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UKCAA MODULE 4 ELECTRONIC FUNDAMENTALS > SERVOMECHANISMS > Flashcards

SERVOMECHANISMS Flashcards

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1
Q

What is a servomechanism (servo)?

A

A servomechanism (servo) is an electromagnetic device that converts electricity into precise controlled motion using negative feedback mechanisms.

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2
Q

In which systems are servomechanisms commonly used?

A

Servomechanisms are commonly used in aircraft systems such as control surfaces, helicopter rotor heads, and aircraft wheel steering.

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3
Q

What is a synchro?

A

A synchro is a type of rotary electrical transformer sensor used to measure the angle of a rotating machine, transforming the angular position of a part into a voltage or vice versa.

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4
Q

Give an example of an open-loop system.

A

An example of an open-loop system is a power-assisted braking system where the driver’s judgement determines the braking force.

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4
Q

Describe an open-loop system.

A

An open-loop system is a control system where human interface between input and output determines accuracy, response time, and stability, with no automatic feedback.

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5
Q

Describe a closed-loop system.

A

A closed-loop system is an automatic error-actuated power control system that uses feedback to limit overshoot, improving accuracy, response time, and stability.

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5
Q

Give an example of a closed-loop system.

A

An example of a closed-loop system is an automatic clothes iron that regulates its temperature to stay within a specified range using feedback from a temperature sensor.

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6
Q

What is deadband in control systems?

A

Deadband is an area of a signal range where no action occurs, used to prevent oscillation or repeated activation-deactivation cycles.

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6
Q

What is null voltage in servomechanisms?

A

Null voltage is the induced voltage in the rotor at the minimum coupling position due to mechanical imperfections, winding errors, and distortions in the magnetic circuit.

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7
Q

Explain time lag in servomechanisms.

A

Time lag is the delay between the input signal and the actual movement of the load, which can be influenced by the weight of the load and the gain of the servo amplifier.

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8
Q

Define feedback in the context of servomechanisms.

A

Feedback refers to the system providing information about its load to the operator or input, allowing the system to correct differences between the state of the receiver and the transmitter signal.

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9
Q

What is hunting in servomechanisms?

A

Hunting refers to the overshoot and undershoot that occurs as the receiving device tries to match the transmitter signal, resulting in oscillation around the desired point.

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10
Q

What does follow-up mean in servomechanisms?

A

Follow-up is the behavior where a servomechanism returns the load position to the demanded position upon detecting an error.

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11
Q

What is damping in servomechanisms?

A

Damping is a force within an oscillating system that opposes motion, reducing hunting in servomechanisms to enhance stability without significantly reducing efficiency.

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12
Q

What are the two types of synchro systems?

A
  1. Torque systems: Provide mechanical output sufficient to align indicating devices or move light loads without power amplification.
  2. Control systems: Provide voltage for conversion to torque through an amplifier and servomotor, driving heavier loads.
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13
Q

What is the primary function of a torque synchro system?

A

A torque synchro system provides a mechanical output sufficient to align an indicating device, actuate a contact, or move a light load without power amplification.

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14
Q

What are the two main categories of synchros based on their load?

A
  1. Torque synchros: Used to produce turning force or torque to move light loads like dials and indicators.
  2. Control synchros: Used to move heavy loads, requiring additional components like amplifiers and motors.
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15
Q

What is the primary function of a control synchro system?

A

A control synchro system provides a voltage for conversion to torque through an amplifier and servomotor, driving heavier loads such as flight control surfaces.

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16
Q

How are synchros used as transmitters and receivers?

A

Synchros can be used as transmitters to provide input to a position control system or as receivers to provide an output from a position control system.

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17
Q

What distinguishes a synchro receiver from a synchro transmitter?

A

A synchro receiver has low friction bearings to accurately follow the movement of the transmitter and includes a damping mechanism to prevent oscillation.

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17
Q

What are the main components of a synchro’s construction?

A

A synchro consists of a laminated stator core carrying three coils in a star configuration and a laminated rotor with a winding connected via slip rings to external connections.

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18
Q

What are the advantages of using synchros?

A
  1. Long-distance capabilities
  2. Easily routed wiring
  3. Low electrical power usage
  4. Lightweight
  5. Cost-effective
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19
Q

What is the purpose of a torque transmitter (TX)?

