4.3 Servomechanisms Flashcards
There are two different types of synchro systems what are they
torque systems and control systems
In a control system, a synchro will provide a voltage for conversion to torque through an amplifier and a servomotor. This torque can be large enough to drive devices such as flight control surfaces
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In a torque system, a synchro will provide a mechanical output sufficient to align an indicating device, actuate a contact or move a light load without power amplification.
In a torque synchro system, the transmitted signal does the usable work without further amplification.
Synchros can be used as transmitters or receivers depending on whether they are providing an input or an output to a position control system.
The two devices are very similar in construction; the main difference is that the receiver has low friction bearings to follow the movement of the transmitter accurately, and some form of damping mechanism designed to prevent oscillation.
A synchro is an electromechanical transducer. A mechanical input, such as a shaft rotation, is converted to a unique set of output voltages. This set of new input voltages is used to turn a second synchro rotor to a matching position
Simple synchros can be used to turn light loads, e.g., dials and pointers
Heavier loads require additional components, such as
Amplifiers
Motors
There are two main categories of synchro’s, depending on the load in the system
torque synchros
control synchros
Torque synchros are used in systems that need to produce a turning force or torque necessary to move light loads (dials and indicators); they include transmitters, receivers, and differentials.
Control synchros are used in systems to move heavy loads; they include transmitters, transformers, differentials, and resolvers.
A synchro can be likened to a long metal shaft that transmits motion from one point to another
Turning the shaft at one end causes the other end to turn in the same manner
Same time
Same speed
Same amount
Placing a pointer and a dial at each end of the shaft produces the same dial readings (provided they were the same at start) irrespective of:
Speed
Direction
Displacement
A flexible shaft (Teleflex cable) could be used, e.g., speedo cables that transfer the turn of the wheels to the speedometer. The problems with these, however, are:
Friction
They cannot drive heavy loads as the shaft begins to twist
the answer to these problems is the synchro, which is essentially an electric motor with wires (typically five) coming out if it. When two synchros are connected, they form a synchro system.
for the terminals, the schematics for aircraft use the Aeronautical Radio Incorporated (ARINC) terms:
S1: X blue
S2: Z black
S3: Y yellow
R1: H red and white line
R2: C black and white line
Synchros have the following advantages:
- Long distance capabilities
- Routing easily accommodated
- Uses little electrical power
- Lightweight
- Cost effective
Servo mechanisms exist in two basic forms to provide both manual and automatic control of linear and angular positions:
- Open-loop systems have a few applications in aircraft systems.
- Closed-loop systems are commonly used in aircraft air data, flight data and autopilot/flight path control systems, and indicators.
Open-loop System
There is no positional feedback, and the pilot does not know if the rudder has adopted the position requested
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In the closed-loop system, the demand is made in the same way as the open-loop system. In a basic system, positional feedback is given to the pilot to adjust accordingly, but this is not practical with systems such as aircraft flying controls.
.An output position transducer has been added to the servomotor and feeds back any difference between input demand and output to an error detector.
Closed-loop servo mechanisms are Alternating Current (AC) or pulse operated and can be classified by
Application
Method of damping
Degree of damping
Damping refers to the means of limiting overshoot.
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Basic Requirements of a Closed-loop Servo System
-Accept an order.
-Evaluate the position of the input and output.
-Evaluate the magnitude, direction, and rate of change of any “error”.
-Carry out correcting orders.
Types of Closed-loop Servo Mechanisms
-Positional servo: control of position
-Rate servo: control of rate and direction
-Computing servo: computes output functions from input information
Synchro’s and resolvers work like induction regulators. Their primary coils are usually represented by the rotor and the secondary coil by the stator windings, which are placed at 120° intervals
the induced voltage into the stator, from the energised rotor, depends on the position of the rotor relative to the stator.
The synchro transmitter converts the angular position of its rotor (mechanical input) into an electrical output signal.
When a 115 V AC (Alternating Current) excitation voltage is applied to the rotor of a synchro transmitter, the resultant current produces an alternating magnetic field around the rotor winding.
The flux lines cut through the turns of the three stator windings and, by transformer action, induce a voltage across the stator coils
The amplitude of the Electromotive Force (EMF) induced in the secondary winding depends on what
the degree of “magnetic coupling” between the primary and secondary windings.
When the maximum effective coil voltage is known, the effective voltage induced into a stator coil at any angular displacement can be determined.
