Control Precis 2 Flashcards
A SERVO is defined as ?
Servo:
Error actuated
Power amplified
Feedback/control system
What are the Characteristics of a Tacho Generator?
- No mechanical input, no electrical output
- Linear
- Polarity dependant on direction of rotation
SERVO equation?
Ve = Vi - Vo
Ve = feedback
Vi = Volts in
Vo = Volts out
TACHO Generator formula?
Vo = (V/RPM) x o/p RPM
Vo = Volts out
V = Volts
RPM = Reps per min
Name 2 features of a Closed Loops control (Servo)
- Error actuated (tachogenerator)
- Power amplifying control system (Op Amp)
What is meant by a RATE SERVO?
We are able to vary the RATE OF SPEED
What is the role of the Tachogenerator in a rate servo?
Turning mechanical energy into electrical (V) to be used as feedback
What will happen to the rate of movement on a DC RATE SERVO if…
Large I/P voltage = ?
Small I/P voltage = ?
Large I/P voltage = Fast rate of movement (speed)
Small I/P voltage = Slow rate of movement (speed)
What term is used to alter the speed of a RATE SERVO ?
SPEED-GRADING
What is SPEED-GRADING ?
Technique used to ALTER THE SPEED of the system by keeping the INPUT VOLTAGE the same.
Name two ways in which speed grading can be achieved?
Altering:
- Gain of the amplifier
- Amount of tachogenerator feedback
What are the effects of varying the GAIN on SPEED-GRADING?
0.8V/1000rpm
Gain (amplifier) x1 = 0.8V
Vo = Vi - Ve Vo = 40 - 0.8 Vo = 39.2V = 3920rpm
Gain (amplifier) x10 = 0.08V
Vo = Vi - Ve Vo = 40 - 0.08 Vo = 39.92V = 3992rpm
Increasing the amplifiers gain x10 will increase the SPEED-GRADING
But has a small speed increase for a large gain increase
What are the effects of varying the % FEEDBACK on the TACHOGENERATOR on SPEED-GRADING?
0.01V/1000rpm
Vo = Vi - Ve Vo = 30 - 0.01 Vo = 29.9V = 2990rpm
Set Tachogenerator to 50% F/B
0.01V/2
(New) 0.005V/1000rpm
TG FORMULA: Vo = (V/rpm)
Vo = (29.9/0.005) =5980rpm
Halving the Tacho feedback doubles the motor speed (SPEED-GRADING)
Small change for a big increase
What is meant by a DC POSITIONAL SERVO?
By aligning an O/P SHAFT with an I/P SHAFT, the position being determined by a input voltage
Describe the function of the following diagram?
P3-21
The A and B coils are winded in opposition to rectify the error Voltage coming in (achieve equilibrium)
How can we overcome a POSITIONAL SERVO from over shooting or going into a PENDULUM ?
By the use of DAMPING
Why do we require DAMPING?
- Overcome inertia
- Reach a steady state as fast as possible
What do we need to add to the system achieve DAMPING?
FRICTION has to be added to the system to achieve DAMPING
Name 3 types of FRICTION?
- Static friction (Stiction)
- Coulomb friction
- Viscous (fluid) friction
What is STATIC FRICTION (STICTION)
Use or draw a diagram to explain
P3-25 (figure.1)
All the friction is upon START, once overcome it drops down to nothing
P3-25 (figure.1)
What is COULOMB FRICTION
Use or draw a diagram to explain
P3-25 (figure.2)
Equipment friction(rubbing) like a bearing.
Lubricated bearing = Less friction
Un-lubricated bearing = more friction
But both their individual friction amounts will stay the same throughout
Show the effects of COULOMB FRICTION on a positional servo by use of a diagram?
Step input - coulomb friction - deadzone
P3-26 (Figure. 3)
What is VISCOUS (FLUID) FRICTION
Use or draw a diagram to explain
P3-27 (figure.4)
Like running in water:
Faster speed = Higher friction(drag)
Slower speed = Less friction(drag)
Friction is proportionate to the speed
What is a typical response to a STEP INPUT?
Step input
Coulomb friction
Deadzone
Draw and describe the following types of STEP INPUT RESPONSES?
Undamped Under damped Critically damped Over damped Ideal
Undamped - Oscillate indefinitely
Under damped - Has two or more overshoots
Critically damped - No overshoot(2nd fastest)
Over damped - Very slow
Ideal - One overshoot (Fastest)
What is a typical response to RAMP INPUT (constant velocity input)?
Ramp input
Viscous friction
Velocity lag
Draw a diagram and describe VELOCITY LAG?
P3-28 (Figure.7)
- Ramp input (constant velocity input)
- Viscous friction (present)
- Velocity lag (caused)
What type of DYNAMIC BRAKING do we use and how is it used?
