# A Wireless Hand Grip Device for Motion and Force Analysis
## 2 Materials and Methods
### Hardware Implementation  
Figure 2 depicts the hardware components of the device, which are briefly described below. * Power of the system: This part includes a battery that provides a 3V voltage, which is converted to 5V through a DC/DC regulator. * Sensors: This block contains a gauge that measures the force made by the user's hand; four sensors are able to detect the pressure of the fingers; a gyroscope and an accelerometer measures hand movements (velocity and acceleration) in order to detect user's tremors. * Control of the system: A microcontroller circuit is in charge of controlling the system. This circuit presents A/D converters at some inputs, and then analog inputs are presented to the microcontroller. These inputs are necessary since the output of some of the considered sensors is analog and they must be processed by the full system. * Communications block: The hardware also includes a RF circuit that is able to communicate with a laptop. This feature allows data presentation on the laptop's screen. It is a desirable element in the design of games with ludic or rehabilitation therapy objectives. One of the most important sensors is dedicated to the measurement of the force that is applied by the user's hand. In this prototype, a force cell based on a Wheatstone bridge  
Figure 1: Shaders prototype and their components: (**a**) controller case, (**b**) Arduino, (**c**) Instrumentation Amplifier (AD627), (**d**) IMU sensor, (**e**) force sensors, (**f**) Interlink FSR sensor, and (**g**) battery. Figure 2: Hardware components of the device. composed by four gauges has been considered. In particular, two identical cells are placed at both ends of the device. Each cell is capable of measuring a force up to 45.35 kg, thus the total force is the sum of the forces applied two both cells, that is 90.7 kg. Based on experimental studies performed in [29; 30; 31; 32] that show the development of a normative data table for hand grip and pinch strength and a summary of grip strength measurements obtained recently, we propose several electromechanically devices that allow the capture of hand motion and force. We also propose a comparative evaluation of our solution against the commercial dynamometer used in [32]. Another important contribution is related to the ergonomically evaluation of biomechanical hand function presented in [33]. Ref. [34] presents a review of the measurement of grip strength in clinical and epidemiological studies, whereas this paper proposes a standardized method for a more consistent grip strength measurement. These cells give us an output of 16-24 mV depending on the applied force. An instrumentation amplifier (AD627) is used for signal conditioning to obtain a voltage range compatible with the control block (analog inputs of the microcontroller). This instrumentation amplifier is rail-to-rail, presents a low consumption (85 microamperes), and the gain can be controlled by using a resistor between pins 1 and 8 of the chip. The response time is 135 microseconds. All these features are adequate and comply with the device requirements. Figure 3 shows a picture of the sensor used. Another component of the sensor block is dedicated to the detection of hand movement. In this part, an inertial measurement unit (IMU), which includes a gyroscope (ITG-3200 model) and an accelerometer (ADXL345 model), has been considered. This unit, produced by SparkFun Corporation, allows for six measurements (Roll, Pitch, Yaw, X, Y, Z). With this data, we can estimate the movements of the hand in terms of position, velocity and acceleration. This information is very important in the detection of tremors. The two chips (accelerometer and gyroscope) are placed on a PCB with reduced dimensions and incorporated to the device. The communication between these sensors and the control block is done through I2C protocol. The last set of sensors is resistive force sensors. These sensors control the adequate positioning of the fingers. In this case, a high precision sensor is not necessary.