FAQ
2. Measuring inclination with an accelerometer
3. Ratiometricity and ratiometric error
7. Sensitivity and sensitivity error
8. Zero point and zero point (offset, bias) error
11. Functional form of the inclinometer output
14. Calibration of VTI's Products
15. Connection of VTI's Sensor
Measuring direction of the vertical sensor is parallel to earth's gravity. For a horizontal sensor the measuring direction is orthogonal to the gravity field. The measuring direction is specific to the product family: e.g. SCA61T and SCA610 Series parallel to the mounting plane, SCA620 and SCA320 Series orthogonal to the mounting plane.
2. Measuring Inclination with an Accelerometer
The acceleration sensor is sensitive to inclination, i.e. earth's gravitation (DC response), and the inclinometer is sensitive to acceleration. When the accelerometer is used as an inclinometer, it measures the component of earth's gravitation in the measuring direction giving a sine function of the inclination angle as the output.
Q: What is the difference between an accelerometer and an inclinometer?
A: VTI's inclinometers and accelerometers are
all based on the same MEMS capacitive measuring technology. Different product
versions are optimized for different purposes, determined by the g-range in
which they are going to be used. The maximum g-range of the inclinometers is
earth's gravitation (approximately 1g). VTI's accelerometers measure low-g
ranges of ±1g up to ±12 g.
3. Ratiometricity and Ratiometric Error
Ratiometricity means that zero point and sensitivity are proportional to the supply voltage. If the supply voltage is fluctuating the output will also vary. The idea is that when using the same reference voltage for both the sensor and the measuring part (A/D-converter), an error in the reference voltage is automatically compensated for.
Ratiometric error is the relative deviation
(%) from the proportional behaviour described above, i.e. if the output voltage
changes 3% at a supply voltage change of 5% we have a 2% ratiometric error.
4. Frequency Response
Depending on the product the frequency response is essentially either 1st or 2nd order low pass (LP): Products with an over-damped element have a 1st order response (-20dB/decade). Products with a wide band element and with internal filtering (400Hz or 1kHz) have a 2nd order frequency response (-40dB/decade). The customer is normally not able to change internal filtering.
Q: How does the frequency response look for VTI's inclinometers?
A: In the SCA61T series the -3dB point is at 18 ±10Hz, 20dB/dec roll off. Attenuation at 1Hz is approx. 0,1%
In the CBxH1G parts in the SCA610 series the -3dB point is at 6 ±4Hz, 20dB/dec, giving an attenuation of approx. 5,6% at 1Hz
Q: Does the inclinometer SCA61T-FAHH1G have a single pole frequency response?
A: Yes, SCA61T-FAHH1G has a single pole frequency response. The frequency response of SCA61T-FAHH1G is 8-28 Hz. In addition, the internal ASIC has a 400 Hz low-pass filter.
Q: What is the damping factor of the inclinometer SCA610-CB1H1G above 1000 Hz?
A: The damping factor of SCA610-CB1H1G is -35dB at 1.0 kHz and the low pass filter's cut-off rate after that is 20dB/decade.
Q: Is it possible to decrease SCA 61T's frequency response with an additional capacitor? Is it possible for customer to make its frequency response lower by using additional electronic, e.g. high order low-pass filters?
A: Yes, it is possible to add active or passive filtering to the sensors output. Resistive and capacivite load on the output must be within specified limits.
Q: VTI has mentioned, that SCA61T and SCA100 have 8Hz frequency response with 0.002° resolution of analog output and in datasheets the frequency response of both of them is said to be 8...28Hz. What is the difference?
A: The SCA 61T and SCA100T are similar regarding the original frequency response. The frequency response in the Data-Sheet (8...28 Hz) describes the max variation of the -3dB point between individual parts and it is the same for the both of the sensor types.
5. Available Measurement Ranges
The accelerometer measurement ranges extend from 0.5g (30°) inclinometer to 12g accelerometer.
We offer sensing elements in die form and on a substrate, PCB mountable and stand alone products as well as customer specific solutions.]
Q: I would like to get some package information of your products on a component level. Is there a lid over the top or is the part filled with solid material? If there is a void (air or gas) inside how large is it?
A: Package of the VTI's component is one-piece molded plastic package. There is a lid over the top and the volume of the gas cavity is approximately 20 mm3 in SCA 61T and app. 70 mm3 in SCA100T. In our stand-alone SCA125T module the cavity is app. 3000 mm3.
Q: There are holes in the bottom of the component. Can I still mold the component into a plastic?
A: Yes, you can.
7. Sensitivity and Sensitivity Error
Sensitivity is defined as the slope of the straight line through two defined points in the measuring range. The points depend on the sensor in question:
Sensitivity Error is the relative deviation (%) of the slope of the line through these points, compared to the optimum, or the zero-deviation slope.
Q: How much does the amplification of the inclinometer SCA610-CB1H1G fluctuate between 0-1 Hz in percentage?
A: The amplification decreases 5.6% between
0-1Hz.
