In the past, developers of inertial systems have placed some important demands on manufacturers of gyroscopes. Not in the least was that gyroscopes should be smaller, more reliable and less expensive. The result was that gyroscope designers moved away from a purely mechanical approach towards solid state devices. Today, these solid state designs have been in successful production for sometime and have been used very effectively for platform stabilisation, remotely piloted vehicles, pointing, navigation and control systems.
But the market never stands still. And today, a new set of requirements exist that are driving the technology for the next generation of gyroscopes. This time around, it is not the military market that is asking for a low cost gyroscope system, but the commercial marketplace. Developers of vehicle dynamic control systems, navigation systems and active suspension systems all need gyroscopes which are more than an order of magnitude less expensive than their present day counterparts.
To address these needs, gyroscope designers have no alternative but to embrace new manufacturing technologies. In effect, this means that they must turn to micromachined silicon components for the mechanics of the gyroscope. They must then closely integrate the micromachined result with control electronics built from a single ASIC in order to meet the price/performance criteria that are required.
In the development of these micromachined gyroscopes, techniques and technologies previously employed in the design of earlier systems can be effectively employed – and then cost reduced. Such was the case with the new silicon micromachined vibrating structure gyroscope from British Aerospace Systems and Equipment (BASE).
Now in production, this new gyroscope will be delivered at a competitive price in high volumes, an astonishing order of magnitude less than was achievable from older generation designs, (Figure 1).
With the development of its earlier VSG2000 gyroscope, BASE designers developed an innovative patented mechanical ring structure to take the place of previous more expensive, mechanical rotating structures such as gimbals and bearings. It was certainly cheaper. This new solid state gyroscope worked by making use of the Coriolis force – a force that is observed to act to a moving element in a rotating body.
In operation, the mechanical ring gyroscope was made to oscillate by the application of alternating force to the ring. When this oscillating body was placed in a rotating reference frame, the Coriolis force came into play, producing a secondary oscillation orthogonal to the primary oscillating motion, (Figure 2).
Measuring the size of the resultant vibration provided a signal proportional to the rate of movement of the gyroscope. So the unit was developed so that the amplitude of the movement of the resonating structure was detected by a pick off positioned 45 degrees from the input drives.
The problem with this approach was that the specification of the gyroscope could be limited by the characteristics of the material used in the ring. As the rate of rotation increased, the ring motion would increase and the output become non-linear.
To solve that problem, a patented closed loop gyroscope technique was developed that amplified the signal from the pickoff and then fed it back to the ring as a nulling voltage, providing a force that cancelled the effect of the Coriolis force. In this way, the material used in the gyroscope ring did not cause errors. The output of the gyroscope was obtained by demodulating the nulling voltage output from the sensor to obtain a dc voltage directly proportional to the angular rate that was being applied.
Re-use of the ring oscillator approach, coupled with the integration of the gyroscope control electronics into a single ASIC, have helped BASE engineers with the development of the next generation design, the silicon vibrating structure gyro. But the most revolutionary aspect to the new design is the way that the entire mechanical gyroscope has now been integrated into a small micromachined element smaller than a penny. This is, in fact, the first time that micromachining has ever successfully been used to build a commercial gyroscope.
The technology used to build the small micromachined rings that form the heart of the new gyroscope was obtained through an agreement between BASE and Sumitomo Precision Products (SPP), a Japanese consortium who specialise in the production of silicon processing equipment, including etching machines for micromachining. Through the use of such machines, SPP were able to optimise the manufacture of small micromachined rings used in the new design using a process known as deep trench etching.
The new gyroscope comprises the micromachined ring itself supported in an IC style package by eight small legs. Each of the eight legs carry three conducting wires onto the surface of the ring. Two wires carry current onto conducting surfaces on the ring, while the third connects to a ground plane. One wire carries current clockwise to `drive’ the ring, while the anticlockwise wire is used as a `pick up’, (Figure 3).