A

A torque transmitter (TX) converts the rotor position into an electrical signal from the stator, which is then supplied to a torque receiver (TR), torque differential transmitter (TDX), or torque differential receiver (TDR).

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20
Q

What is required for torque to be present in a synchro system?

A

Torque is only present when current flows through the stators, generating magnetic fields. For this to occur, there must be a differential angle between the transmitter rotor and the receiver rotor.

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21
How does the torque moment change with the rotor angle difference?
The torque moment increases as the rotor angle difference increases up to 90°, where it reaches its maximum value. After 90°, the torque moment starts to decrease as the angle approaches 180°.
22
What effect do bearing friction and brush friction have in a synchro system?
Bearing friction and brush friction have a negative effect on precision, especially when the differential angle between the transmitter and receiver rotors is small.
23
Why must a synchro system be designed with low electrical resistance?
The system must be designed with low electrical resistance to allow sufficiently large currents to flow, as torque moments are proportional to the currents.
24
How does a differential transmitter (TDX) differ from a torque transmitter (TX)?
A differential transmitter (TDX) has three coils wired in a star shape on the rotor and is used to add or subtract angles. Unlike a torque transmitter (TX), it does not generate torque but rather transmits the difference or sum of angles.
25
What happens in a TX-TDX-TR system when the TDX is at electrical zero?
When the TDX is at electrical zero, it passes the voltages applied to its windings without change. The TX stator voltages are the same as those at the TDX rotor, and the TDX rotor voltages equal and oppose the TR stator voltages, resulting in no current flow and no torque in the TR rotor.
26
What is the effect of turning the TX rotor to 240° in a TX-TDX-TR system?
Turning the TX rotor to 240° induces maximum voltage into the S1 winding of the TX, which is then passed through the TDX to the TR. The TR rotor turns anti-clockwise to the 240° position, following the TX rotor exactly when the TDX rotor is at 0°.
27
How do you subtract angles using the TX-TDX-TR system when the TDX rotor is at 120°?
When the TDX rotor is at 120°, the system behaves as follows: The maximum voltage is induced in R1 of the TDX, which is connected to S1 of the TR, causing the TR rotor to turn to the position calculated by subtracting the TDX angle from the TX angle.
28
What happens in a TX-TDX-TR system when the TDX rotor is at 30° and the TX rotor is at 75°?
The TR rotor turns to 45° because the TDX subtracts the 30° from the 75° of the TX rotor, resulting in a difference of 45°.
29
Describe the setup for addition in a TX-TDX-TR system.
For addition, the S1 and S3 leads between the TX and the TDX are reversed, and the R1 and R3 leads between the TDX and the TR are also reversed. This setup causes the TR rotor to turn to a position that is the sum of the TX and TDX rotor angles.
29
How does a Control Transformer (CT) differ from a torque receiver?
A Control Transformer (CT) does not generate torque but instead provides a voltage that is proportional to the differential angle between the transmitter and receiver rotors. It is used to control a power amplifying device that moves heavy equipment.
30
: What is the primary function of a Control Synchro system?
A Control Synchro system uses a transmitter signal to control a servo motor at the receiver, which moves heavy loads. The receiver rotor’s signal is amplified and used to drive the servomotor.
31
What happens to the CT rotor's voltage when it is rotated 30° clockwise?
When the CT rotor is rotated 30° clockwise, the stator magnetic field induces an EMF across the rotor winding. This induced voltage is proportional to the rotation and in phase with the AC supply voltage.
31
What is a resolver and what is its primary function?
A resolver is a type of rotary electrical transformer used to measure degrees of rotation by resolving an angular input into its sine and cosine components. It is an analogue device that has digital counterparts like digital resolvers and rotary encoders.
32
In aircraft, what are some applications of synchro/resolvers?
Synchro/resolvers are used to monitor and control the position of a flap on an aircraft wing, the control stick of a helicopter, and the throttle position of an aircraft.
32
Explain the phase relationship between AC supply voltage and CT output voltage for anti-clockwise rotation.
For anti-clockwise rotation of the CT stator magnetic field, the output voltage across the rotor is still proportional to the amount of rotation but is 180° out of phase with the AC supply voltage.
33
How does the resolver control transmitter differ from the resolver control transformer?
The resolver control transmitter consists of a two-phase stator and a two-phase rotor, with the rotor free to turn within the stator’s field. The resolver control transformer is electrically identical to the transmitter, with the transmitter and stator windings connected in a similar manner.