8. Zero Point and Zero Point (Offset, Bias)
Error
The sensor is at zero point, when it is mounted in the prescribed position (measuring direction) and no acceleration is acting on it. For horizontal inclinometers and accelerometers it is at 0g (with the defined face up) and for vertical accelerometers at 1g. Zero Point Error is the deviation of the output, when the device is in its zero position, compared with the optimum value.
Q: Is the internal temperature compensation of SCA100T turned on automatically?
A: Yes.
Q: Is it possible for user to compensate zero point temperature drift using built-in temperature sensor the readings of which are available through an SPI interface?
A: Yes, you can use the internal temperature sensor to do 2nd or 3rd degree compensation.
Q: What is the stability of offset calibration accuracy in the SCA61T inclinometer?
A: Maximum offset calibration error in room temperature is +/- 5mg and typical value is +/- 2mg (stable) and typical drift is 1mg after 500 hours of having power connected to the inclinometer. Non-repeatability (hysteresis) is 0.25mg when cycling temperature.
Non-linearity is defined as the deviation of the output from the straight line defined by sensitivity. Non-linearity should not be mixed with the effect of angular mounting errors and cross-axis sensitivity on the sinusoidal behaviour of the accelerometer in the gravitational field.
Q: Is it also possible to get inclinometers with linearity error of no more than 0.5mg in +/-0.1g measurement range and what is linearity change as a function of temperature (if any)?
A: Yes, please ask the measuring results of
linearity from VTI. Results are from between +/-30 degree. For example in +/-
0.1g measuring range the linearity is better than 0.5mg.
10. Cross-axis Sensitivity
Cross-axis sensitivity is the maximum sensitivity in the plane perpendicular to the measuring direction relative to the sensitivity in the measuring direction. It is calculated as the geometric sum of the sensitivities in two perpendicular directions (Sx and Sy) in this plane:
Q: If the max accelerometer cross-axis sensitivity is ± 5%, what is the angle between the direction of measurement (z axis) and the primary axis of sensitivity?
A: ± 5 % cross-axis sensitivity corresponds to approx. 2,8° mounting error. Angle between secondary axis (z-axis) and primary axis (x-axis) is 87,2°.
Q: As VTI's datasheet indicates, accelerometers cross-axis sensitivity can be up to 5%, when used for dual axial tilt measurement application. How can I measure and compensate for this effect?
A: If you have two VTI´s inclinometers on a PCB which are perpendicular to each other, you can compare both of their outputs when tilting them one axis at the time. The other axis´ sensor is a control sensor. The change in the output of the control sensor indicates the cross-axis error. The compensation is easiest to do by testing different mounting positions of sensors and trying to find the best position where the cross-axis effect is as small as possible.
11. Functional form of the Inclinometer Output
Inclinometers measure the effect of earth's gravity on the silicon proof mass. This is a sinusoidal function of the inclination angle. However, to get the best and most linear measurement result one should add the angular error caused by the Cross-axis Sensitivity component in the inclination plane and mounting errors.
The most important factors, which affect the total accuracy of a motion sensor are resolution, ratiometric error, sensitivity error, zero point (Offset, Bias) error, non-linearity and cross-axis error. These factors depend on operating circumstances, sensor's assembly into an end-user application and end-user application's calibration possibilities. Due to these reasons, it is easier to define the accuracy one factor at a time. The value of the each factor is available from the Data-Sheets www.vti.fi/products. For example resolution is an ultimate limit for accuracy. Other factors reduce the accuracy, but with the right compensation their impacts are reasonable.
Q: If your resolution is 0.1% (of full scale), it means that you have a resolution of 0.001*90 = 0.09 Degrees. However, your repeatability is 0.03 Degrees. How can repeatability be less than resolution?
A: It cannot be. The 0.1% resolution is the digital output resolution and the repeatability is measured from the analog output which has much higher resolution than 0.03 degrees. Based on the specification the analog output resolution over an 8Hz bandwidth is 20µg *sqrt (8) = about 60µg => 0.003 degrees. With this bandwidth the analog output resolution is 10 times better than the repeatability. From the digital output point of view the repeatability and the resolution are the same (+/-90 degrees = 180 degrees change. 180degrees /1024=0.18 degrees / bit).
Q: Is the analog output capable of higher resolution than the SPI interface?
A: Yes, the resolution of the analog output is higher than the digital one. The analog resolution is limited by the output noise density of SCA61T. The noise is 15 µg / √Hz and it yields the following resolutions: 0 - 0.1Hz=>resolution 5 µg, 0 -1 Hz=> resolution 15 µg, 0 - 10 Hz=>resolution = 50 µg, 0 - 30Hz=>resolution is 80 µg
Q: May I have some exemplary characteristics of linearity, zero point temperature dependency, and sensitivity error as a function of temperature.
A: These values are available on the Data-Sheets of VTI's products www.vti.fi/products . If more detailed information is needed, please contact VTI to get the figures which show the required characteristics.
