# A Wireless Hand Grip Device for Motion and Force Analysis
## 2 Materials and Methods
### Hardware Implementation
The use of these sensors, based on a polymer film (PTF) that shows a reduction of its resistance value when a force is applied, is acceptable. For ludic applications of the device, only finger position will be required; for therapeutic applications, the system would also detect changes in the force generated by the patient. These force sensors are connected to the inputs of the control block in a pull up configuration, where each sensor is connected between the power supply and a 10K grounded resistor. A good comparative study of the performance evaluation of five commercially available low-cost piezo resistive sensors with potential application in compression therapy is provided in [35]. Figure 3: Force sensors, IMU sensor, and Interlink FSR sensor. The microcontroller considered in the control block is the ATmega328. A PCB Arduino Nano, which contains the ATmega328 microcontroller, has been used. This platform has 8 analog inputs, which are enough to read the voltage values of the analog outputs of sensors. The communication between the microcontroller and the computer is done at two levels:  
* USB: This option is only used to upload the software into the Arduino Nano PCB. Once the program is loaded, this communication is no longer needed. This communication is not necessary when the device is performing ludic or therapeutic tasks since it works as a real wireless device. The device's application will be defined in the program design. In general, the microcontroller will read data from the different sensors and, depending on the data, will perform different actions. These actions include data visualization, data storage, data analysis or its interaction with ludic or therapeutic games. * RF communication: This is a new part of the device complying with the communication block mentioned above. It is composed by a BlueSMIRF Silver V2 Bluetooth wireless modem by SparkFun Corporation. The communication range is 18 m, good enough for the specifications of the device. The modem is placed on a PCB with reduced dimensions that could be integrated inside the device. The last part of the hardware is the power supply. Considering the wireless requirement of the device, the main element of the power supply is a battery. We have considered a rechargeable 3.75V ion-lithium battery with a capacity of 2.2 Ah. The rechargeable circuit is the MAX1811 integrated circuit, which is compatible with the battery used and can be charged via USB connection. The dimensions of the battery are 69 mm \(\times\) 21 mm \(\times\) 18.5 mm, and its weigh is 55g. All the circuits are biased between 0-5 V (note that the instrumentation amplifier is a rail-to-rail 0-5 V), and this facilitates the power supply implementation. The last component of the power supply is a DC/DC converter, which converts the 3.75 V of the battery to 5 V. The MAXIM683 chip is considered. This circuit is based on a charge pump, has a yield of 85%, a fixed output voltage of 5 V and a current output of about 150 mA. All these parameters are adequate for the complete device. All PCBs, designed and fabricated for the device, must satisfy the following requirements:  
* Reduced size: in order to be placed inside the device allowing for a comfortable and useful prototype;
* Easy connectability between them;
* Noise reduction techniques must be considered. In Figure 4, we show a view of the PCB (printed circuit board) implemented. The motion sensor communicates with the microcontroller using the I2C protocol is connected to pins 4 and 5, corresponding to the SDA and SCL lines, respectively. For wireless communication, Bluetooth is connected to the RX and TX lines of the Arduino Nano that correspond to TX and RX, respectively. The pressure sensors, resistors that depend on the pressure made, are connected in series with a resistance and act as a retainer splitter. This signal is read by pin 0 (analogue inputs to SI) up to pin 3 for the other sensors.