Engr. Hayat wali
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Abstract— PLC and AC Servo motor drive system is basically a set of some instruments, which have been interfaced with each other to form a system configuration. Generally we see just a drive with Servo motor for its operation, the project completed by us allows user to perform different experiments while communicating with the servo motor through human machine interface (HMI) and Programmable logic controller (PLC).
I. INTRODUCTION
In the modern era, the importance of motors can’t be denied. There use can be seen in simple house hold applications to big industries. Now-a-days working load is being transferred to machine to reduce human efforts. The machines which are used the most are motors. To perform different tasks accurately, some of the parameters of machines are also kept in mind. These parameters are speed control of motor, position control and torque.
The system which we have designed is capable of performing the above tasks very accurately. Our system contains PLC and HMI modules which allows the user to interact with the machines in the way the user wants. The processes going inside the PLC is programmed using any software where ladder programming is done. The Whole process can be seen on a display screen of Human machine interface.
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SYSTEM MODEL
The system which we have designed is a complete package. It consists of four important modules which are HMI, PLC AC Servo drive and AC Servo motor. For a certain use initially PLC program is written in software’s, (Winproladder) which is then downloaded to the PLC and HMI. The HMI is also programmed using software (easy builder) for graphical user interface. Using the graphical display the system is then operated.
This interconnection of blocks shows that how different components of the whole system are interconnected with each other. Input is sent from one block. This input moves through different parts and converted to output.
3 SYSTEM OPERATIONS
There is an input signal that loads the counter of servo drive with information regarding to the position. This information is enough to represents the position of the load that must be moved to. With the acceleration of the motor, the pulses that are being emitted from encoder of the servo motor.
While the motor is running, the pulses coming at a fix rate that are directly related to speed of the servo motor. During this time counter is counting the pulses which are being generated from the encoder. The signals are generated to set the position of motor at a desired position. While setting the position we keep in mind that the motor should not overshoot from a demanded position.
When load is connected at the output of the motor, the motor draws more current to keep the speed of shaft constant , when we goes on increasing the load rpm remain the same but more current produces heat in motor which burns the motor or damages the motor.
4. SERVO SPEED CONTROL
A. Pulse Width Modulation Based Speed Control.
The input from HMI goes to the PLC to execute it’s programming to perform a specific task. If the PLC is programmed to control the speed of a load attached at the output of servo motor, then this speed is maintained through a technique called Pulse Width Modulation. Operation is as follows:
Pulse width modulation is a technique of controlling speed very precisely, during which pulses of variable width are generated. Basically these pulses show a changing in voltage.
When the generated pulses have large width it means that the average voltage level is large and as a result the speed of motor increases. Usually small power dissipation is observed in the transistor because of its two conditions whether it is fully “on” or fully “off”.
The concept of pulse width modulation is shown in diagram below, which shows pulses of variable width that controls the speed of a servo motor.
PULSE WIDTH MODULATION
This pulse diagram shows that initially the pulses less wide means that the transistor is on for a smaller period of time, due to which the current going towards the load is less and the motor is moving with a smaller rotation per minute. But later on the pulses are wide that shows that the transistor is on for larger period of time, due to this the amount of current flowing towards the load is larger. This large current enables the load to move with a higher speed.
B. Semi conductor Based speed control
This method of controlling the speed of Servo motor is based on a semi conductor device. The commonly used semi conductor is silicon controller rectifier (SCR). Here the SCR is connected to an AC line voltage. This method of speed control is applied where a large amount of power is involved. If an increase in speed is desired, the SCR must be turned on to apply large pulses of instant power, and when lower speeds are desired, power is removed and a gradual coasting down in speed occurs.
SCR is used to convert this type of AC line input to a regulated value of output which increases or decreases the speed of motor.
5. APPLICATIONS
The project on which we have set efforts is basically designed for industrial automations. In industries of all types the involvement of motors and other such machines is important. Motors are generally used for precise speed control and torque control. Industrial robots need to have accurate controlling; the servo motor in this project ensures the precise control of speed and torque.
It is an industrial trainer, on which certain practical experiments can be performed. Therefore it is equally important for industries and technical institutions.
AC servo motor is commonly employed in various control applications such as robot actuator, machining centre, computer numerical control, machine and precise industrial robot. Due to the presence of electrical, mechanical properties and a high efficiency, AC servo system is demand to have an accurate response for the position tracking and a rapid recovery for the external disturbances or load variations. Because of the all above applications, servo motor bears a good place in industries. Our project is also an important project for industries, where speed control and load interfacing with PLC are done. The project is consisted of PLC, HMI, AC Servo Drive, AC Servo motor.
