High strength angular rate sensor at low cost

A new Coriolis-based rotational motion rate sensor fabricated in single crystal silicon gives an angular rate sensor product at a fraction of the expense, coupled with extraordinary strength

Gyroscopes (Angular Rate sensors) have traditionally been exclusively regarded as being core components of navigation equipment.

In fact, mechanical gyroscopes have been used by the Admiralty since 1913.

The main driver for this has been cost and bulk, due mainly to the sophisticated nature of the sensor.

Even today, scale of performance can be measured against price.

The fundamental consideration for performance is +zero drift rate, ) the smaller the drift the more costly the rate sensor.

For example, a laser rate sensor used in an inertial navigation system might drift at a rate of.01 degrees per hour, (ie, over 100 hours of continuous operation, it will only have deviated from the perfect heading by one degree) but they are commensurately expensive, costing many tens of thousands of Euros.

The ability to use the Coriolis effect for a rotational motion detector has in the past been exploited in a number of products using a variety of materials, such as ceramic, quartz and metal.

A Coriolis-based rotational motion rate sensor fabricated in single crystal silicon exploits silicon micromachining and other solid state fabrication techniques, giving an angular rate sensor product at a fraction of the expense, coupled with extraordinary strength.

One of the key considerations when using a rate sensor, is its environment.

All designs of Coriolis rate sensor are good at measuring an angular rate and are fine when there is pure angular motion, the performance of simple beam oscillator or tuning fork sensors degrades under shock, linear acceleration and vibration.

The main advantage of a shell resonator is that it can be made shock-insensitive.

The ring structure of the silicon rate sensor is ideally suited to high volume manufacture and a wide range of applications.

By taking advantage of the wafer processing technology developed for the electronics industry, a high volume, low cost and robust manufacturing process can be designed.

An important contribution is derived from the mechanical properties of silicon, which can be a surprise to those unfamiliar with the material.

In its crystalline state, silicon has a fracture limit of 7Gpa, which is higher than the majority of steels.

Coupled with this is a low density of 2330 kg/m3, resulting in a very robust material under its own weight.

In order to produce a robust and reliable manufacturing process, a reduction in the processing steps through simplicity in design is key.

As such the sensing element requires only three mask levels for the silicon fabrication.

Two conventional processes lay down electrical conductors on an insulating oxide layer.

This is followed by mask level 3, where photo-resist is spun and patterned to define the resonator structure.

This is then etched with a dry, deep trench etch process.

Following etching to produce the mechanical component, the photo-resist is stripped away leaving a wafer of silicon resonators.

This design avoids the necessity for small gaps between the resonator and surrounding material and, therefore, problems with +stiction, are avoided.

Another advantage of the design is that the ring resonator has all its vibration in one plane, with no requirement to couple motion from one plane to another.

This permits close control of key parameters over temperature.

Because of its robust design, the silicon sensor is ideally suited to high volume manufacture and has already demonstrated satisfactory performance in a wide range of applications.

Also, (at the cost of some external components), performance can be considerably improved by the use of more complex control algorithms, and selected parts for use in highly demanding applications.

Whenever there is a system incorporating rotary motion, unless it has the crudest form of control, it is necessary to know, not only the error with respect to the demanded angle, but also how fast it is travelling ) the speed at which it is turning.

Now, with the new class of rate sensors being commercially affordable, it is practical to have that angular velocity term in the control as well as angular error.

Sensor gain and bandwidth characteristics are easily and inexpensively altered to suit virtually any control loop requirement.

Rate sensors are being used in anything from machine tools and adaptive cruise control systems in cars, to antenna stabilization, medical devices and toy helicopters.

Research is already underway to resolve the problems of the future.

Particular areas such as higher levels of integration, smaller devices and digital control loops are being actively pursued.

For a high performance-to-cost ratio in angular rate measurement, silicon micromachined rate sensors represent the future and the technology is available now for exploitation in many application areas.

Back to top