34
What is the role of the resolver in a system where the rotor is rotated through 360° from a datum position?
As the rotor is rotated through 360°, it induces voltages across the stator coils that vary sinusoidally. These voltages correspond to the sine and cosine of the rotor’s angle, representing the angular position in terms of Cartesian coordinates.
34
What happens to the voltage induced across the stator coils in a resolver system when the rotor is aligned with one of the stator coils?
When the rotor is aligned with one of the stator coils, maximum voltage is induced across that stator coil. The voltage across the stator coil perpendicular to the rotor's flux will be zero.
34
How does the resolver system convert polar coordinates to Cartesian coordinates?
The resolver system converts polar coordinates to Cartesian coordinates by using one rotor coil (R1 R2) with an alternating voltage and short-circuiting the other coil (R3 R4). The resulting voltages from the stator coils represent the Cartesian coordinates.
35
Describe the configuration of a simple resolver.
A simple resolver has a rotor with a single winding and a stator with two windings at 90° to each other. The voltages appearing on the stator terminals vary depending on the angular position of the rotor.
36
What is the difference between a Cartesian and a polar coordinate system in the context of resolvers?
Cartesian coordinates represent positions using x and y values, while polar coordinates use a magnitude and angle. Resolvers can convert between these systems by outputting sine and cosine components for Cartesian coordinates or using angular displacement for polar coordinates.
37
What does it mean if no EMF is induced across a coil in a resolver system?
If no EMF is induced across a coil, it means that the rotor flux is perpendicular to the coil and hence the coil is "in line" with the flux direction.
38
What are the main types of LVDTs (Linear Variable Differential Transformers) and their respective advantages?
The main types of LVDTs are DC-operated and AC-operated. DC-operated LVDTs are easier to install, have simpler data conditioning, and can operate from dry cell batteries. AC-operated LVDTs are smaller, more accurate, and perform well at high temperatures.
39
What are the characteristics of an unguided armature in an LVDT?
An unguided armature fits loosely in a bore hole, has frictionless movement, and is suitable for short-range high-speed applications. It requires alignment between the LVDT body and the armature.
40
How does a captive (guided) armature differ from a spring extended armature in an LVDT?
A captive (guided) armature is restrained and guided by a low friction bearing assembly and is suitable for longer working ranges but may face alignment issues. A spring extended armature is also guided by a low friction bearing assembly but has an internal spring that maintains reliable contact and pushes the armature to its maximum extension.
41
What physical principle does an LVDT operate on?
An LVDT operates on the principle of electromagnetic induction.
42
What are the key components of an LVDT?
The key components of an LVDT are the primary coil, secondary coils, and a ferro-metallic core.
43
How does the ferro-metallic core affect the LVDT’s operation?
The ferro-metallic core increases the strength of the magnetic field produced by the primary coil.
43
What is the purpose of the primary coil in an LVDT?
The primary coil generates a magnetic field that induces voltages in the secondary coils.
44
Describe the 'NULL' position in an LVDT.
In the 'NULL' position, the movable iron core is centered between the secondary coils, resulting in equal and opposite voltages induced in the secondary coils, leading to zero output.
45
What happens to the output voltage when the iron core moves away from the NULL position towards one secondary coil?
The output voltage will have an amplitude dependent on the deflection magnitude and will be in phase with the primary coil if the core moves closer to that secondary coil.
46
What is a Rotary Variable Differential Transformer (RVDT)?
An RVDT is similar to an LVDT but is used to measure rotational movement rather than linear movement.
47
What are the advantages of using an LVDT?
Advantages of using an LVDT include friction-free operation, infinite mechanical life, infinite resolution, electromagnetic coupling, low risk of damage, null point repeatability, and environmental robustness.
48
What is the difference between a step input, ramp input, and accelerating input in a servo system?
A step input is a sudden change in position; a ramp input is a constant velocity change; an accelerating input involves a constant rate of acceleration.
49
What is the function of a closed-loop LVDT system?
A closed-loop LVDT system detects and corrects errors in the output signal by using feedback to maintain the core in a NULL position.
50
What is the role of the indicator light in a capacitive sensor?
The indicator light shows when a target is within the sensor’s sensing range, turning on when the target is detected.
