Introduction to Kionix KXM Tri-Axial Accelerometer

Transcription

1 Author: Che-Chang Yang( ); recommendation: Yeh-Liang Hsu ( ). Introduction to Kionix KXM Tri-Axial Accelerometer The Kionix KXM tri-axial accelerometer, as shown in Figure 1, is a high performance silicon micro-machined linear accelerometer consisting of a sensing element and a CMOS signal conditioning ASIC(Application Specific Integrated Circuit) packaged in a standard 5 1.8mm DFN(DualFlat Non-lead). The device functions on the principle of differential capacitance. Acceleration causes displacement of a silicon structure resulting in a change in capacitance. A signal-conditioning CMOS technology ASIC detects and transforms changes in capacitance into an analogue voltage which is proportional to acceleration. The general specification of Kionix KXM is listed in the Appendix. Figure 1. Kionix KXM52 series accelerometers Figure 2 shows the functional diagram of KXM52. Three sensors measure the accelerations along X, Y, and Z axis. The charge amplifiers transfer the differential capacitance (charge) into analog signal and then amplify the signal to a higher voltage level. The oscillator generates differential charge and results in specific outputs when self-test mode is activated. The built-in 32kΩ resistors and the external, user-defined bypass capacitors comprise low-pass filters for output signals. 1

2 Figure 2. The functional diagram of Kionix KXM52 1. Typical schematic Figure 3 shows the typical schematic that is used in the postural sensory module. The output will change with supply voltage(v dd ) variations ranging from 2.7(2.5)V to 5.5V. 3.3V is recommended for the reason that the output of KXM is programmed and tested at 3.3V power supply. In this schematic, a 0.1µF capacitor (C 1 ) across V dd to ground is recommended to decouple the noise of the power supply. For more decoupling, a ferrite beads or small resistors may be inserted into the supply line. 2

3 Figure 3. The typical schematic of KXM52 Capacitors C 2 to C 4 across the output pins to ground are used to adjust the output bandwidth of each axis. These capacitors with the 32kΩ internal resistor comprise RC low-pass filtering circuits. The desired bandwidth f BW is determined by the Equation (1): C C, C 2, 3 4 = C BW 1 = 2π (32kΩ) f BW (1) The output bandwidth is an important parameter when using the accelerometer. For KXM , the output bandwidth of X and Y axis ranges from DC to 3kHz, and that of Z axis is from DC to 1.5kHz. The choice of desired bandwidth depends upon the application. For example, a higher output bandwidth is advisable for the applications which require fast and sensitive motion detection. As for human postural detection required in this research, only low bandwidth is needed. In Figure 3, 0.1µF capacitors are adopted, which limits the bandwidth at about 50Hz (Equation (2)). 1 f = BW = Hz (2) 6 2 π (32kΩ) ( ) Pin 9 of KXM is the power shutdown pin. When this pin is left floating or grounded, the KXM52 is shut down and draws very little power. When tied to V dd, the unit is fully functional. Pin 10 is the self-test pin, and 0.25V dd, 0.5V dd and V dd can be fed to it to check the output signals. If not in use, this pin must be tied to ground. Figure 4 shows the sensing module and its PCB layout whose size is 25 20mm. This small module can be connected to specific receivers like DDS or any compatible devices. 3

4 Figure 4. The sensory module and its PCB Layout 2. Output response Figure 5 illustrates the coordinates of acceleration in KXM , and Figure 6 and 7 show the response of this unit with respect to different directions of accelerations and with respect to gravitational direction in different positions, respectively. If there is no acceleration applied along an axis, the output voltage V off equals half V dd 1. If acceleration exists toward positive direction, the output voltage increases (V out > V off ) and vice versa. The sensing range of KXM of each axis is 2g and the output varies with acceleration linearly in the rate of 660mV/g. Equation (3) shows the relation: V( X, Y, Z ) = Voff ( offset) ± 660mV g (3) For example, when KXM is put horizontally and still, there are no gravitational effects along X and Y axes, and V outx =V outy =V dd. Only 1g vertical acceleration along the negative direction of Z-axis is detected whose effect can be regarded as the accelerometer moving in positive direction of Z-axis. In this condition, the output voltage at pin 14 is V outy >V off +660mV(at 3.3V V dd ). 1 0g (V off ) offset changes linearly with temperature and therefore will not always equal half V dd. The maximum 0g offset tolerance is 167mV under extended temperature range of -40 to 125, and 100mV under -40 to125. 4

5 Figure 5. The coordinates of acceleration of KXM ` Figure 6. Output response due to directions of accelerations 5

6 Figure 7. Output response due to gravitational directions in different positions Tilt/inclination sensing is a common application for low-g accelerometers. Figure 8 indicates the tilt assignments (pitch and roll), which φ, ρ, and θ stand for the tilt angle with respect to X, Y and Z axes relative to ground. Equation (4) identifies the relation between the tilt angles and accelerations of each axis. A simple performance test was conducted using Equation (4). KXM rotates about each axis from 0 o to 90 o, and the data is shown in Table 1 and Figure 9. One g accelerations yields an output of about 2.24V, and 0 g yields about 1.6V. When φ, ρ and θ is 0 o, the outputs of X and Y axis have the maximum sensitivity while Z axis has the minimum sensitivity. φ = arcsin( X accel ) ρ = arcsin( Y accel ) θ = arccos( Z accel ) (4) 6

7 Figure 8. Pitch and roll assignments relative to ground Table1. Output voltages of KXM w.r.t specific tilt angles Angle( ) X accel (g) X(V) Y accel (g) Y(V) Z accel (g) Z(V) V off

8 Figure 9. Tilt angle vs. output voltage of each axis. 8

9 Appendix. Kionix KXM general specification Performance Specifications Parameters Units Condition Range g 2.0 Sensitivity mv/g 660 0g offset vs. temp mv 100 C -40 to 85 Over temp range Span mv Noise Bandwidth µ g / Hz 35(x and y) 65(z) Hz 0 to 3000 max (x and y) 0 to 1500 max (z) Output resistance Ω 32k typical Non-linearity % of FS 0.1 typical ( 0.5 max) Ratiometric error % 1.0 typical ( 1.5 max) Cross-axis sensitivity % 2.0 typical ( 3.0 max) Power supply V 3.3-3dB V -0.3 (min) 7.0 (max) Absolute min/max ma 1.5 typical (1.8 max) Current 3.3V µa <1.0 Shutdown pin grounded ms 1.6 Power-up 500Hz Environmental Specifications Parameters Units KXM52 Condition Operating Temp Storage temp C -40 to 125 Powered C -55 to 150 Unpowered Mechanical shock g 4600 Powered or unpowered, ESD V 3000 Human body model 9