A Torque Transmitter (TX) and a Torque Receiver (TR) make up a simple torque-synchro system
The AC supply is normally 115 V AC or 26 V AC
Torque systems only drive light loads.
A turning moment is required to move the rotor in the receiver to match the position of the rotor in the transmitter. This is created by the interaction between the magnetic fields of the stator and the rotor
When the two rotors are aligned with respect to the stators, the excitation current in the rotors induces a voltage across the stators. For each pair of stators, this voltage is equal and opposite, since they are wired in parallel, and no current flows in the stators.
If the transmitter rotor is turned, the voltage induced across each pair of stators is no longer equal, and a current flows through each stator.
As the receiver rotor turns, the induced voltage across each pair of stator coils, again, approaches equality; therefore, the torque
decreases
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
the torque moment for the forces working on the rotor begins to get smaller after it passes an angle of 90°.
The task of a differential transmitter is to add or subtract angles. As opposed to a torque transmitter, it has three coils wired in a star shape on the rotor, which has a circular cross section so that it does not influence the magnetic field of the stator.
If both shafts are turned in the same direction, subtraction is the result; if the shafts are rotated in opposite directions, an addition results. The TR indicates the addition or subtraction by the position of its rotor.
the synchro chain can also be constructed as a steering system instead of a torque system. Instead of TX, TDX, and TR, it can use CX (Control Transmitter), CDX (Control Differential Transmitter), and CT (Control Transmitter)
In a differential transmitter, the coupling and translations are chosen in such a way that the maximum voltages between two stator terminals are of the same magnitude as the maximum voltages between two rotor terminals
Control transmission systems are so called because the displacement signal from the transmitter is used to control a servo motor at the receiver. Any signal in the receiver rotor is amplified and the amplified output used to drive a servomotor. The servomotor is geared to the indicator mechanism and to the receiver rotor, which is turned until no signal is induced across it (the null position)
There is a 90° difference between the relative rotor/stator positions of the transmitter and the control transformer when there is no servo signal (the transformer rotor is at its null point).
Current still flows in the stator windings of control synchros even when at rest. Note that the rotor of the Control Transmitter (CT) only has an induced EMF from the stator windings. It is not connected to a separate power source
Unlike torque synchros, the receiver of a control synchro chain does not generate torque but instead delivers a voltage that depends on the differential angle between the transmitter rotor and the receiver rotor
Control synchros cannot move heavy loads by themselves. However, they are used to ‘control’ servo systems, which carry out the actual movement.
There are three types of control synchros:
Control Transmitter (CX)
Control Transformer (CT)
Control Differential Transmitter (CDX)
The control transmitter (CX) and the control differential transmitter (CDX) are identical to the TX and the TDX discussed previously, except that the windings in the CX and CDX have higher impedance.
The higher impedance windings are because control systems draw far less current and provide an output voltage to a control transformer that outputs an error signal that controls a servo motor, which can drive a large load.
Torque systems are based on having an internal current to provide the driving torque needed to position an indicator.
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Control Transformers (CT)
A CT is a device that accurately governs some type of power amplifying device used for moving heavy equipment.
The CT compares two signals: the electrical signal applied to its stator and the mechanical signal applied to its rotor. Its output is a difference signal that controls a power amplifying device and, thus, the movement of heavy equipment
The unit construction and physical characteristics of a control transformer are like those of a control transmitter or torque receiver, except that there is no damper, and the rotor is a drum or wound rotor, rather than a salient-pole rotor
The rotor is never connected to an AC supply and, therefore, induces no voltages in the stator coils. As a result, the CT stator currents are determined solely by the voltages applied to the high-impedance stator windings.
Resolvers
A resolver is a type of rotary electrical transformer used for measuring degrees of rotation. It is considered an analogue device and has digital counterparts, such as the digital resolver and rotary (pulse) encoder.
On aircraft, synchro/resolvers allow you to monitor and control:
The position of a flap on an aircraft wing
The control stick of a helicopter
The throttle position of an aircraft
The resolver control transmitter consists of a two-phase stator and a two-phase rotor, free to turn within the field of the stator. In each case, the two windings are physically at right angles to one another
The transmitter rotor in use is energised from a 26 V/115 V 400 Hz single phase AC supply
Analogue Transducers
LVDT is an acronym for Linear Variable Differential Transformer. It is a common type of electromechanical transducer that can convert the rectilinear motion of an object (to which it is coupled mechanically) into a corresponding electrical signal.
