Revision Questions Flashcards
Explain the following terms:
Free gyro
Gimbal lock
Caging
Nutation
1) A Free gyro consists of a relatively heavy motor mounted in such a way that it has two degrees of freedom, TILT and TURN.
2) Gimbal lock: The spin axis of a 2 degree of freedom gyroscope is precessed to align with the outer gimbal axis. The results is uncontrolled torques causing in violent processional movements. Fixed by fitting stops that restrict gimbal movement
3) At low speeds when starting up, the gyro is highly susceptible to small amounts of torque causing high presession. CAGING locks the gimbal axis in place until the gyro wheel is up to speed.
4) Nutation: A sharp blow is applied to a gyro causing oscillation of the spin axis. Lower angular momentum and a larger torque cause greater nutation. Minimised by keeping the angular momentum high and shock mounting the gyroscope
What factors is precessional rate dependant upon?
Precessional rate = Applied torque/Angular momentum
Define precession
Precession is the resulting angular velocity acquired by the spin axis of a gyro when a torque is applied in a plane perpendicular to the plane of the rotor. A TILT causes a TURN and TURN causes TILT
What is the equation for Horizontal Earth Rate (HER)
HER °/hr=15 COS(Lat) SIN(phi)
Apparent TILT of spin axis caused by earth rotation
Max at equator
Max when displacement from the meridian is 90°
What is the equation for Vertical Earth Rate?
VER °/hr = 15 SIN(LAT)
Apparent TURN/drift of spin axis due to earths rotation
Max at Poles
What is the relationship between output and input for a RATE and RATE INTEGRATING gyro
RATE: Output angle proportional to input rate
RATE INTEGRATING: output angle proportional to input angle
Explain the requirement for a stable platform in an inertial navigation system mentioning accelerometer corrections. what other type of initial navigation system could be employed and how does this correct for accelerometer tilt?
The accelerometers are mounted on a set of gimbals and gyroscopes so that they maintain a fixed reference regardless of vehicle movements. This allows for corrections to be made to the measured acceleration to account for gravity, centripetal force, the coriolis effect and convergence.
A strap down system can also be used where accelerometers and gyros in the X,Y and Z directions are all rigidly mounted to the vehicle body. The gimbals in the stable platform are replaced with a computer and simulated electronically.
Draw a ring laser gyro and explain its operation
Two laser beams sent clockwise and anti-clockwise around the ring, reflected by mirrors.
Wavelength selected such that path length is an integer number of wavelengths long.
As the cavity is rotated one laser will find its path made shorter as the mirrors approach it, the other laser will experience the opposite.
This will cause the two lasers to arrive back at the detector out of phase.
From the phase difference the angle of rotation can be calculated.
The Sagnac effect is the measurement of rotation when no reference is available.
Lock-in is an issue with RLG, rectified by constantly rotating RLG by a small amount
What are the advantages and disadvantages of INS?
Advantage:
Resistant to jamming
No emissions
Disadvantage:
Requires initial location
Accuracy degrades over time
How is INS used to engage a target?
Before launch: provide initial location, target location, flight path, orientation.
During flight accelerometers used to track position by integrating from felt acceleration. Gyroscopes used to determine orientation.
Both metrics used to determine if the missile following the flight path and corrections to course are made as necessary.
WTAD show how a free gyro can be made to seek and maintain the north meridian
DIAGRAM:
Forces anti clockwise
Level and meridian
Precessional control, tilt up tilt down @ top and bottom
HER east and west
Precessional damping towards level
Undamped the north end will never settle on the meridian.
Damping will precess the gyro in opposition to HER
Loss of VER compensation will cause spiral
