View
11
Download
1
Category
Preview:
DESCRIPTION
the basics behind servo systems in terms of aircraft technology.
Citation preview
Malaysian Institute of Aviation Technology
1
Topic 13:
Servomechanism
Malaysian Institute of Aviation Technology
2
Objectives
After studying the material in this chapter, you should be able to:
1. Explain on the classification of the open and closed loop control system.
2. Explain on the feedback/follow-up system of a closed loop controller.
3. Explain on the meaning of the term : null, hunting,dead band and damping
4. Describe on the degree and method of damping
Malaysian Institute of Aviation Technology
3
Definition
A servomechanism is a force amplifier mechanism where the output accurately follows the input but with greater power.
Control system can be divided into two basic types:
a. Open Loop Control System
b. Close Loop Control System
Malaysian Institute of Aviation Technology
4
a. Open Loop
A system whereby external action is required to
control the loop manually.
Output controlled by the input only
Malaysian Institute of Aviation Technology
5
b. Close Loop
The control of loop is automatic within the system. Output controlled by the input with some form of
feedback or follower.
Malaysian Institute of Aviation Technology
6
Basic system of Servomechanism consist of following components:
Malaysian Institute of Aviation Technology
7
The unit that control ailerons, horizontal stabilizer and rudder are called servo motors.
They are class of devices that include synchros and resolvers.
Servos for the flight control systems (FCS) have important characteristics; they will not jam or cause other parts to become
entangled in the motor.
If an FCS fails, the aircraft returns to manual control.
Servo mechanisms, also called SERVO SYSTEMS or SERVOS for short, have countless applications in the operation of
electrical and electronic equipment.
Introduction to Servo Systems
Malaysian Institute of Aviation Technology
8 Main Flight Control Surface
Malaysian Institute of Aviation Technology
9 Synchros
Malaysian Institute of Aviation Technology
10
It is often necessary to operate a mechanical load that is remote from its source of control in working with:
Radar and antennas Aircraft control surface Flight Directors Computing devices Many other equipments
Introduction to Servo Systems (contd)
Servo motor
Malaysian Institute of Aviation Technology
11
To obtain smooth, continuous, and accurate operation, the mechanical loads are normally best controlled by synchros.
As you already know, the big problem here is that synchros are not powerful enough to do any great amount of work.
This is where servos come into use.
A servo system uses a weak control signal to move large loads to a desired position with great accuracy.
The key words in this definition are move and great accuracy.
Introduction to Servo Systems (contd)
Malaysian Institute of Aviation Technology
12
Elevator and rudder control cables
Malaysian Institute of Aviation Technology
13
In many large aircraft, control surfaces are moved with motors.
A pilot does not have the strength to move control surfaces in some aircraft and, like power steering in an automobile
The controls are servo assisted.
In the most advanced aircraft;
Control surfaces are manipulated by a simple wrist action controller called a side stick
Surface are moved only by servo, with no mechanical connection to the controller
This is fly by wire, where signals from the side stick are transmitted to a computer which adjusts the control surfaces
Introduction to Servo Systems (contd)
Malaysian Institute of Aviation Technology
14
Fly by wire - A321 Cockpit
Malaysian Institute of Aviation Technology
15
The difference between fly by wire and servo-assist is that fly by wire uses digital data and control surface positions are determined by a computer.
Conventional servo assist typically uses analog signals and does not involve a computer.
Servo assist, even in the largest aircraft, is a simple feedback control system.
The important point in servo-assisted control system is that there are motors already in place.
Introduction to Servo Systems (contd)
Malaysian Institute of Aviation Technology
16
In general, synchros are excellent for sensing angular position, but are not effective as torque-producing motors.
To overcome this problem,
An AC or DC motor, possibly even with a gear reduction drive, to provide ample torque
The synchro to measure the position of the indicator
The position of the indicator mechanism is compared to the desired position, and the error is amplified and applied to the driving motor.
This technique applies to a broad class of electrical or electrical-mechanical systems called servo systems.