An LVDT is used to transfer linear movements and angular movements, such as rudder deflections.
Advantages of DC Operated LVDTs
-Ease of installation
-Simpler data conditioning than AC operated LVDTs
-They operate from dry cell batteries (good in remote locations without a power supply)
-Lower system cost than AC operated LVDTs
Advantages of AC Operated LVDTs
-Smaller than DC operated LVDTs
-More accurate than DC operated LVDTs
-They operate well at high temperatures
The physical principle by which the LVDT operates is electromagnetic induction
The primary coil (red) is connected to a power source.
The secondary coils (blue) are connected in parallel, but with opposing polarity (Lenz’s Law).
The primary coil’s magnetic field (black) induces a current in the secondary coils.
The ferro-metallic core (yellow) increases the strength of the magnetic field due to the primary coil.
Underlying Principle of an LVDT
The iron core is mechanically hinged to the part whose movement is to be detected. The coupling between the primary and secondary coils changes according to the relative position of the iron core – the amplitude and phase position of the output voltage is dependent on this
Rotary Variable Differential Transformers (RVDT)
RVDTs and E and I bar transducers work on the same principle as the LVDT
each device is used to produce an electrical signal from a mechanical movement. The RVDT produces an electrical signal proportional to a rotational movement, and the E and I bar can be used to produce a signal from both linear and rotary movements
The E and I bar gets its name from its physical construction. It is laminated to reduce the effects of eddy currents.
Inductance Transmitters
When measuring position, there are two main elements: the sensor and the target
a common application of an inductance transmitter is a proximity switch
Magnetic proximity switches are usually classified into two types: an integrated type, e.g., vane type, and a separate type.
Vane Type Transmitter
Detection of an object can be achieved without any physical contact – it enters or passes by the groove of a U-shaped structure. In general, the object is flat and made of a ferromagnetic material, such as iron.
With its high detecting accuracy, the switch exhibits less constraint conditions and greater ease-of-handling.
Separate Type
The switch unit is fixed and the magnet unit is mounted to the moving object to be detected. Approach or passage of the magnet unit is detected without contact. This type of switch does not require a separately mounted detecting unit. Furthermore, one magnet unit can energise several switch units.
typical proximity switch mounts. They are normally cylindrical or rectangular in shape. The leads are also colour coded red and blue, although this can vary by manufacturer
A capacitive sensor is an electronic device that can detect solid or liquid targets without physical contact
To detect these targets, capacitive sensors emit an electrical field from one end. Any target that can disrupt this electrical field can be detected by a capacitive sensor.
A capacitive sensor can detect many types of solid materials, including all types of metal and plastic, wood, paper, glass, and cloth.
Capacitive sensors have four main parts what are they
the sensor body,
the sensing face,
the indicator light
the cable or cable connection end.
DC Servo Systems are not so common today, mainly due to:
- Weight
-Increased servicing required – maintenance of the brush gear of the DC motors.
-Reliability of potentiometers and input/output transducers
-Temperature stability of DC amplifiers
pilot could no longer see the control surfaces from the cockpit, and mechanically operated instruments became severely limited in application
This requirement was met using electrically remote reading and indicating systems, in which a sensor detects changes in a measured quantity or position and transmits this information to a remote indicator in the cockpit for the pilot to view.
The system introduced was a synchronous system consisting of a transmitter at the source (the medium to be measured) and electrical, synchronous information about the medium being measured (e.g., surface position), which is transmitted to the indicator remotely.
The earliest of these systems is the Desynn system, which requires DC at either 12 V or 24 V, depending on the design requirement.
There are three types of Desynn transmitters:
Toroidal resistor: rotary motion for the indication of position
Micro Desynn: linear motion for the indication of pressure
Slab Desynn: also used for pressure measurement
The toroidal resistor transmitter is the basic system; the others are derivatives of it.
toroidal transmitter. Note that the stator windings of the indicator act like three phases.
A servo (servomechanism) is an electromagnetic device that converts electricity into precise controlled motion using negative feedback mechanisms. Servos can be used to generate linear or circular motion, depending on their type
. They control large loads through changes to low-power systems, and automatically correct the performance of a mechanism to ensure accuracy. More specifically, they are electronic control systems in which a hydraulic, pneumatic, or another type of controlling mechanism is actuated and manipulated by a low-energy signal.
