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YAW RATE STIMULUS FOR GYROSCOPES
In principle, stimulating a gyroscope sounds easy. You just induce a constant yaw rate and test the
gyroscope during the rotation. However, this is not the reality when targeting to very accurate
stimulus. Afore has wide experience in testing of high grade automotive gyroscopes and combos.
We know that there are several major issues to be taken into account. The real accuracy of the yaw
rate stimulus depends on:
1. Yaw rate accuracy
2. DUT alignment accuracy
3. Measuring time vs. accuracy
1. YAW RATE ACCURACY
If we look the yaw rate during one second, it may vary tremendously. Also the yaw rate accuracy
within one revolution may be excellent, but the variation within 1 degree rotation may not be
acceptable for high accuracy gyroscope calibration. Same applies to the yaw rate range. Typically,
high yaw rate accuracy is easier to reach than low yaw rate accuracy. In order to achieve accurate
yaw rate stimulus in wide range, the combination of servo motors and controller must be perfect.
Unfortunately today, there aren’t any standards on how a vendor should present the yaw rate
accuracy and therefore the vendor values may not be comparable. The way to present the yaw rate
accuracy figure as well as the verification methods may differ from vendor to vendor. Afore uses a
fiber optic gyroscope to verify the accuracy of the yaw rate, because we believe that it is the most
suitable and accurate measuring tool for that purpose. In addition, use of 1000 Hz sample rate
makes it possible to investigate the actual yaw rate in a very short period. Having said that, it is true
that also the fibre optic gyroscope has its own accuracy and therefore we cannot claim that our test
cell is more accurate than the measuring tool itself, even if it was true.
Noise may destroy even an ideal stimulus accuracy. The noise may be either mechanical or electric.
In case of yaw rate, the mechanical noise comes from vibrations of the test system. In order to
minimize mechanical noise, the rotating unit should have adequate bearings, stiff mechanical
structure and have means to dampen the possible vibrations. In addition, the rotation unit should be
isolated from any source of mechanical vibration such as piece handling units. The balancing of
rotating mass will reduce the vibration and makes the yaw rate control easier.
Figure. Measurement results of Afore Kronos 2nd axis. Measured with fibre optic gyro.
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2. DUT ALIGNMENT ACCURACY
If the sensor axes are not co-aligned with the axes of the rotation unit, it will have an instant impact
to the stimulus accuracy. In fact, if angular alignment of the DUT is faulty, the stimulus of one axes
can turn out to be stimulus of two axes.
Sometimes the alignment of the DUT is harder to control than the stimulus itself. Sensor size in
general turning into smaller and smaller makes DUT alignment even more challenging. The picture
below shows the comparison of alignment of single sensor and a strip.
Figure. An example of how clearance of 0,05 can affect to angular positioning, single DUT vs. Strip.
As can from the picture above, even a very small clearance in a socket will cause a major angular
error of the DUT. In reality, the situation escalates, when the DUT size decreases. On the other
hand, the socket cannot be very tight assuming that the handling is done with P&P equipment. The
problem may be solved by using aligning sockets, which remove the clearance when closing.
Aligning sockets, however may cause unnecessary extra stress to the DUT, which may have
negative effects to the MEMS structure and sensor performance. It should be noted that, even if the
alignment in a socket is assumed to be accurate, it is in practice very hard to guarantee that the
socket in PCB is well aligned. Therefore we at Afore believe, that it is definitely better to let the
DUT to be in a “free state”. The use of strips or precision carriers allow positioning clearance
without major impact to the angular accuracy.
In wafer testing (for example un-diced WLCSP) Afore has another approach - testing on film frame.
In case of WLCSP, the wafer is placed on the tape and diced on it. Testing and calibration follows
seamlessly by using the dicing tape. We are aware that wafer dicing may shift the DUT’s original
position on tape, however the dicing has only a minor effect to the angular position. When aligning
the whole wafer to the handler axis, the angular accuracy of a single DUT is excellent.
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3. MEASURING TIME VS. ACCURACY
In high precision gyro calibration, the yaw rate must be stabilized before any measurement can take
place. If the turning range of the test system is limited, i.e. to one revolution, the system must be
accelerated and decelerated within that range. With higher yaw rates, the time for constant yaw rate
between acceleration and deceleration may be very short and inadequate for accurate measurement.
This means that the system may not be applicable for accurate testing of high yaw rates even though
it could reach the desired yaw rate.
As an example, KRONOS test cell for motion sensor testing has an infinite turning range for both
gyro axes. Once the system has reached the desired yaw rate, which can be up to 750 deg/s, it keeps
on rotating accurately as long as needed. The measuring time of the DUT is therefore not limited.
Multiple touchdown contacting method combined with infinite turning range is a key to extremely
high test cell UPH capacity.