Introduction to Servo Systems (contd)
Malaysian Institute of Aviation Technology
17
Synchro Controlled Servomechanism
The CT rotor is at 90 to CX rotor and so the system is in the null condition
Malaysian Institute of Aviation Technology
18
The CX rotor has been rotated so that an error signal has been produced This will drive the motor (clockwise) until the null position reached
Malaysian Institute of Aviation Technology
19
The CX rotor has been rotated so that an error signal has been produced This will drive the motor (counterclockwise) until the null position is reached
Malaysian Institute of Aviation Technology
20
In figure above:
A synchro-controlled servo system is used as an input
The output is made up of an indicator and other associated rotating components (load)
The mass or friction would not allow a simple synchro to be used
The input synchro signal is connected to a synchro that is coupled to the indicator shaft.
When used in this application, special synchros, called control transformers (CTs), are used.
The rotor and stator impedances of the control transformer are higher than those of a simple synchro, which allows them to interface more
easily with electrical circuits.
Introduction to Servo Systems (contd)
Malaysian Institute of Aviation Technology
21
If the input synchro and the control transformer in the indicator were set to exactly the same angle, the rotor output from the control transformer
would be zero.
When theres an angular difference between the two synchros, there will be rotor voltage,
The amplitude is proportional to the sine of the error angle and the phase is indicative of the algebraic sign of the error angle.
If the error angle is positive, the output from the synchro rotor is in phase with the driving voltage;
If it is negative, the two are out of phase.
Introduction to Servo Systems (contd)
Malaysian Institute of Aviation Technology
22
If the AC voltage from the rotor of the synchro were amplified and applied to a two-phase AC motor, the motor would operate so as to drive the servo system toward a zero error angle.
At this point, the output from the synchro would be zero and the motor would cease to run.
There are two positions at which the error voltage is zero; one at the desired angle and one 180o from the desired angle.
Introduction to Servo Systems (contd)
Malaysian Institute of Aviation Technology
23
When the servo system is near its desired position, the motor is still running at a relatively high speed.
Because the motor and the other components of the servo system have an inertia, the high motor speed near the desired servo position will cause the servo to overshoot the position and end up on the opposite side of the desired null.
The error voltage will cause the motor to change direction and approach the null point again.
Depending on the amount of inertia, the system could overshoot again and the process be repeated.
A servo system that oscillates continuously is said to be unstable.
Servo Behavior
Malaysian Institute of Aviation Technology
24
Continuation of the overshooting about the null position is called hunting. Damping of the system can reduce the oscillations and prevent hunting.
A servo system in which oscillations exist, but cease after a few passes of the null point, is said to be underdamped.
If the system approaches the null point without any oscillations in the minimum time, the system is said to be critically damped.
If the system approaches the null point without any oscillations but requires an excessive amount of time, the system is said to be overdamped.
The damping of a servo system can be controlled either electrically or mechanically.
Usually it is easier to control damping electrically, and most systems set the characteristics of the servo by means of electrical components.
Malaysian Institute of Aviation Technology
25
Degree of Damping
Malaysian Institute of Aviation Technology
26
Method of Damping
a. Viscous Frictional Damping Consist of a thin disc metal (copper or aluminium) on the
output shaft rotating between the pole of a permanent magnet.
Rarely used because: i. Consume power
ii. Causes or widens dead band which is the amount of
error that can exist without correction.
Malaysian Institute of Aviation Technology
27
b. Velocity Feedback Damping
By a tacho-generator attached to the output shaft which provide a small AC to produce voltage proportional to the
angular velocity of the shaft (motor).
Cosume less power.
Malaysian Institute of Aviation Technology
28
Introduction to Servo Systems (contd)
Malaysian Institute of Aviation Technology
29
Summary
1. Define of open and closed loop control
system.
2. Explain on the meaning of the term : null,
hunting,dead band and damping?
3. Describe on the degree and method of
damping?
