Sensing Textiles

Customized data acquisition hardware

For TextileUX, we developed several custom devices to operate our textile sensors. This was required due to the largely varying properties of the fabrics, and to cover various use cases, which came with different demands for the driver hardware. For achieving general-purpose solutions, we focused on these aspects:

  • scalability
  • connectivity
  • sampling frequency
  • accuracy and consistency
  • low PCBs footprint
  • versatility of transport protocols

Resistive Sensing

Since the RESi yarn, a metal wire coated with piezoresistive enamel, was the initial building block of TextileUX, our focus was on the development of driver electronics that are capable of a high number of resistive sensors that are embedded in textiles.

The first hardware we designed with scalability and high readout speeds in mind. It features a modular design and consists of a single main board for fast data acquisition and multiple extension PCBs that allow routing of a large number of sensor electrodes. This setup is highly flexible in terms of sensor dimensions as it is able to scale seamlessly from single sensors to matrices of up to 64×64 sensors. To sample the resistive sensors, we use shift registers in combination with analog matrix switches. The shift register applies voltage to individual rows, while the matrix switches sequentially connect each column to the ADC via a voltage divider. We furthermore include a digital potentiometer, to dynamically adjust the pull-down resistance in the firmware. This allows for adaptive sensing by adjusting to individual sensors with varying resistance changes. An Espressif ESP32 microcontroller is used to acquire sensor data, perform basic pre-processing, and sending data to a host device via USB, Wifi, or Bluetooth.

Following up on this hardware design, we developed a readout electronics variant for embroidered textile solutions. It was designed to be directly placed and mounted on the textile to circumvent fragile hardware connections. One major issue with e-textiles in general is the connection between the flexible embedded electrodes inside the augmented textile and the rigid data acquisition hardware. Inspired by the Arduino Lilypad, we designed a PCB that provides conductive mounting holes for individual electrodes so they are easily connectable by stitching with conductive yarn. Using a computerized embroidery machine, we fully automated the process of fixing this PCB onto the substrate fabric and connecting the sensor electrodes to the mounting holes directly. Unused mounting holes can be used for additional fixating with standard yarn. The hardware features 16 transmitter and 16 receiver connectors allowing for up to 256 individual sensor readouts per board, which are performed by an ESP32 and can be transmitted via Wifi or Bluetooth. Further connections for power and ground are also provided, for powering the board via embroidered traces, eliminating the need for cable linkage or including of batteries.

We introduced another hardware iteration focused on adaptability to different sensors and measurement accuracy while still keeping scalability in mind. One of the main differences to the resistive readout electronics is the introduction of a 24Bit delta-sigma analog-to-digital converter capable of reaching sampling speeds of up to 12800 sps. Additionally we employed a wheatstone bridge circuit to perform differential readouts which can now be configured for maximum flexibility through the introduction of programmable digital rheostats. This allows to balance the bridge for different sensors with different resting resistances which in return helps to achieve better measurement results especially when the inbuilt programmable gain amplifier is used to boost weaker signals. Additionally, we directly mounted four connectors to the PCB to provide stable connections to sensors, which is especially important for sensors that exhibit only small changes in resistance upon actuation. With this setup, up to 1024 individual sensors can be measured in a 32×32 sensor configuration.

Capacitive Sensing

Over the course of TextileUX we also experimented with and developed our own sensor systems based on capacitive measurement. In Sound of Textile, for instance, we employed self capacitance measurement to achieve hover interactions. For TexYZ we used mutual capacitance sensing to support multi touch.

We created another hardware with a focus on capacitive sensing. Many applications also required a small footprint PCB. The PCB was therefore designed around the tiny chip “CYBLE-022001-00” (Cypress Semiconductor). The hardware platform features a JST plug for connecting lithium-ion batteries, a charge controller that will automatically charge the battery if power is supplied. A major advantage and one of the main reasons for the creation of this PCB, is the module “CapSense” for capacitive sensing. Packed into the small-scale chip, it provides great signal-to-noise ratios, fast scan times and advanced features such as gesture recognition and an automatic tuning algorithm to ensure that changes in the environment do not affect the sensing process. The chip supports both self- and mutual capacitive sensing modes for different application areas. In addition, this hardware is also able to perform measurements of resistive sensors using the onboard ADC and supports communication via BLE.