into a control system. The module was designed such that three sensors may be placed within the actuator to track the indentation to ensure additional accuracy against interference from the actuator's high voltage, as shown in Figure 3d. Moreover, a low-pass software filter was applied to the sensor's analog data stream to reduce noise introduced by the high voltage line. To characterize the proposed sensor's resistance with respect to bend angle, a testing platform was constructed, as shown in Figure 4. The sensor was fixed to the stationary base and the sensor tip was magnetically secured to a platform attached to a stepper motor. As the motor turns, the sensor bends. At each angle, the sensor's resistance is read using an analog input. Figure 3: (**a**) Proposed bending sensor; (**b**) bending sensor construction; (**c**) sensor working principle and circuit diagram; (**d**) haptic module with ER actuator and bending sensor. To visualize the actuator’s internal components, the ER fluid that fills its volume is rendered as transparent. Figure 5 shows the characterization curve for the bending sensor: resistance (k\(\Omega\)) vs. bending angle (deg). As shown, for positive angles (i.e., expansion), the sensor exhibits a nearly linear response from about 250 k\(\Omega\) at 0 deg to 850 k\(\Omega\) at 90 deg. Across the angles tested, the sensor elastically returns to its initial state. Thus, the sensor is predictable and may be integrated into our haptic module.