Malaysian Institute of Aviation Technology
30
Topic 5: Servomechanism ii
Malaysian Institute of Aviation Technology
31
Objectives
After studying the material in this chapter, you should be able to:
1. Explain the principle and operation of a synchro
transmission system namely:
i. Control synchro
ii. Torque synchro
iii. Differential synchro
iv. Resolver synchro
Malaysian Institute of Aviation Technology
32
Synchro Transmission
System
Malaysian Institute of Aviation Technology
33
Control Synchro
Used as error detectors in servo mechanism. Comprises of two synchro units, a control transmitter (CX) and
a control transformer (CT).
Malaysian Institute of Aviation Technology
34
Example of synchro transmitter
Control Synchro
Malaysian Institute of Aviation Technology
35
Operation
Reference signal applied to CX rotor and created a magnetic field link to CX stator coils.
CX stator EMFs produce currents through the coils of CT stator coils and set up magnetic fields in the CT.
EMFs induced in CT rotor depend on angle between rotor and stator field.
R1 aligned to S1 of the CX and the rotor of CT at 90 degrees to S1 is the Null Position. (No error)
Malaysian Institute of Aviation Technology
36
Malaysian Institute of Aviation Technology
37
Malaysian Institute of Aviation Technology
38
Malaysian Institute of Aviation Technology
39
Torque Synchro
Used in instrument repeater systems (data indicating).
No amplification of torque takes place.
Movement of an input shaft is converted into an electrical signal and transmitted to move a pointer on a meter.
Malaysian Institute of Aviation Technology
40
Operation
AC applied to TX rotor and created a magnetic field in TX rotor which is the primary of a transformer.
This primary field effect the three coils S1,S2 and S3 (secondary transformer).
If the rotors misaligned, the currents will set up a resultant magnetic field in TR stator to which the TR rotor will align.
When the rotors are aligned, the stator EMFs on the TX and TR are the same. No current flows. (Null Position)
TR rotor follows TX shaft.
Malaysian Institute of Aviation Technology
41
Principle of torque synchro measurement
Malaysian Institute of Aviation Technology
42
Resolver Synchro
Commonly used in older analog computers, flight director and remote indicating compasses system on the aircraft
They are used in dealing with problems that are often occur in navigation: fixing the relative position of the two points.
Has two sets of stator coils arranged at 90 degrees and two set of rotor coils also at 90 degrees
Malaysian Institute of Aviation Technology
43
Positions are fixed in two way:
1. Measuring range and bearing (Polar coordinates)
2. Measuring X and Y coordinates (Cartesian coordinates)
Malaysian Institute of Aviation Technology
44
Polar to Cartesian Conversion Operation
= x
= y
Malaysian Institute of Aviation Technology
45
Cartesian to Polar Operation
Malaysian Institute of Aviation Technology
46
Differential Synchro
Used when the output required to be the difference between two input shaft angles.
The rotor outputs are connected to the three stator coils of TR or CT
May be used in either torque,control or resolver synchro system.
Malaysian Institute of Aviation Technology
47
Operation
TDX acts as a three winding transformer.
The induces EMFs of TX stator applied to stator coils of TDX which produces a resultant magnetic field.
Induces EMFs TDX rotor coils applied to TR stator coils.
The resultant field produced in TR stator is combination of both TX and TDX rotor position.
TR rotor moves to align with the resultant field (difference between two shaft angles).
Malaysian Institute of Aviation Technology
48
Example of combination of input and the resultant output
Malaysian Institute of Aviation Technology
49
Summary
1. Difference of Torque synchro & Control synchro?
2. How differential synchro operate?
3. Resolver synchro used for what application?
Malaysian Institute of Aviation Technology
50
Objectives
After studying the material in this chapter, you should be able to:
1. Explain the meaning of transducer.
2. Explain the operation and state the use of E and I Transformers.
3. Explain the operation and state the use of Inductive Transmitter.
4. Explain the operation and state the use of Capacitive Transmitter
Malaysian Institute of Aviation Technology
51
Transducer Transducer
A device, component machine, system or combination of
these that is used to convert one form of energy into
another.
Example types of transducers:
1. Temperature transducer convert temperature changes into electrical voltages (or mechanical switching)
2. Pressure transducer change barometric pressure into electrical voltage
3. Motor convert electrical signal into mechanical (rotary/linear) .
Malaysian Institute of Aviation Technology
52
E and I Transformers
Construction E core the centre limb is the primary coil and
the 2 outer are the secondary coils
Secondary windings are connected in series opposition, therefore the voltage induces in
them will oppose each other.
