Wireless Tuning Fork Gyroscope for Biomedical Applications

Transcription

1 Wireless Tuning Fork Gyroscope for Biomedical Applications Jose K. Abraham, Vijay K. Varadan, Pennsylvania State University University Park PA 16802, USA A. Whitchurch and K. Sarukesi Bharathiar University, Coimbatore, Tamil Nadu, India ABSTRACT This paper presents the development of a Bluetooth enabled wireless tuning fork gyroscope for the biomedical applications, including gait phase detection system, human motion analysis and physical therapy. This gyroscope is capable of measuring rotation rates between -90 and 90 and it can read the rotation information using a computer. Currently, the information from a gyroscope can trigger automobile airbag deployment during rollover, improve the accuracy and reliability of GPS navigation systems and stabilize moving platforms such as automobiles, airplanes, robots, antennas, and industrial equipment. Adding wireless capability to the existing gyroscope could help to expand its applications in many areas particularly in biomedical applications, where a continuous patient monitoring is quite difficult. This wireless system provides information on several aspects of activities of patients for realtime monitoring in hospitals. Keywords: Gyroscope, Wireless system, Wireless Gyroscope, Bluetooth, biomedical applications 1. INTRODUCTION Recently, there has been a dramatic increase in the utilization of wireless technologies for sensing systems particularly in biomedical applications. Medical instrumentation, driven by rapidly evolving MEMS and microelectronics technologies, is changing from its bulky and analog components to smaller, smarter, more reliable and more amenable monitoring and control systems. One of the components behind the revolution of the medical instrumentation is the Sensors. Generally sensors are the integral parts of many monitoring and control devices. They help to monitor factors influencing a process or experiment such as temperature, acceleration, rate of rotation, stress/strain etc. Sensors are also widely used in biological diagnosis such as monitoring of electrophysiological signals (EEG, ECG), heartbeat, blood pressure, blood glucose level etc. One of the disadvantages of the current sensors for biomedical applications is the complex nature of the wires to be connected between the sensor and a monitoring system, which makes uncomfortable for many patients. Cables can be cumbersome, hazardous and prone to failure. Micromachining is one of the most important technologies for the development of many physical sensors such as gyroscope and accelerometers because it offers smaller size, mass production and cost effective fabrication. Several designs of micro-gyroscopes are presented in literature: tuning fork gyroscope [1-4], vibratory gyroscopes [5-7] and surface acoustic wave gyroscopes [8-10]. The main restriction in these gyroscopes is the wire connecting to the sensors and the monitoring system [11-12]. This can be overcome by integrating the MEMS devices with wireless technology, which can make possible to design cost-effective, user friendly devices [13-14]. Bluetooth has been one of the fast growing technologies in wireless area. But most of the applications developed using Bluetooth is focused on normal communication in an office environment or cable replacement solutions for connecting 338 Smart Structures and Materials 2003: Smart Electronics, MEMS, BioMEMS, and Nanotechnology, Vijay K. Varadan, Laszlo B. Kish, Editors, Proceedings of SPIE Vol (2003) 2003 SPIE X/03/$15.00

2 computers with various peripherals. Bluetooth has not yet been used for any such commercially available wireless remote sensing system. Existing wireless telemetry sensors use their own mode of communication and are not compatible with other devices. Usually, normal monitoring systems use cables to connect the system to the sensor unit. Wireless sensors have applications in a wide range of areas. Adding wireless capability to existing sensors could even help to discover new applications for many sensors. This paper presents the design and development of a wireless gyroscope using Bluetooth technology. The system enables to be connected continuously using any Bluetooth-enabled Notebook computer without any wires. The remote transmission unit is based upon a microcontroller which controls various functions and transmission of the signals obtained from the gyroscope connected to it. The wireless unit has the capability of monitoring five discrete channels simultaneously. The monitoring application can run on a computer and receive signals from transmitting units, this signal could either be manually monitored by a user, or a system could be developed for automatic monitoring and detection of events from the signals. Characteristic of the Bluetooth makes it ideal for the many medical applications. The use of Bluetooth technology for the wireless communication and the elimination of wires ensure system reliability which is particularly important in biomedical or other critical monitoring systems. It also minimizes chances of any errors which may occur due to improper signal acquisition. In this system, the data acquisition process is done on the sensing unit itself, and hence noise level and signal deterioration are found to a minimum. The use of Bluetooth standard for the communication ensures system reliability and compatibility with any Bluetooth capable PC. 2. MEMS GYROSCOPE The working principle of the gyroscope is based on a vibratory tuning fork gyroscope which is micromachined on quartz substrate. One set of tines is electrically driven to large amplitude of oscillation. This drive mode vibration can establish required linear momentum for the Coriolis forces to be developed. When these sensing elements rotates about an axis parallel to the tines, the forces perpendicular to tine velocity and input angular rate results a Coriolis force which can be written as F = 2 mω v where m is the equivalent mass of the driving tines, v is the velocity and Ω is the angular rate. The Coriolis force due to rotation is sensed by piezoresistors. 3. WIRELESS NETWORK In many cases, the main restriction in using sensors is the wire connecting the sensors to the monitoring system. The system consists of a wireless sensing unit and a monitoring system which is a Windows application running on any computer as shown in figure 1. The microcontroller-based sensing unit interfaces to up to 5 different sensors and is capable of acquisition, digitization and wireless transmission of the signals using the Bluetooth standard. The Bluetooth protocol stack has been built for the sensing unit with a microcontroller and Ericsson ROK /21E point-to-point Bluetooth module as well as the monitoring unit using a notebook computer. Proc. of SPIE Vol

