//Logo Image
Author: Che-Chang Yang(2006-01-02); recommendation: Yeh-Liang Hsu (2006-01-03).

Introduction to Kionix KXM52-1050 Tri-Axial Accelerometer

The Kionix KXM52-1050 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 5×5×1.8mm 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 KXM52-1050 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 32kW resistors and the external, user-defined bypass capacitors comprise low-pass filters for output signals.

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(Vdd) variations ranging from 2.7(2.5)V to 5.5V. 3.3V is recommended for the reason that the output of KXM52-1050 is programmed and tested at 3.3V power supply. In this schematic, a 0.1mF capacitor (C1) across Vdd 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.

Figure 3. The typical schematic of KXM52

Capacitors C2 to C4 across the output pins to ground are used to adjust the output bandwidth of each axis. These capacitors with the 32kW internal resistor comprise RC low-pass filtering circuits. The desired bandwidth  is determined by the Equation (1):

                                                          (1)

The output bandwidth is an important parameter when using the accelerometer. For KXM52-1050, 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, 0.1mF capacitors are adopted, which limits the bandwidth at about 50Hz (Equation  (2)).

                                           (2)

Pin 9 of KXM52-1050 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 Vdd, the unit is fully functional. Pin 10 is the self-test pin, and 0.25Vdd, 0.5Vdd and Vdd 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.

       

Figure 4. The sensory module and its PCB Layout

2.     Output response

Figure 5 illustrates the coordinates of acceleration in KXM52-1050, 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 Voff equals half Vdd[1]. If acceleration exists toward positive direction, the output voltage increases (Vout> Voff) and vice versa.

The sensing range of KXM52-1050 of each axis is ±2g and the output varies with acceleration linearly in the rate of 660mV/g. Equation (3) shows the relation:

                                                                     (3)

For example, when KXM52-1050 is put horizontally and still, there are no gravitational effects along X and Y axes, and VoutX=VoutY=Vdd. 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 VoutY>Voff+660mV(at 3.3V Vdd).      

`

Figure 5. The coordinates of acceleration of KXM52-1050

Figure 6. Output response due to directions of accelerations

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 f, r, and q 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). KXM52-1050 rotates about each axis from 0o to 90o, 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 f, r and q is 0o, the outputs of X and Y axis have the maximum sensitivity while Z axis has the minimum sensitivity.

       

       

                                                                                            (4)

Figure 8. Pitch and roll assignments relative to ground

Table1. Output voltages of KXM52-1050 w.r.t specific tilt angles  

Angle(°)

Xaccel(g)

X(V)

Yaccel(g)

Y(V)

Zaccel(g)

Z(V)

90

1.00

2.24

1.00

2.28

0

1.6

80

0.985

2.24

0.985

2.24

0.174

1.72

70

0.940

2.24

0.940

2.24

0.342

1.8

60

0.866

2.2

0.866

2.2

0.500

1.92

50

0.766

2.12

0.766

2.08

0.643

1.96

45

0.707

2.12

0.707

2.08

0.707

2.04

40

0.643

2.04

0.643

2.00

0.766

2.04

30

0.500

1.96

0.500

1.88

0.866

2.16

20

0.342

1.84

0.342

1.8

0.940

2.16

10

0.174

1.76

0.174

1.68

0.985

2.24

0

0

1.64

0

1.6

1.00

2.24

Voff

 

1.64

 

1.6

 

2.24

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

Appendix. Kionix KXM52-1050 general specification

Performance Specifications

Parameters

Units

 

Condition

Range

g

±2.0

 

Sensitivity

mV/g

660

 

0g offset vs. temp

mV

±100

Over temp range

°C

-40 to 85

Span

mV

±1320

@3.3V

Noise

35(x and y) 65(z) typical

@500Hz

Bandwidth

Hz

0 to 3000 max (x and y)

-3dB

0 to 1500 max (z)

Output resistance

W

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

 

V

-0.3 (min) 7.0 (max)

Absolute min/max

mA

1.5 typical (1.8 max)

Current draw @ 3.3V

mA

<1.0

Shutdown pin grounded

ms

1.6

Power-up time @ 500Hz

Environmental Specifications

Parameters

Units

KXM52

Condition

Operating Temp

°C

-40 to 125

Powered

Storage temp

°C

-55 to 150

Unpowered

Mechanical shock

g

4600

Powered or unpowered,

ESD

V

3000

Human body model

 



[1] 0g (Voff) offset changes linearly with temperature and therefore will not always equal half Vdd. The maximum 0g offset tolerance is ±167mV under extended temperature range of -40 to 125, and ±100mV under -40 to125.