A good example of a servomechanism is the cruise control in a modern car – the system repeatedly checks that the speed of the vehicle is the same as that selected by the driver and corrects its speed if this is not the case.
Servos are used in many aircraft systems, e.g.
aircraft control surfaces, helicopter servo actuators on the rotor head/blades/swashplate, or aircraft wheel steering.
Synchro-mechanism (Synchro)
A synchro is a type of rotary electrical transformer sensor used to measure the angle of a rotating machine, such as an antenna platform
The term, ‘synchro’ can describe any electrical device in which the angular position of a rotating part is transformed into a voltage, or vice versa. When used to transform a voltage into the rotation of a physical component, it can be considered a servomechanism
An open-loop system is a control system in which human interface between input and output determines system accuracy, response time, and stability.
manually operated servo systems tend to suffer from overshoot, leading to poor response times and instability
A simple example of an open loop is a power-assisted braking system; there is no automatic feedback system, just the driver’s judgement.
In an open-loop system, the input position is converted to an electrical signal; this demand signal is amplified and sent to the motor to position the load. The final position of the load is dependent on various factors over which the system has no control. These include friction in the system and variations in the power supply or amplifier gain.
The final position of the system is not accurately controlled – this makes an open-loop system unsuitable for close tolerance use. Open-loop systems are entirely reliant on the judgement of the operator for accuracy.
A closed-loop system is an automatic error-actuated power control system. Error is the difference between the required state and the actual state of the load.
Closed-loop systems have a circuit or means of automatically limiting overshoot; this significantly improves system accuracy, response time, and stability in comparison to manually operated open-loop systems.
A simple example of a closed loop is an automatic clothes iron. An automatic iron regulates the temperature of iron itself in such a way that the temperature for a cloth stays in a specified temperature range
Input: The desired temperature setting is the input to the system. It represents the target temperature at which the iron should operate.
Process: The process refers to the iron’s heating element, which converts electrical energy into heat. The heating element is responsible for raising the iron’s temperature.
Controller: The controller is responsible for comparing the desired temperature (input) with the actual temperature (output) measured by the sensor. It calculates the error, which is the difference between the desired temperature and the actual temperature.
Actuator: The actuator in this case is the control mechanism that adjusts the heating element’s power based on the controller’s instructions. It controls the heat generation in response to the error signal.
Feedback: The feedback loop is established by the sensor, which continuously monitors the temperature of the iron. It provides the necessary information about the system’s current state.
the difference between the demanded position and the actual position. This difference, called the “error signal”
Null Voltage
When the transducer stator and rotor windings are exactly perpendicular, there should be no induced voltage. However, mechanical imperfections, winding errors, and distortions in the magnetic circuit cause some voltage to be induced in the rotor at the minimum coupling position. This is known as the “null voltage”.
Deadband
A deadband (sometimes called a “neutral zone”) is an area of a signal range or band where no action occurs. Its purpose is to prevent oscillation or repeated activation-deactivation cycles (called ‘hunting’ in some control systems). The deadband principle is used in voltage regulators and other controllers.
Time Lag
Because a servo compares an input signal with a feedback response, there is always a time lag between the input signal and the actual movement of the load. The weight of the load can also introduce an additional time lag.
The time lag of the servo can be decreased by increasing the gain of the servo amplifier
Feedback
In the context of servomechanisms, feedback refers to the system providing information about its load to the operator or its input. This allows the system to correct differences between the state of the receiver and the transmitter signal.
The lack of feedback in an open-loop system is the fundamental difference between an open-loop and closed-loop
Hunting
Hunting refers to the overshoot and undershoot that occurs as the receiving device tries to match the transmitter signal. Without the damping device, the receiver would go slightly past the desired point, and then return past the desired point in the opposite direction. This would continue, by smaller amounts each time, until the receiver came to rest at the desired position.
Feedback prevents hunting, slowing down the approach to the desired point.
Follow-up
Follow-up is the behaviour of a servomechanism upon detection of an error. If the load position deviates from the demanded position, it immediately returns to it.
Damping occurs when there is a force within an oscillating system that is proportional to, and opposes, its motion. This can be deliberately introduced, for example by placing a mass-spring system in oil, or an undesired consequence that designers try to minimise, such as friction
In servomechanisms, damping is desirable, to some degree, to help reduce hunting, but not so much that it significantly reduces the efficiency of the system.
what does TDX stand for
Torque Differential Transmitter
what does TX stand for
Torque Transmitter