REGENERATIVE BRAKING:
Generates a current which is fed to the DC batteries to act as a heavy load to the armature and helps bring it to a stop.
Why do we want to use a system that uses ERROR RATE CONTROL ?
To overcome VELOCITY LAG
ERROR RATE CONTROL
Advantages ?
Disadvantages?
Advantages:
- No increase in Velocity lag
- Acceleration boost over NVFB
- Cheaper than a Tachogenerator
Disadvantages:
- Not suitable for low speed control
- Adjustment not easy
- complicated in AC servos
What does the INTEGRATOR NETWORK DO?
Sums up the error over time.
What does a Three Term Controller (PID) do?
Produces an overall improvement in:
- Immediate response (system GAIN)
- Long term errors
- Short term errors
Draw a THREE TERM CONTROLLER (PID)
P3-51 (Figure. 7)
P3-51 (Figure. 7)
What does the (P) in PID controller do?
Proportional control:
- Responds immediately to changes
- Insensitive to errors
- Improves the ACCURACY and SPEED of RESPONSE (Gain)
What does the (I) in PID controller do?
INTEGRAL control:
- Responds to LONG TERM ERRORS
- Minimises / Eliminates steady state errors
What does the (D) in PID controller do?
DERIVATIVE control:
- Responds to SHORT TERM ERRORS
- Increases SPEED of response
- Controls overall system DAMPING (remember D for DAMPING)
What is a SYNCRO SYSTEM?
a General term used to names devices that uses a VRT(variable ratio transformer)
What are the Principles of a VRT(Variable Ratio Transformer)?
That of VARYING FLUX LINKAGE between two coils
- One fixed (with AC power)
- Other free to rotate (output voltage taken from its coils)
If a VRT rotates what are the PHASE relationship of the Voltage?
0 degrees
90 degrees
180 degrees
0 degrees. - Voltages are IN-PHASE
90 degrees. - ELECTRICAL ZERO
180 degrees. - PHASE INVERSION
What are the Disadvantages of a VRT?
Disadvantages:
- Voltage can only represent TWO ANGULAR positions
- Can only read +/- 90 degrees
Name two SYNCRO used in servo systems?
CX (controller transmitter)
CT (control transformer)
What are a CX-CT link used for ?
They are used to provide continuous error detection over 360 degrees
(Instead of 90 like the VRT)
Draw a AC POSITIONAL SERVO
P4-11 (Figure. 8)
P4-11 (Figure. 8)
- CX input 30 degrees / CT output 0 degrees
- CX has a AC input voltage / CT has a induced EMF created by the system(error voltage)
- ERROR voltage IN PHASE - Motor turn CW
- ERROR voltage ANTI-PHASE - Motor turn CCW
- Motor will STOP if the induced EMF coils fall to ZERO
Explain how the 2 PHASE A.C servo motor works?
P4-16
P4-16
- Stator carries two COILS at right angles (90 degrees)
- Coil A is more CAPACITIVE (so will LEAD) - Carries the A.C voltage
- Coil B is more INDUCTIVE (LAG behind) - Carries ERROR signal from the amp
- Result being a COMBINED FLUX VECTOR POSITION
Explain the AC TACHO( induction generator) diagram?
P4-17 (Figure. 11)
P4-17 (Figure. 11)
- Input windings(AC ref Voltage) are 90 degrees to the output windings
- When the IRON CORE rotates, EDDY CURRENTS are induced which DISTORTS the ref field
- Causing a VOLTAGE to appear at the output PROPORTIONAL to SPEED
- Output will be IN-PHASE for CW / ANTI-PHASE for CCW
What are the requirements for a A.C TACHO (induced) GENERATOR?
- ZERO in for ZERO out
- Voltage PROPORTIONAL to SPEED /
- IN PHASE (+) = CW
- ANTI-PHASE (-) = CCW
Wiring Faults (CX-CT link)?
ROTOR:
CX rotor O/C CT rotor O/C R1 and R2 reversed on CX-CT S/C across R1 and R2 on CX S/C across R1 and R2 on CT
CX rotor O/C - Nothing is going to happen
CT rotor O/C - Nothing is going to happen (maybe a dither)
R1 and R2 reversed on CX-CT - 180 degree out
S/C across R1 and R2 on CX - Nothing is going to happen (blown fuse)
S/C across R1 and R2 on CT - Nothing is going to happen (no response)
Wiring Faults (CX-CT link)?
STATOR:
O/C stator coil S1
O/C stator coil S2
O/C stator coil S3
O/C stator coil S1 - STOP in line with S1
O/C stator coil S2 - STOP in line with S2
O/C stator coil S3 - STOP in line with S3
Wiring Faults (CX-CT link)?
