A New PCB-Based Low-Cost Accelerometer for Human Motion Sensing

Transcription

1 Proceedings of the IEEE International Conference on Automation and Logistics Qingdao, China September 2008 A New PCB-Based Low-Cost Accelerometer for Human otion Sensing Dapeng Qiao, Grantham K.H. Pang Department of Electrical and Electronic Engineering, Industrial Automation Research Laboratory, The University of Hong Kong, Pokfulam Road, Hong Kong { dpqiao, gpang }@eee.hku.hk Abstract - This article presents a new low-cost PCB-based accelerometer. A metal film is adhered to PCB board, which forms the two electrodes of the sensing capacitor of the accelerometer. The sensor signal is proportional to the change of capacitance, and it is obtained by a read-out circuit. This signal is compared with the output signal of a commercially available accelerometer, ADXL330, by Analog Devices. The result shows that the new low-cost accelerometer can fulfill the requirement of human motion sensing. Index Terms - accelerometer, PCB-based, human motion sensing, read-out circuit. I. INTRODUCTION Accelerometers have found many interesting applications in biomedical engineering, navigation and toys. For example, the success of Wii, with sophisticated remote controls for video game, has demonstrated the tremendous potential for toys in the application of this technology. Currently, the accelerometer used in the remote control of Wii is ADXL330, a ES silicon accelerometer. Although its price has already dropped to only a few US dollars in volume purchase, it is still considered expensive when used together with the custom-designed chips and interface electronics associated with the remote control. Hence, a new generation of low-cost accelerometers that can trigger more diverse and wide spread applications is needed. To drive down cost, accelerometer designed and fabricated using low cost PCB processes and materials are explored.. Wii is the latest video game by Nintendo and it s game console is known for its wireless controller, called Wii Remote [1]. The handheld device is among the few innovative advanced gaming devices that detect acceleration in three dimensions. Its distinguishing feature enables the user to play interactive sport games such as tennis and badminton against the computer, and brought new levels of excitement to digital entertainment. The heart of the Wii Remote is an accelerometer (ADXL330) by Analog Devices [2], which is used to sense the player s hand position in three-dimensions (3D). Ideally, an accelerometer is a device whose output is directly proportional to the acceleration it is measuring. Very often, this fixed ratio of output to input is valid only over for a small range of operation, and for a small temperature range. ui an Kit, David C.C. Lam Department of echanical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong medcclam@ust.hk In recent years, the cost and size of accelerometers have been reduced with advances in microelectromechanical system (ES) technology. Other than their recent usage in the video game industry, wide applications have been found in Inertial Navigation System (INS), Hard Disk Drive (HDD) head positioning control system, virtual reality system and automobile control system. The ES silicon accelerometers can be divided into two categories. In one category, the acceleration is sensed and converted into capacitance or resistance value, which is then measured and output as voltage. Another category would translate the sensed acceleration into a change of the resonant frequency in mechanical resonators [3]. The frequency is measured and output as voltage. Both categories of accelerometers are based on the mechanical design structures of the polysilicon device. The parameters such as bias and scale factor stability changes with temperature,and the performance of the accelerometer is affected. When accelerometers are used in toys, remote controls for video game or applications in human limbs, the requirements of motion sensing are varied for different parts of human limbs. A summary of the acceleration and frequency range of human motions is shown in Fig. 1 [4]. Fig. 1 Acceleration and frequency of different parts of human body /08/$ IEEE 56

