CRS39 was designed for use in borehole surveying where temperature is not likely to be very stable. Since MEMS sensors can be quite sensitive to changes in temperature, temperature ramp and changes in the temperature ramp, temperature measurements are critical for optimal performance. Three temperature sensors were therefore built into CRS39 to enable temperature gradients to be monitored across the CRS39 and track changes to the temperature environment. Using these sensors, it is possible to compensate for dependency on temperature, temperature ramp, and temperature gradient across the device and also changes to the temperature ramp. The actual compensation process will be dependent on the mounting of the CRS39 as well as the environment the device is subjected to.

Basically, our gyroscopes use a silicon (MEMS) ring which is setup and controlled to vibrate in a precise manner. When the gyro is rotated, the resonance pattern changes; the way it changes being proportional to the rotation rate applied to the gyro. Electronics around the ring control the resonance of the ring and also sense the motion of the ring.

When the gyro is subjected to changes of temperature, the bias and scale factor of the gyroscope can change.

We therefore provide two outputs which can be used to sense the change of temperature.

A temperature sensor is included to sense the temperature of the electronics within the gyroscope.

The ring's resonating frequency is also sensed and output as a digital signal. The frequency of the ring is proportional to the temperature of the ring. The frequency of the ring is proportional to the temperature of the ring. This ring frequency is nominally 14 KHz, with the FRQ signal being two times this frequency, that is , nominally 28 KHz. The frequency changes with temperature at -0.76 Hz/degC.

By subjecting the gyroscope to a changing temperature, it is possible to measure the errors (bias and scale factor) of the gyroscope against the frequency output and the temperature sensor output. Using look up tables or fitting polynomials to these errors, it is possible to compensate for the errors, by subtracting the derived error from the output of the gyroscope.

The temperature sensors will be more responsive to changes in temperature of the environment than frequency because of the longer thermal path to the MEMS ring. Equally, in a highly (angular) dynamic environment where the ring will be heated by the nulling action of the secondary loop, the frequency output will be more responsive. In a fairly stable environment, where the temperature across the whole of the gyroscope is stable, then both methods are equivalent.

In general terms, the simplest method for temperature compensation is to use the on-board temperature sensor as the first step. This can be regarded as the primary (coarse) thermal error correction. A further refinement of thermal compensation can be achieved using the ring frequency (FRQ) since this is a measure of the temperature of the ring.

At normal room temperature (+25degC) operation, the FRQ signal is between 27.4kHz and 28.6kHz. The ring gets 'stiffer' as the temperature drops and thus the frequency will increase, and vice versa. The temperature coefficient of the ring is between -0.82 and - 0.70Hz/degC (nominally -0.76Hz/degC). So, if the value of FRQ drops by, say, 7.6Hz then you can assume the ring temperature has increased by 10degC to +35degC [-7.6 / -0.76 = +10degC]. The silicon ring is supported on a glass substrate surrounded by an inert gas inside a sealed metal can. So it is quite well insulated, thus there is a lag between a change in the ambient temperature and the temperature (and thus frequency) of the ring. The temperature sensor on the board reacts quicker to the ambient temperature fluctuations.

CRS39 was designed for use in down-hole surveying applications with a 1” diameter requirement, hence the unusual form factor, and as such has a thin, but long characteristic shape. The mounting arrangement of the CRS39’s brass posts means there is a possibility that the resonant mode of the PCBs will be excited when operating in a vibrating environment.

It is therefore advisable that the CRS39-01 be mounted in such a way that the lower PCB is securely clamped. This can be achieved when the unit is mounted as it was designed to be; in an inch diameter tube, with the lower PCB located in and constrained by the tube wall.

It has been observed that the CRS39-02 (packaged variant) exhibits a similar resonant mode, so it is advised that the CRS39-02 is not subjected to a vibrating environment which exceeds 1.8 KHz.  

Sampling and averaging of the gyro rate data, guided by Allan Variance analysis, is key to getting the best out of CRS39 and to allow the earth rate signal to be drawn out of the noise.

Two sampling schemes that we would recommend are:

(a)  In our test chambers we use a16 bit (successive approximation register ADC) National Instruments card. We sample the CRS39-03 fully differentially (Rate and Ref), at 10KHz, without any anti-aliasing filters. We then average every set of 10 samples to produce data at 1 KHz. This data is then analysed for AV and Noise.

(b) In our IMUs, we use 24 bit sigma delta ADCs, outputting sampled data at 10 kHz. Actual sampling at the sensor end is around 192KHz. Again the CRS39-03 is sampled differentially (Rate and Ref). We average every 10 samples to produce a data set at 1KHz.

Analysis of the resulting data using Allan Variance techniques will determine the optimum averaging time.  However, the optimum averaging time may be longer than the user can accept and we have typically used 15s averaging for each compass point (90deg separation) measurement.

Minimising temperature variation over measurement cycle will improve accuracy, either by thermal shielding or provision of thermal mass.  Improvements may also be found by fully enclosing the gyro in a metal enclosure, minimising any 'metal detector' or field effects.

Temperature compensation is recommended - linear or third order may be required depending on the actual conditions seen by the gyro - temperature range, rate of change of temperature, as well as actual rates applied and temperature.

Averaging measurements taken at index positions of 180deg to each other can help by draw out the real bias of the gyro by removing earth rate.  Temperature fitting is also possible by comparing changes between measurements at the same index position against bias and temperature.

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