FAQ

What is the principle of operation for Sensonor gyros?

The principle of operation for Sensonor gyros is based on the Coriolis Effect, or what is also sometimes referred to as the "Coriolis force". This is in the physics the comparable (or actually the same) effect as can be felt when trying to change the orientation of a rotating wheel. A force on the axis of the rotating system is felt (e.g. for a rotating wheel), and it appears (unexpectedly) to be a force applied 90 degrees offset from where applied by the user. (This effect is called precession.) For Sensonor gyros, the dual masses of the sensor element is not rotating, however they are being excited with a movement back and forth, vibrating at roughly 9.5 kHz. The input rotation (around the sensitive axis of the gyro) results in a detecting movement, a vibration around the axis which is both normal to the excitation axis AND the sensitive input axis of the gyro. The force of the detection movement is proportional to the angular rate applied, and this is further processed and calibrated by the gyro internally in order to give digital real-time expressions for the angular rate applied.

Dual masses of Sensonor gyro element:

Sensonor Gyro Element

 

Is it possible to modify some parts of the housing?

No. There is a high risk that it could degree the performance of the product. The housing needs the current stiffness for avoiding resonance frequencies for critical parts of the product.

Can STIM products be repaired?

If the failure requires removal of the top cover and reassembling, Sensonor cannot guarantee for the performance accordingly to the product specification and therefore not recommended to be used in application.

Are certificate of compliance available for STIM?

Sensonor standard CoC is available with each shipment for customers that require this. The CoC contain product identifications and serial number for all units in the shipment.

Is it possible to update STIM micro controller firmware?

STIM firmware can only be updated at Sensonor.

Do you use a continuous temperature compensation for the gyros?

Yes, the gyros use internal die temperature to continuously compensate the output signal.

What is the start-up time for STIM202/STIM210/300?

STIM202: Time to valid data =max 10s @25°C

STIM210/STIM300: Time to valid data =max 5s @25°C

This is the time needed from power-on or reset to reset of start-up bit (bit 6 in the status byte). During this period the output data should be regarded as non-valid.

What are the functional differences from STIM202 toSTIM210/STIM300?
  • Angular random walk (ARW) is improved from 0.20 to 0.15°/√h
  • Bias error over temperature gradients is improved from 30 to 10°/h rms
  • Maximum sample rate is increased from 1000 to 2000 samples/s
  • Shielded connector improves EMI capability
  • Maximum RS422 transmission bit rate is increased from 921.6 to 1,843kbits/s
  • Total delay is reduced
  • Time Of Validity (TOV) signal for external synchronization
  • Start-up time is reduced from 10 to 5s
  • Scale factor accuracy is decreased from ±2000 to ±500ppm
  • Non-linearity (200°/s) is decreased 200 to 15ppm
How to understand the zero rate/bias accuracy from theSTIM products?

Bias accuracy/range is the output signal when the gyro is at rest. Bias accuracy/range specification of STIM is ±250°/h (±0.069°/s).

How is STIM bias instability measured?

Bias instability is the minimum point of the Allan Variance curve over a 20 hour test period.

What are the conditions during STIM biasinstability measurements?

Bias instability is measured by Allan Variance method, according to IEEE std 952.

  • Modules are placed on vibration damped tables
  • Test temperature is ambient (~25°C)
  • Total measurement time is 20 hours
  • Power is applied 2 hours prior to data collection
  • Data are collected for 18 hours

ARW [˚/√h] is found at τ =1s averaging time
Bias instability [˚/h] is found as lowest point of curve

How is ARW defined?

ARW is the noise component at the output of the gyroscope, derived from the Allan Variance plot at 1s averaging time.

What type of low pass filters are used for STIM?

STIM uses cascaded integrator-comb (CIC) low pass filters with linear phase response. Filter bandwidth is user selectable for each gyro axis, independent of sample rate.

What is group delay?

Group delay is the time taken for processing the signal through the chosen low pass filter in the digital ASIC

What is the total delay to start of datagram transmission?

Total delay = group delay (depending of low pass filter selected, ref. datasheet)+ 0.5ms (internal sampling of 2KHz) + delay from internal time-tick to TOV active (max 6μs)+ delay from TOV active to start of transmission (max 80 μs). If delayed output unit is chosen 5ms must be added to the total delay.

What is the TOV signal of STIM210/STIM300?

Time Of Validity (TOV) is for synchronization of datagram transmission with external events. TOV falling edge occurs synchronous to the sample rate. Raising edge occurs after last bit of datagram.

What is the repeatability and the accuracy of sample rate?

The repeatability and the accuracy of sample rate is ±25ppm (0,0025%) over the full temperature range.

What is the definition of Incremental angle?

The change in angle/sample. E.g: (100°/s)/ (125sample/s) =0.8°

What is the definition of Integrated angle?

Cummulative sum of angle movement/sample. Integrated angle (n-1) +incremental angle, up/down to the level of ±4°is reached then a wraparound occurs

How does the wraparound for integrated angle work and how comethe output is designed to ±4° ?

The background for the chosen ±4° wrapping: the maximum output-rate = 480°/s, the lowest output rate = 125 samples/s -> the largest change in angle between two samples = 480/125 °/s = 3.84°/sample. The clipping range must be twice this in order to be able to differentiate between a change and a wrap. The ±4° range has been used in order to achieve the highest possible resolution.


The total angle can be kept track of by counting number of wraps, e.g:
The following parameters/variables are defined:


• "#wraps" is number of wraps
• "s_curr" is current sample
• "s_prev" is previous sample
• "Total_angle" is total integrated angle

Add following logic/calculations:
If(s_curr - s_prev) <= -4, then #wraps = #wraps + 1
If(s_curr - s_prev) > 4, then #wraps = #wraps - 1
Total angle= s_curr + 8 * #wraps

Can the STIM300 output angle of degree?

No, not directly. For calculating angle of degree the user will need an algorithm additionally to the output of the IMU. Besides angular rate the gyro can output incremental angle or integrated angle. The accelerometer/inclinometer outputs in g or in m/s/sample.

What will be the accuracy of a position change measurement usingSTIM300?

It will be dependent of how the IMU is integrated in the user's system/application together with an algorithm. One could check out our collaborating partner results where STIM300 is integrated: http://www.novatel.com/assets/Documents/Papers/SPANBrochure.pdf

What is the frequency response for STIM300?

Nominal bandwidth (-3dB) Gyro: 262Hz, Accelerometer: 214Hz, Inclinometer: 17Hz

What is the cut off frequency for STIM products?

It is dependent of the low pass filter settings from 16Hz-262Hz ref. datasheet for STIM products

What is the accuracy of the accelerometer and inclinometer inSTIM300?

Accuracy would be the contribution from different parameters like scale factor, non-linearity, bias error over temperature, bias instability, velocity random walk and misalignment

What is the resolution for the different g variants availablefor STIM300?

2g: 0.5μg, 5g: 1.0μg, 10g: 1.9μg, 30g: 3.8μg, 80g: 15.3μg

STIM Evaluation kit: The indicator for "Data arriving fromdevice" light is green but there is no output from the STIM

1) Verify that the sample rate settings are in accordance with chosen datagram as specified in STIM300 datasheet

2) Verify that the evalution kit is supporting the requested bit rate (ref. user manual)Table 6-9: Mximum Sample Rate