2 The following is the characteristics of the different types of devices: - Head devices (e.g. head phone): average 3.5Hz frequency with maximum frequency up to 8Hz; and tilt less than 60 degree per second. - Hand, arm, upper-body devices (e.g. tennis racket, baseball bat): acceleration ranges from 0.5g to 9.0g with frequency less than 12Hz. - Hand, wrist, finger devices (e.g. pen, cell phone): acceleration ranges from 0.04 to 1.0g with frequency less than 8-12Hz. - Foot-leg devices (e.g. walking distance measuring device): acceleration ranges from 0.2g to 6.6g with frequency less than 12Hz. The range of acceleration for different human parts suggests that one single design of the accelerometer may not effectively meet all specifications. The designs of accelerometers should be tailor-made for various applications. One major field of motion sensing in consumer products is the motion of human hands. Game controllers and computer remote presenters sense the swinging and waving motions of human hands. easurements of human hand acceleration were carried out by Verplaetse [4]. In the experiment, hand accelerations of arm or hand swinging motions were measured and were found to range from 0.49g to 9.02g. According to the experimental results, Verplaetse concluded that most of the acceleration activities were concentrated near the mean value of 2.2g. The frequency range of the swinging of the human arm is from 0 to 12Hz. Thus, to design an accelerometer for sensing human hand motion, the range of operation must cover the acceleration of 2.2g; and the frequency should be 12Hz or below. The accelerometer used for human motion sensing in game control should be inexpensive and PCB-based accelerometer can be used as a substitute for costly silicon accelerometer. Examples of successful micro-systems demonstrated to be successfully transferred from a siliconbased technology to PCB based technology, include RF switches, pressure sensors, micro-actuator and micro-fluidic systems [5]-[8]. The material and processing technologies was changed from silicon based substrates to organic substrates, for example, epoxy, polyimide, and Liquid Crystal Polymer (LCP). Silicon accelerometers can be categorized into piezoelectric, capacitive and thermal types. In a piezoelectric accelerometer, a proof mass is suspended by piezoelectric-active springs. Upon acceleration, the spring changes its shape and changes the output electric signal. In the capacitive type, the movement of the proof mass changes the gap distance between the two plates of the capacitor. In thermal accelerometers, temperature sensors are used to measure the temperature of the gas surrounding a heater. With acceleration, the gas moves and the change in gas temperature is detected by the temperature and correlated with acceleration. In this paper, the performance of a capacitive PCB-based accelerometer is characterized. II. PRINCIPLE OF ACCELEROETER SENSOR When an accelerometer is fixed to a moving object, the model of the accelerometer can be simplified and shown as a schematic diagram in Fig. 2. Fig. 2 Schematic diagram of a typical accelerometer The proof mass experiences an acceleration force from the object movement and the distance of displacement of the seismic mass is y when measured against the universal reference. The free body diagram of the seismic mass is shown in Fig. 3. Fig. 3 Free body diagram of mass The motion of the seismic mass can be described as, y + Dz + Kz = 0 (1) where, z = y x (2) and K is the spring constant and D is the damping constant. After substitution, equation (1) becomes, ( z + x) + Dz + Kz = 0 (3) and D K z + z + z = x (4) By applying Laplace transform, the second order transfer function of the seismic mass is, 2 2 Zs () s s H() s = = =, (5) 2 2 X() s 2 D K s + s+ s + 2ζωn + ωn where the natural resonance frequency is, K ω n = (6) and the damping ratio of the system is, 57

3 D ζ = (7) 2 K The relationship between the output displacement of the sensor and the input motion is shown in equation (5). The sensor sensitivity is dependent on the seismic mass, the damping effect of the damper D and the stiffness of the suspension structure K. The damping of the accelerometer originates from the squeeze film damping of the gas film underneath the proof mass. The damping factor D of the gas film is dependent on the viscosity of the medium, the gap height and the overlap area between the proof mass and the base. The stiffness of the suspension structure K is determined by the material properties and the geometric configuration of the structures. Under a constant acceleration a, equation (1) becomes, a + Kz = 0 (8) and, zstatic = (9) a K Equation (9) represents the static sensitivity of the sensor and the negative sign denotes the opposite moving direction of the acceleration relative to the mass displacement. From the equation, the sensitivity of the sensor is directly proportional to the ratio of the weight of the seismic mass to the stiffness of the suspension. To reduce the damping in the accelerometer, one approach is to reduce the gas pressure inside the package in order to reduce the viscosity of air. This approach requires costly advanced packaging with good sealing. An alternative is to use porous plates to vent the gas underneath the plate away during compression. In micro-machined accelerometers, an array of holes is fabricated on the movable thin-film electrodes to reduce the damping of the accelerometer. III. ACCELEROETER SENSOR The design specification of the PCB accelerometer sensor is defined based on the human motion sensing requirements. As most of the activities of the arm are at 2.2g, this will be identified as the target measurement of the PCB prototype. Characterization range of 5g with 2.2g at approximately midway of the full range was used in this study. The bandwidth of the sensor must cover the frequency of arm motion, which is 12Hz. The size of the accelerometer is the other design issue to be considered. It should be minimized in order to reduce the footprint occupied by the sensor and the cost of material. The prototype discussed in this paper is a capacitance type of accelerometer, which consists of a proof mass, a tap and a piece of PCB board. A piece of metal film, that was etched into a center plate with suspension attached to a frame, is adhesively attached to the PCB board. The metal film and the etched copper pattern on the PCB board facing the metal plate forms the two capacitor electrodes separated by a gas film. When accelerated, the proof mass moves and the gap distance changes with acceleration, and changes the capacitance of the sensor. The change in the capacitance value would be converted to a voltage output by the interface electronic. Fig. 4 and Fig. 5 are the front view and the back view of the prototype. A sensor and circuit that is part of interface circuit are placed on the PCB board. Fig. 4 The front view of the prototype Fig. 5 The back view of the prototype IV. EXPERIENTS AND ANALYSIS In the experimental setup, the sensor was connected to the read-out circuit which produced an analog voltage signal proportional to the capacitance of the sensor. The source waveform for the sensor was a 5k Hz square wave which had a maximum of 0.5V and minimum of -0.5V. This square wave was produced by a waveform generator, and DC power supply served the power source of the read-out circuit. The analog voltage of read-out circuit was read by oscilloscope. A 3-axis silicon accelerometer from Analog Device (ADXL 330) with analog voltage output was used as a reference device. The ADXL330 has signal conditioned voltage outputs in a single monolithic IC design. It measures acceleration with a minimum full-scale range of 3g with a sensitivity of 300mV/g. It can measure the static acceleration of gravity in tilt-sensing applications, as well as 58