51
How does a capacitive sensor detect a target?
A capacitive sensor detects a target by measuring changes in capacitance caused by the presence of the target disrupting the electric field between the sensor's plates.
52
What is the main difference between a vane type and a separate type proximity switch?
The vane type switch detects an object passing by its groove without contact, while the separate type uses a magnet unit and detects without direct contact.
53
What is the principle of operation for an inductance transmitter?
An inductance transmitter operates by measuring changes in inductance caused by the movement of a target in relation to the sensor.
54
What is a Desynn system used for?
A Desynn system is used for remote indication of angular positions, such as flap, rudder, and elevator positions on aircraft.
55
Describe the basic setup of a toroidal resistor-type Desynn transmitter.
It consists of a continuous resistance ring with fixed tappings and a transmitter with two spring-loaded sliding contacts that provide varying DC current to the receiver.
56
What is the function of the E and I bar in a servo altimeter?
The E and I bar assembly in a servo altimeter provides accurate measurements of altitude changes by detecting the angular position of the I bar relative to the E bar.
57
How does the slab Desynn system differ from the standard Desynn system?
The slab Desynn system provides higher accuracy with less friction and a sine wave output, and is not interchangeable with the standard Desynn system.
58
What is the purpose of laminations in the E and I bar construction?
Laminations reduce eddy currents and associated joule heating, improving efficiency and reducing heat generation.
59
How does the proximity switch output change when the target is near vs. far?
When the target is near, inductive reactance increases, current decreases, and output is low (0). When the target is far, inductive reactance decreases, current increases, and output is high (1).
60
What is hunting in synchro systems?
Hunting refers to the overshoot and undershoot that occurs as a receiving device tries to match the position signal of the transmitter, causing oscillations until the receiver settles at the desired position.
61
How can hunting be corrected in synchro systems?
Hunting can be corrected by introducing damping, which slows down system response and avoids excessive overshoots or overcorrections. This can be done through electrical resistance, mechanical brakes, or oil damping
62
What are the types of damping in synchro systems?
The two basic types of damping are Coulomb (Dry) friction and Viscous friction. Coulomb friction is a constant force independent of speed, while viscous frictional forces are proportional to velocity.
63
What is Velocity Feedback Damping?
Velocity Feedback Damping involves using a tacho generator to provide a feedback signal that opposes the control signal, reducing oscillations and helping the system stabilize faster.
64
How does time lag affect synchro systems?
Time lag in synchro systems occurs due to the delay between the input signal and the actual movement of the load. It can be reduced by increasing the gain of the servo amplifier, but too much gain can cause overshoot and instability.
65
What is deadband in a synchro system?
Deadband is a range of input values where the output remains zero. It prevents oscillation or repeated activation-deactivation cycles, requiring a certain threshold of input change for the system to respond.
66
What are typical symptoms of synchro faults?
Symptoms include no receiver movement (power failure), receiver displaced 180° or other angles from the transmitter (connection errors), reverse rotation (stator connections reversed), and oscillation between two points (open or short circuits).
67
What is the effect of reversing rotor connections in a synchro system?
Reversing one pair of rotor connections will cause the receiver to be displaced 180° from the transmitter, with correct rotation.
68
What happens if there is a short circuit between two stator lines?
A short circuit between two stator lines causes the receiver to be displaced in 180° steps.
69
How does reversing two stator connections affect synchro system output?
Reversing two stator connections causes reverse rotation of the receiver relative to the transmitter.
70
What are the operational reasons for changing the direction of rotation on a synchro receiver?
Operational reasons may require displaying the direction of rotation differently, such as clockwise instead of counterclockwise, which can be achieved by cross-connection.
71
What are the remedies for an open circuit on one stator line?
The remedy is to carry out a continuity check on the stator lines to identify and fix the open circuit.
72
what type of transducer is likely to be used to monitor the temperature of a liquid
an analogue transducer
73
what does the phase of the voltage induced into the rotor of a CT in a control syncro system depends upon
the direction in which the CT rotor is turned

UKCAA MODULE 4 ELECTRONIC FUNDAMENTALS (5 decks)

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