Dynamically tuned gyroscope

Dynamically Tuned Gyro (DTG) is a kind of two degree of freedom inertial sensor with a specially flexible joint. It has two input axes which are mutually orthogonal and which lie in a plane which is perpendicular to the spin axis of the gyroscope. Work to demonstrate this form of technology was underway at the Royal Aircraft Establishment, Farnborough by Philpot and Mitchell during the late 1940s. Although demonstration of the tuning phenomenon took place in the early 1950s, it is only since the 1970s that this type of gyroscope has been fully developed. The original concept was developed for stabilized platform applications, but has been applied to strapdown systems since the mid- to late 1970s in many types of vehicle. Because of its advantages of simple structure, small size and low cost, DTG is widely used in military and civil fields, such as guidance and orientation of aviation, spaceflight, navigation, land vehicle or exploitation of oil mines.

The sensor consists of three major sub-assemblies as indicated in Figure 1:

• the body block, which consists of the spin motor and angle pick-off arrangement;
• the rotor assembly, which also includes the torque generator magnets and the Hooke's joint suspension;

• the case and torque generator coil assembly.

The rotor is connected to the drive shaft by a pair of flexure hinges to an inner gimbal ring. This inner ‘gimbal’ is also connected to the drive shaft by a pair of flexure hinges, the two axes of freedom being orthogonal. This is often called a Hooke's joint or a Cardan joint and allows torsional flexibility. This is an internal type of gimbal and is far more compact than the external gimbal. At the other end of the drive shaft is a synchronous motor.

Rotation of the gimbal causes a reaction at the rotor that is equivalent to negative torsional spring stiffness. This effect occurs when the angular momentum of the shaft does not coincide with that of the rotor, the angular momentum of the gimbal jumping between that of the shaft and the rotor, at twice the speed of the rotor. Thus, careful selection of the torsional stiffness of the gimbal components and the rotational speed of the rotor, allows the rotor suspension to have a net zero spring stiffness at a particular rotor speed, known as the tuned speed. Under these conditions, the rotor is decoupled from the motion of the rest of the sensor and hence is 'free'. In practice, this condition is usually adjusted or trimmed by the use of screws set into the inner gimbal ring that allow minor changes in the mass properties of the gimbal.

Normally, the decoupling of the rotor is not complete or perfect and residual elastic restraints restrict the useful angular range of movement of the rotor. Therefore, the sensor is usually used in a torque re-balance mode allowing only very small deflections of the rotor. Deflections of the rotor are sensed about two orthogonal axes, and are directly proportional to the motion of the gyroscope case about the respective axes in inertial space.

As in the case of the rate-integrating gyroscope, this sensor is sensitive to linear and angular accelerations, vibratory motion, stray magnetic fields and temperature changes, all of which give rise to errors in measurements. This type of sensor is sensitive to vibrations at integer multiples of the spin speed, not only at the spin frequency, as in the single degree of freedom gyroscope, but also vibrations at twice this frequency. Vibration about the input axis interacts with the gimbal angular momentum, and is rectified to give a fixed bias.

The dynamically tuned gyroscope offers a number of significant advantages for many applications, when compared with the rate-integrating gyroscope. These are usually quoted as fewer parts, a fluid free suspension, no flex lead torques, simplified spin motor bearing design and a fast warm up characteristic. Of course, it offers the ability to measure angular motion about two axes, and additionally, its construction allows the sensor either to be re-worked more easily, or to have its performance optimized before final sealing of the case. One potential drawback is its susceptibility to disturbances and oscillations at the tuned frequency and harmonics of this frequency. Its suspension is analogous to a mass on a spring. For this reason, careful design is required to ensure that mechanical resonances do not interact with the suspension and destroy it. For reliable performance in a harsh environment, careful design of the suspension and mounting is crucial.