Q: When using the SPI to communicate with the inclinometer, what is the maximum sampling rate?
A: The maximum practical sampling rate is approximately 500 Hz.
Q: In the SCA61T Series, the digital output has an 11 bit resolution. In the SCA 103T the resolution is 12 bits? Is this correct?
A: The effective digital output resolution of
the SCA103T is really 12 bits. This is due to its X-X sensor configuration and
the use of the two channels in differential mode. This is how the effective
resolution of 12 bits can be achieved.
13. SCA Demo Kit
The SCA Demo Kit is designed for VTI Technologies (MEMS) accelerometers. It can be used for research and evaluation purposes in different applications where physical movement will be converted to an electrical signal. The board contains two single-axis inclinometers and will support two external channels for extra sensors. No additional components are required to use the evaluation kit.
Q: Is it possible to open your circuit drawings of SCA Demo Kit?
A: Yes it is, please contact VTI Technologies. You can find the open circuit drawing in the CD-ROM of the Demo Kit.
Q: Is it possible to get a source code in C++ language for the Demo Kit's control program?
A: The advice source code is designed for the features of SCA Demo Kit test board, but VTI Technologies has not made the source code and unfortunately we do not have the resources to support with the code.
Q: Please advice whether you have an SCA Demo Kit for 3 (X, Y and Z) axes type?
A: The SCA Demo Kit itself does not include
the sensor for the third channel (for the z axis), but you can connect an
external accelerometer to the external sensor port and monitor all three axes
at the same time. Actually you can monitor four axes at the same time by adding
two external accelerometers to the external sensor port of the SCA Demo Kit.
You can connect maximum three external accelerometers to the Demo Kit, but in
that case the other built-in accelerometer in the Demo Kit will be replaced.
14. Calibration of VTI's Products
VTI's products are factory calibrated, but additional calibration can be done with the end-user application. Please see Application Note 14, Offset Calibration
Q: Is there a way to save calibration data into SCA 61T/100T via SPI by customer?
A: Calibration data can be programmed into device memory. This feature is used during factory calibration. The re-calibration at customer site requires special agreement between VTI and customer.
Q: How does the field programmable temperature compensation work?
A: Temperature compensation is done internally in the ASIC. The type of compensation is linear and the compensation is calibrated and programmed during factory testing. Thus it is not field programmable, but coefficients are stored in the same memory than the other calibration data and they can be changed by re-programming.
Q: I would like to do the field programmable mounting error compensation for the SCA100T and SCA61T series inclinometers, and programming the eeprom using the SPI interface.
A: Programming the mounting error compensation
and the eeprom is done only at the factory and thus they cannot be done by a
customer without special agreement.
15. Connection of VTI's Sensor
The basic information for the connection of VTI's sensor is available in the Data-sheets www.vti.fi/products
Q: May I get an SPI manual for reading the t, x, y information from the SCA100T?
A: Please see Technical Note "SPI Interface Specification" from http://www.vti.fi/support and contact VTI Technologies for further information.
Q: How can inclination angles be read from SCA100T in digital format?
A: Please see Technical Notes "SPI Interface Specification" and "Angle vs. Digital Output" from http://www.vti.fi/support
Q: Are the SCA610/620 products known to have any EMI emission problems?
A: Parts are automotive tested and approved. Classification has been done according to CISPR22 class B requirements. The results are on the level of 20dBuV/m with the bandwidth of 100kHz - 1GHz. Our customers have not indicated any EMI problems so far.
Q: What is the input for the self test pin of SCA610-products family?
A: Voltage, ST= 0V (ST off) and ST=Vdd (ST on)
Q: Is the self test pin input DC or AC and what is the maximum input quantity?
A: DC voltage and the maximum input quantity is Vdd+0.3V (absolute maximum rating)
Q: Does VTI have recommended circuits between an ECU and the self test pin of SCA610- product family?
A: No special circuitry is needed. Self test can be driven directly from ECU. Plese see application note for the Self Test function.
Q: Why does the output signal of SCA61T inclinometer fluctuate in my application?
A: Stability of supply voltage source is important, due to the ratiometric feature of VTI's products. Please note connection considerations: Analogue output load is min. 10 kOhm, max. 20 nF. Digital output load is max. 1 nF. Supply voltage stabilizing needs to be done with min. 100nF capacitor. If external A/D- converter is used, the power for the SCA61T needs to be supplied from the same place than the reference voltage for the A/D-converter. The risk of EMC (high voltages, electric motors etc. near SCA 61T) needs to be prevented.
Q: Does the SCA61T and SCA100T work with 3V power supply?
A: No, the ASIC is designed so that it will shut down the circuit when supply voltage is under ~ 4V. So, minimum supply voltage is 4V. Between 4.75 -> 4 V device will operate but the specification can not be guaranteed.
Q: Does the SCA610 work with 3V power supply?
A: Yes, but performance over temperature range is not guaranteed. Below 2,8V ASIC functionality (internal blocks) is not guaranteed.