STATOR:
S/C stator coil S1 and S2
S/C stator coil S2 and S3
S/C stator coil S3 and S1
S/C stator coil S1 and S2 - 90 degrees S3 (one not effected)
S/C stator coil S2 and S3 - 90 degrees S1 (one not effected)
S/C stator coil S3 and S1 - 90 degrees S2 (one not effected)
Wiring Faults (CX-CT link)?
STATOR:
CROSS Connection of any TWO (2) coils
CROSS Connection of all THREE (3) coils - shifted by 1
CROSS Connection of all THREE (3) coils - shifted by 2
CROSS Connection of any 2 coils
- Direction of the load will go ANTI-CLOCKWISE to the input
CROSS Connection of all 3 coils (S1/s2 - S2/s3 - S3/s1)
- Stators SHIFTED by 1
- Turn CW
- 120 degrees out from I/P
CROSS Connection of all 3 coils (S1/s3 - S2/s1 - S3/s2)
- Stators SHIFTED by 1
- Turn CCW
- 240 degrees out from I/P
Advantages of
A.C and D.C positional servos?
A.C Positional Servo
- Synchros provide 360 degree continuous error detection
D.C Positional Servo
- D.C motors have a higher POWER/WEIGHT ratio and A.C motors
Draw a combined A.C and D.C system that uses PSD?
Explain how it operates
P4-26 (Figure. 18)
P4-26 (Figure. 18)
- AC ref voltage is comes from the CX (input shaft)
- A.C Ve (Voltage error) comes from the CT (output shaft)
- Both fed into a PSD (PHASE SHIFT DEMODULATOR)
- A smoothing circuit smooths the D.C voltage which is fed to the D.C motor
What are the requirements for a PSD(PHASE SHIFT DEMODULATOR)?
- ZERO in for ZERO out
- AC in for DC out
- AC proportional to DC
- AC phase error =
IN PHASE / pos + DC output. (CW)
ANTI PHASE / neg - DC output. (CCW)
Explain how the DIODE PSD works?
P4-30
P4-30
During a CLOCKWISE rotation of the shaft
- AC supply Positive HALF cycle
- AC error signal IN PHASE with supply
- Diodes will conduct and allow POS + current through
- Driving the DC motor CW
During a ANTI-CLOCKWISE rotation of the shaft
- AC supply
- AC error ANTI-PHASE with the supply
- Drive the DC motor CCW
What are RESOLVERS used for?
May be used to resolve right angled triangles in a system
Draw a RESOLVER diagram showing CW and CCW motion of:
Vin=5sinwt
Diagram P4-33 (Figure. 2)
Diagram P4-33 (Figure. 2)
Draw a SPACE or FREE gyro and name a few properties?
P4-43 (Figure. 1)
P4-43 (Figure. 1)
- 2 degrees of freedom (2 axis)
- Spatial rigidity (stays rigid in whatever position you leave it)
- Direction finding (used in aviation)
Draw a RATE gyro and name a few properties?
P4-44 (Figure. 2)
P4-44 (Figure. 2)
- 1 degree of freedom
- Uses the Application of precession (with the sprigs)
- Angular velocity (works out speed of change in movement)
- SPERRY’s rule to work out rotation of PRECESSION
What does the RATE gyro measure?
Angular velocity
Used to keep a tank turret locked onto a target no matter the movement of the tank
What happens to the Application of PRECESSION when a force is applied to INNER or OUTER GIMBAL them?
Precession is when:
Force applied to INNER gimbal = OUTER gimbal moves
Force applied to OUTER gimbal = INNER gimbal moves
What is SPATIAL RIGIDITY?
The ability for a spinning GYRO to maintain its position in space
Describe how SPERRY’s RULE acts when applying a force to a GYRO?
Remember:
Force applied to INNER gimbal = OUTER gimbal moves
- Apply a force on inner gimbal
- Follow direction of spin 90 degrees (ie. left)
- Outer gimbal will move
Force applied to OUTER gimbal = INNER gimbal moves
- Apply force to the outer gimbal
- Follow the direction of spin 90 degrees
- Inner gimbal will go the opposite direction of the applied force
How does a NORTH SEEKING GYRO avoid drifting off its Axis?
By the use of some gravity control using a PENDULUM
Explain the operation of a RING LASER GYRO?
- 3 Optical Laser beams travel in opposition to each other around an optical cavity.
- As the GYRO moves the optical paths become longer or shorter
- Thus for a given rotation, one FREQ will fall and the other will rise
- The difference is called the BEAT FREQUENCY, which changes the FRINGE pattern and gives a digital O/P
What does the 3 OPTICAL paths measure in the Ring Laser Gyro?
3 Optical Paths:
Role
Pitch
Yaw
How can a LOCK-IN occur?
Output has to be LINEAR
(but is not linear at low frequencies)
Causing a LOCK-IN
How do we overcome a LOCK-IN on a RLG (ring laser gyro)?
By using MECHANICAL DITHER:
2 x greater than the LOCK-IN rate