4 dynamic acceleration resulting from motion, shock, or vibration [9]. The experimental setup was as follows: sensor ADXL330 PCB-based accelerometer read-out circuit Reference accelerometer Evaluation board Fig.6 Block diagram of the experiment setup oscilloscope ADXL330 has 3-axes acceleration output: X, Y, and Z. In this experiment only X-axis was compared with the output signal of the PCB-based accelerometer which has only one axis. The bandwidth of ADXL330 was set to 50Hz. An evaluation board (EVAL-ADXL330Z) provided by Analog Devices was used to obtain an output voltage from ADXL330. To compare the output of these two accelerometers under identical acceleration, they were affixed onto the same substrate in the experiment. Since the prototype was designed for human motion sensing, thus these two accelerometers were vibrated by hand to simulate human motion acceleration. The ADXL330 would give an output voltage at X-axis of around 1.2V at zero g. The X-axis sensitivity varied from 270mV/g to 330mV/g, and averaged 300mV/g. The PCB-based accelerometer s circuit was designed to be comparable with ADXL330 and gives -0.2V at zero g. Fig. 7 Static acceleration responses of PCB-based accelerometer compared with the response of ADXL330 Fig. 7 shows the output of these two accelerometers under static acceleration. The waveform at the top is the output signal of ADXL330, while the lower one is the output of the PCB-based accelerometer. From time 0 to 0.6 second, the two accelerometers were hand held in vertical position. Then, the two devices were placed in a horizontal up position until 2.5 second was reached. Afterwards, the two devices were flipped to a horizontal down position. The waveforms of these two accelerometers are not straight since it was hand held. It can also be seen that the waveforms are nearly the same. Both have a response of ~0.3V for 1g of change, which is close to the designed specification. In a second evaluation, the two accelerometers were shaken suddenly by hand, and their responses to abrupt shaking are shown in Fig. 8. Voltage [v] Output signal of ADXL330 Output signal of PCB-based acceleromter Time [s] Fig. 8 Abrupt shake responses Three sudden shake motions are shown in the figure. The first shake occurred at around 1.1 second and the two accelerometers moved from left to right, and then suddenly stopped. At about one second later, the two devices went from right to left, and then suddenly stopped again. At around 4.5 second, they were shaken again from left to right with more force by the hand. These motions have been clearly shown in the figure. Similar waveforms by both the PCB-based accelerometer and ADXL330 have been recorded. The bandwidth of the read-out circuit is designed to be 16Hz, which covers the common frequency of human motion. Fig. 9 shows the quick shake responses, and again, the waveform of PCB-accelerometer is essentially identical to that of ADXL330. The shake s frequency is about 8Hz, which is near the maximum of usual arm movement. From the figure, it can be seen that the PCB accelerometer can measure rapid arm shake. From the signal of ADXL330, the peak from the base line is about 0.8V. From the specification sheet provided, the sensitivity of ADXL330 is 0.3V/g, the maximum acceleration is about 2.7g. 59

5 Output signal of ADXL330 accelerometer. Consequently, the expected ratio of performance over price will be highly attractive. This will enable PCB-based accelerometers to be adopted in toys, biomedical and navigation applications that have low measure range, low temperature requirements with no strict limits on structure size. Voltage [v] Output signal of PCB-based acceleromter Time [s] Fig. 9 Quick shake responses V. CONCLUSIONS A low-cost PCB-based accelerometer has been designed and fabricated. The performance of the PCB based accelerometer is shown to be comparable to a commercial accelerometer (ADXL330) in human motion sensing. With a simple structure and no custom-design chips, we can expect that this new accelerometer would cost less than one USD, or around one tenth of the price of silicon REFERENCES [1] [2] [3]. Helsel et al., A navigation grade micromachined silicon accelerometer, Proc. IEEE Position Location and Navigation Symp., Las Vegas, pp , [4] C. Verplaetse, Inertial proprioceptive devices: Self-motion-sensing toys and tools, IB Systems Journal, Vol. 35, Issue 3-4 (1996), pp [5] R. Ramadoss, A. Sundaram, L.. Feldner, RF ES phase shifters based on PCB ES technology, Electronics Letters, Volume 41, Issue 11, 26 ay 2005, Pages [6] J. N. Palasagaram, R. Ramadoss, ES capacitive pressure sensor array fabricated using printed circuit processing techniques, IECON Proceedings (Industrial Electronics Conference), Volume 2005 (2005), pp [7] E. T. Enikov, K. Lazarov, PCB-integrated metallic thermal microactuators, Sensors and Actuators, A: Physical, Volume 105, Issue 1, 15 June 2003, pp [8] A. Wego, S. Richter, and L. Pagel, Fluidic microsystems based on printed circuit board technology, J. icromech. icroeng., Vol. 11, Issue 5 (2001), pp [9] 60