3 wireless Sensing Unit with Ericsson Bluetooth Module Bluetooth enabled Notebook PC Gyro Inputs (up to 5 channels) Figure 1. Schematic diagram of th0e wireless gyroscope. Control Layer (App Layer) Host Controller Interface L2CAP Layer HCI-UART Transport Layer Baseband (Physical Layer) Fig. 2: Layers of the Bluetooth protocol stack 3.1 THE BLUETOOTH COMMUNICATION The Bluetooth protocol uses a combination of circuit and packet switching. Slots can be reserved for synchronous packets. Bluetooth can support an asynchronous data channel, up to three simultaneous synchronous voice channels, or a channel which simultaneously supports asynchronous data and synchronous voice. Each voice channel supports a 64 kb/s synchronous (voice) channel in each direction. The asynchronous channel can support maximal kb/s asymmetric (and still up to 57.6 kb/s in the return direction), or kb/s symmetric. Between master and slave(s), different types of links can be established. Two link types have been defined: 340 Proc. of SPIE Vol. 5055

4 Synchronous Connection-Oriented (SCO) link Asynchronous Connection-Less (ACL) link The SCO link is a point-to-point link between a master and a single slave in the piconet. The master maintains the SCO link by using reserved slots at regular intervals. The ACL link is a point-to-multipoint link between the master and all the slaves participating on the piconet. In the slots not reserved for the SCO link(s), the master can establish an ACL link on a per-slot basis to any slave, including the slave(s) already engaged in an SCO link. In this system, ACL links are used to exchange data packets. The HCI driver on the Host exchanges data and commands with the HCI firmware on the Bluetooth hardware. The Host Control Transport Layer (i.e. physical bus) driver provides both HCI layers with the ability to exchange information with each other. The Host will receive asynchronous notifications of HCI events independent of which Host Controller Transport Layer is used. HCI events are used for notifying the Host when something occurs. When the Host discovers that an event has occurred it will then parse the received event packet to determine which event occurred. The HCI Command Packet is used to send commands to the Host Controller from the Host. The format of the HCI Command Packet is shown in Figure 4. When the Host Controller completes most of the commands, a Command Complete event is sent to the Host. Some commands do not receive a Command Complete event when they have been completed. In the protocol stacks developed in both the units for this system, all communication and transfer of packets is performed at the Logical Link Control and Adaptation Protocol (L2CAP) layer. Data from the sensing unit are sent as ACL packets to the receiving monitoring system which processes the data payload within these packets. Executing functions and receiving events are done by sending and receiving appropriate commands to the Host Controller Interface (HCI) layer. The HCI provides a uniform interface method of accessing the Bluetooth hardware capabilities. Here, a master-slave approach is used in communicating between the sensing unit and the monitoring system. A Bluetooth protocol stack has to be developed for each of the units to enable communication between them. 3.2 GYROSCOPE INTERFACE: Most of the sensors to be connected to the Bluetooth system have to be scaled up to +5 V DC using amplifiers. Sensors with output within this range can be directly connected to the system. Figure 3 shows the schematic diagram of the sensor interface. For the present system, a common reference voltage of VREFL (lower reference) of ground and VREFH (higher reference) of +5V was used for the A/D converter to scale the input voltage. This means that the range of 0-5V would be scaled to digital value from to 4096 (since it is a 12-bit A/D converter). So, a change in the digital value of 1 indicates a change of 5/4096 = V. This rule is applied to the data received by the monitoring system to scale the data appropriately for analysis and visualization. The sensors can be connected to any of the analog input pins (AN0 to AN4, AN8, and AN9) of the microcontroller. A common problem which is likely to affect the quality of data acquisition is noise in the input analog voltage. If there is a considerable amount of high frequency noise in the input voltage, aliasing occurs in the digitized signal. So, an anti-aliasing filter should be used to remove unwanted high frequency noise from the input analog signal before it is fed to the analog-digital converter. A simple RC-filter or a high-pass filter built using Sallen-key circuits could be used to act as an anti-aliasing filter. If the voltage produced by the sensor is not in the range of 0-5V, then an external voltage reference should be used to bring the reference level of the A/D converter to the level of voltage produced as output by the sensors. Some sensors provide a built-in voltage reference for this purpose. For visualization of the data obtained, the digital values are plotted on a real-time graph with appropriate scaling to accommodate most of the amplitude of the signals produced by the sensor. These signals are plotted with respect to time on the X-axis. Since the data is already available in a digital format, it is ready for any kind of analysis which needs to be performed on the data. Appropriate processing algorithms could be applied for the acquired data to obtain more Proc. of SPIE Vol

5 information from the signal. All 5 channels are visualized at the same time to allow simultaneous monitoring of signals from different sensors. Figure 4 shows a screenshot of the monitoring program in action. 4. RESULTS The wireless gyroscope is fixed on a rate table (Ideal Aerosmith 1270VS), which is a one axis rotation table with a maximum velocity of deg/min and a resolution of 0.1 deg/sec. The output voltage is recorded for different rotation rates. Figure 3 presents the measured output voltage for radioman rate varies from -90 to Output V Rate, deg/sec Figure 3. Measured output of the gyroscope for different rates of rotation. 5. CONCLUSIONS A Bluetooth enabled wireless gyroscope for biomedical applications is presented in this paper. This micromachined gyroscope is ideal for many sensing applications including gait phase detection system, human motion analysis and physical therapy applications. The use of Bluetooth technology for the wireless communication ensures reliability which is particularly important in biomedical or other critical monitoring applications. 342 Proc. of SPIE Vol. 5055

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