I bar pivoted in the center and attached to whatever we are trying to measure the movement
of, e.g. in servo altimeter, the I bar is connected
to the capsules.
Changes in the position of the I bar changes the reluctance at the upper of lower arm.
Malaysian Institute of Aviation Technology
53
OPERATIONS
Figure a: I Bar Neutral
The flux in the top and bottom limbs will be the same
The emf induced into two coils B and C will be the same but of opposite phase.
The output will be zero
Malaysian Institute of Aviation Technology
54
Figure (b) : I Bar Position 1
When the I bar is moved by the sensing element, more flux cutting coil B (less air gap) and less flux cutting coil C (larger air gap).
The emf induced in coil B is greater than in coil C. The output is the difference between these two giving an output that is in phase with input.
Malaysian Institute of Aviation Technology
55
Figure (c): I Bar Position 2
When the I bar moved in opposite direction, the emf induced into coil C is greater than coil B
The output will be anti-phase to the input. The amplitude of the output will depend on the amount of movement of the I bar.
Malaysian Institute of Aviation Technology
56
Linear Voltage Differential Transformer (LVDT)
Malaysian Institute of Aviation Technology
57
Applications
These type of sensor is used in:
Servo altimeters Acceleration sensors Servo instruments Air data computers Cabin pressure transducer
Malaysian Institute of Aviation Technology
58 58
ADC Electro Mechanical
Malaysian Institute of Aviation Technology
59
Inductive Transmitter
Construction
Has a coil supplied with alternating current set against two vanes. Voltage will be induced into the secondary coil depending on the inductance of the vane next to the coils.
Amount of inductance depends on the type of vane material used (permeability value,)
(a)
Malaysian Institute of Aviation Technology
60
Operation
A common inductive transmitter is made of aluminium and ferrite material. The null position is when the inductive coil is positioned on the join between ferrite and the aluminium vanes, figure (a).
If the vane is moved so more of the aluminium vane is beside the coils, less inductance results, figure (b).
Conversely when the vane move more of the ferrite vane is beside the coils, greater inductance results, figure (c).
One of the main application of inductance transmitter is position indication.
Malaysian Institute of Aviation Technology
61
The capacitance of a capacitor depends on:
1. The distance between the two plates
2. The area of the plates
3. The dielectric constant of the material between the plates
Capacitance Transmitter
Malaysian Institute of Aviation Technology
62
Capacitance Transmitter
Construction
Capacitive transmitter are simply variable capacitor (air dielectric)
Movement of the input shaft effectively alters the area of the
plates facing each other.
Has a rotor and stator of intermeshing plates.
Malaysian Institute of Aviation Technology
63
Operation
The amount of stator and rotor intermeshing is controlled by the rotation of a shaft by mechanical output.
As illustrated in figure above, when the stator and rotor plates are fully intermeshed, the capacitance is high.
Conversely, when the stator and rotor plates are partly meshed, the capacitance is low.
Malaysian Institute of Aviation Technology
64
Applications
A common use of capacitance transmitters is a fuel quantity indicator system. As the fuel rises in the tank, air is displaced by fuel and the dielectric changes to increase the capacitance of the unit.
While the fuel level goes down, so the capacitance goes down. Air has a dielectric constant of 1 and aircraft fuel has a dielectric constant of approximately 2.
Simple capacitive tank unit
Malaysian Institute of Aviation Technology
65
Malaysian Institute of Aviation Technology
66
Tank unit installation (B777)
Malaysian Institute of Aviation Technology
67
Example of Fuel Tank Units
Malaysian Institute of Aviation Technology
68
Summary
1. What is E & I bar? The construction?
2. How E & I bar operate?
3. The use if E & I bar?
4. Construction of inductive transmitter?
5. Amount of inductance depends on?
6. Give 3 factors that effect capacitance of
a capacitor?
7. Application of capacitive transmitter?
Recommended