2-phase ac servomotors have been traditionally used for position/speed control applications especially in light weight, precision instrumentation area in airborne systems. These motors, though more expensive than industry standard split-phase induction motors and ac driven stepper motors, have a much better torque speed characteristics
The present unit is designed around a 12 V ac servomotor and exposes the basic characteristics and dynamics of a position control system. A block diagram of the system is shown in. It may be observed that the error detector consists of two servo potentiometers rather than a synchro pair. This is done primarily to limit the cost of the unit to make it affordable to the target market-educational institutions in the country. This variation does not in any way compromise the performance of the unit or the knowledge gained by the students.
Besides introducing the basic features like balanced modulation of the error signal, phase reversal around the set point and phase difference between the reference and control phases of the motor, the experiment involves study of the step response of the closed loop system. Being a mechanical system the response is too slow for a comfortable viewing on a CRO, except on an expensive storage oscilloscope. A microprocessor based waveform capture/ display card in the unit stores the step response in real time and displays the same once steady state is reached.
The AC servomotor is basically a two phase induction motor with some special design features. The stator consists of two pole-pairs (A-B and C-D) mounted on the inner periphery of the stator, such that their axes are at an angle of 90° in space. Each pole-pair carries a winding. One winding is called reference winding and the other is called a control winding. The exciting current in the winding should have a phase displacement of 90°. The supply used to drive the motor is single phase and so a phase advancing capacitor is connected to one of the phase to produce a phase difference of 90°. The stator constructional features of AC servo motor are shown in figure.
The rotor construction is usually squirrel cage or drag-cup type. Rotor Construction of AC Servo motor is shown in figure - 8. The squirrel cage rotor is made of laminations. The rotor bars are placed on the slots and short circuited at both ends by end rings. The diameter of the rotor is kept small in order to reduce inertia and to obtain good accelerating characteristics.
Figure-3 rotor construction of ac servomotor
The Drag-cup construction is employed for very low inertia applications. In this type of construction the rotor will be in the form of hollow cylinder made of aluminum. The aluminum cylinder itself acts as short circuited rotor conductors. (Electrically both the types of rotor are identical).
The stator winding are excited by voltages of equal rms magnitude and 90° phase difference. These results in exciting currents i1 and i2 that are phase displayed by 90° and have equal rms values. This current give rise to a rotating magnetic field of constant magnitude. The direction of rotation depends on the phase relationship of the two currents (or voltages). The exciting current shown in figure-9 produces a clockwise rotating magnetic field and phase shift of 180° in i1 will produce an anticlockwise rotating magnetic field.
The rotating magnetic field sweeps over the rotor conductors. The rotor conductors experience a change in flux and so voltages are induced in rotor conductors. This voltage circulates currents in the short circuited rotor conductors and the currents create rotor flux.
Figure-4 wave form of stator and rotor excitation current
Due to the interaction of stator and rotor flux, a mechanical force (or torque) is developed on the rotor and so the rotor starts moving in the same direction as that of rotating magnetic field.
Electrical generally produce their maximum power at high speed. In other words generally electrical motors exert rather small torques while rotating at high speeds. In consequence appropriate gear is necessary for the electric motor in order for that system to drive large loads at low speeds. For example, robot arms are usually moved at low speeds less than 1 revolution per second, while require maximum torque range in a few Newton-m to several hundred Newton. A large gear reduction is typically required for standard servomotor.
The gear with N1 teeth is called primary gear and the gear with N2 teeth is called secondary gear. Angular displacement of shaft 1 and 2 are denoted by 21 and 22 respectively with the positive direction as indicated in figure. The moment of inertia and viscous friction of motor and gear 1 are denoted by J1 and B1 respectively, and those of gear are denoted by J2 and B2 respectively. TM is the torque developed by the motor. Elasticity of the gear teeth and shaft is assumed negligible.
The controlled variable is the quantity or condition that is measured and controlled. The manipulated variable is the quantity or condition that is varied by the controller so as to affect the values of controlled variable. Normally the control variable is the output of the system. Control means measuring the values of controlled variable of the system and applying the manipulated variable to the system to correct or limit deviation of the measured value from a desired value.
In study control engineering, we need to define additional terms that are necessary to describe control systems. The Block diagram of open loop control system is shown in figure -1 for an open loop control system would not have feedback path i.e feedback path is open.
Figure-05 open loop control system
A plan may be a piece of equipment, perhaps that a setup machine parts functioning together, the purpose of which is to perform a particular operation. In this manual we shall call any physical object to be controlled a plant.
The Merrian-Webster Dictionary defines a process to be a natural progressively continuing operation or development marked by a series of gradual changes that succeed one another in a relatively fixed way a lead towards a particular results or end. Or an artificial or progressively continuing operation that consists of a series of controlled option or movements systematically directed toward a particular result or end. Examples are chemical, economic and biological processes.
A system is a combination of components that act together and perform a certain objective. A system is not limited to physical ones. The concept of the system can be applied to abstract, dynamic phenomena such as those encountered in economics. The word system should interrupt to imply physical, biological, economic and the like systems.
A Disturbance is a signal that tends to adversely affect the value of the output of a system, It is disturbance is generated within the system, it is called internal, while an external disturbance is generated outside the system and is an input.
Feedback control refers to an operation that, in the presence of disturbances, tends to reduce the difference between the output of a system and some reference input and that does .so on the basis of this difference. The block diagram of closed loop control system is shown in figure.
Figure-06 closed loop control system
Control systems Control a certain physical quantity so that it changes in prescribed ways according to arbitrary input changes.
The basic components of an automatic control system are error detector, amplifier and controller, power actuator, system and sensor (or) feedback system. The block diagram of an automatic control system is shown in figure.
Which compares the reference signal Vr with feed back signal? The output is a voltage proportional to the difference between the two signals.
The controller process the error signal and gives an output voltage signal Vc known as the control voltage. This suggests the necessary corrective measures required in the actuating signal Va to be applied to the system.
Which takes the input as the control voltage Vc from the controller and produces the necessary actuating input signal to be applied to the system to achieve the desired output.
The constitutes the output sensor and associated amplifier. The feedback signal Vf is the voltage proportional to the output variable of the system.
For the study of automatic control system, AC motor is used as a system to be controlled. Here the controlled variables are the position of the motor.
The main advantage of the AC servomotor used as system is
In Position Control Systems the reference input will be an input signal proportional to desired output. The feedback signal is a signal proportional to current output of the system. The error detector compares the reference input and feedback signal and if there is a difference it produces an error signal. The error signal is used to correct the output if there is a deviation from the desired value.
A controller is a device introduced in the system to modify the error signal and to produce a control signal. The manner in which the controller produces the control signal is called control action. The controller modifies the transient response of the system. The controller may be electrical, electronics, pneumatics and hydraulics depending upon the system. We are discussed only electronic controller.
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