A small magnet is bonded onto the base of the gyroscope. The magnet passes through the centre of the ring and is capped by a `C-shaped structure’ that directs the flux from the magnet back onto the circular ring, (Figure 4). When current flows through two of the conducting elements in the ring, the cross product of the current flowing through the ring, and the magnetic force field produced by the permanent magnet delivers a force that causes the ring to move outwards, in a similar fashion to the VSG rate sensor. By applying an alternating current to the conductors on the ring, the microminiature ring is made to oscillate.
A voltage is created in the pick off elements on the ring due to the fact they are moving conductors in a magnetic field, and hence produce an output proportional to the amount of movement. As before, when there is no rate of rotation, there is no output on the secondary (orthogonal) axis. As the gyroscope is rotated faster, the output increases. Measuring the amount of secondary axis vibration again provides a measure of the rotation rate of the gyroscope.
Silicon has some very useful material properties for a sensor which is why it is already being used for the design of micromachined accelerometers in airbag systems. Because the silicon ring used in the new gyroscope is both small and light, it has low inertia. Therefore, when the can containing the gyro is moved, the ring easily moves with it giving very high shock resistance. And because it is silicon, the ring itself is relatively strong and very resilient, it does not `stretch’ in the same fashion as metal – providing low hysteresis.
The gyroscope has also been carefully designed so the silicon ring itself has no fundamental vibration patterns at low frequency. All the fundamentals are at high frequency so that it is insensitive to vibration.
The gyroscope manufacturing process first starts with a wafer of silicon. Using industry standard deposition and patterning techniques, the insulators and conductors are laid down on the silicon. A resist is then applied and patterned. Then, through a process of etching, the unwanted silicon is removed, leaving the ring and the legs intact.
The electronics in the single ASIC controller is a testimony to modern levels of VLSI integration. A mixed-signal ASIC comprising both digital and analog electronics contains all the control circuitry that had previously been found in a discrete form in earlier devices such as the aforementioned VSG2000. The ASIC includes the primary loop, gain control, amplitude detector, filter and demodulator. The ASIC also contains built-in test circuits which check the integrity of the sensor to a very high level, an important consideration in braking system design where safety of the system is of paramount importance.
Through effectively leveraging micromachining technology, the BASE Silicon VSG development has met the price/performance ratio that designers of next generation systems, such as car navigation systems are demanding. The deep trench etching process has enabled BASE to exploit the technology to produce gyroscopes on silicon wafers at a low price. This is important in a market which saw over 500,000 car navigation systems sold in Japan last year. And all of these represent a potential home for the new micromachined silicon gyroscope.
Although such systems can simply just use CD-ROM map-based data and global positioning systems (GPS), such systems tend to be unreliable. Over the years, navigation systems designers have realised that, in some places, such as in cities or in canyons, accuracy of such systems can be sacrificed due to loss of signal from the GPS system itself. By coupling a gyroscope into the system, the reliability of the system is enhanced. With a gyroscope, the rate of turn of the vehicle can be measured and used as an additional input to the system. Obviously, a good performance, low cost gyroscope is essential for such applications.
At the moment, navigation systems may be the first application to take advantage of the new technology, but braking systems will not be far behind. In fact, such gyroscopes may be the only alternative that designers have to consider in such an application, since vibrating beam devices have limited performance which does not meet the aspirations of most system designers.
But what sort of performance will the new gyroscopes offer? And how do they compare to their earlier counterparts? The new micromachined gyroscopes give similar performance to their larger more expensive predecessors. Both devices provide a performance of 1 degrees to 2 degrees/s rate error, and 1% to 2% of scale factor error over the full environmental specification. The silicon micromachined gyroscope is, however, much easier to manufacture in high volume at low cost. And, the reliability of the BASE Silicon VSG is superior to the previous devices, due to the low parts count and low stress levels inherent in the silicon ring.
The likely effect of the micromachined gyroscope is that many designs that had previously considered a gyroscope too expensive will now come into fruition.