Simulation of a servo system
A servo motor is an electromechanical device in which an electrical input determines the position of the armature of the motor. The position of armature is determined by the duty cycle of a periodic rectangular pulse train.
This system involves a servo amplifier, a motor, a lead screw/nut, and a sliding carriage. The servo mechanism is typical of that used on the translational axes of a robot.
Servo System Attributes
1. Motor: A,B,C,D,E
2. Screw: A,B,C,D,E
3. Pgain: 3,4,5,6
4. Vgain: 1,2,3,4,5
5. Class: 0.13 to 7.10
The servomotor simulation involves an extremely non-linear phenomenon - predicting the rise time of a servomechanism in terms of two (continuous) gain settings and two (discrete) choices of mechanical linkages."
The output value represents the time required for the system to respond to a step change in a position set point.
NEMA motor characteristics
Motors are designed with certain speed-torque characteristics to match speed-torque requirements of various loads. A motor must be able to develop enough torque to start, accelerate, and operate a load at rated speed. The National Electrical Manufacturers Association (NEMA) has established class designations for motors on the basis of motors’ starting-torque and accelerating loads. The four standard NEMA designs are NEMA A, NEMA B, NEMA C, and NEMA D. NEMA A motors usually are used for applications that require extremely high efficiency and extremely high full-load speed. NEMA A-design motors are special and are not used very often. NEMA B-design motors are considered to be normal-torque motors. They are used for low-starting-torque loads, such as with centrifugal pumps and fans. NEMA C and NEMA D motors are used for applications that require high starting torque (e.g., positive-displacement pumps and compressors).
Starting torque (Fig. 1, A) is also known as locked-rotor torque. It is developed when the rotor is held at rest with the rated voltage and frequency applied, a condition that occurs whenever a motor is started. When the rated voltage and frequency are applied to the stator, there is a brief time before the rotor turns. During this time, a NEMA B motor develops approximately 150% of its full-load torque. Accelerating torque and breakdown torque As a motor accelerates, torque decreases slightly (Fig. 1, A to B) before beginning to increase. As speed continues to increase, torque increases until it reaches a maximum at approximately 200% (Fig. 1, B to C). This torque is referred to as accelerating (or pull-up) torque. If this maximum is beyond the motor’s torque capability, the motor will then stall or abruptly slow down. Point C on the graph in Fig. 1 is referred to as the breakdown (or pull-out) torque. Full load torque Full-load torque is the torque that develops when the motor is
operating with the rated voltage, frequency, and load. The speed at which full-load torque is produced is the slip speed or the rated speed of the motor.Starting current and full load current Starting current also is referred to as locked-rotor current and is measured from the supply line at the rated voltage and frequency with the rotor at rest. Full-load current is the current measured from the supply line at the rated voltage, frequency, and load, with the rotor up to speed. Starting current typically is 600 to 650% of full-load current on a NEMA B motor. As the rotor comes up to speed, the starting current decreases to the rated full-load current (Fig. 2). Multispeed motors and motors used in variable-speed applications are special motors that are uniquely designed or selected to fulfill specific load requirements. NEMA design classifications are not applicable to these specialized motors. Mounting NEMA Dimensions NEMA has standardized frame-size motor dimensions, including bolt-hole sizes, mounting-base dimensions, shaft height, shaft diameter, and shaft length. Existing motors can be replaced without reworking the mounting
arrangement. New installations are easier to design because the dimensions are known. Letters are used to indicate where a dimension is taken. For example, the letter "C" indicates the overall length of the motor, and "E" represents the distance from the center of the shaft to the center of the mounting holes in the feet. Motor manufacturers provide tables in the motor-data sheet that reference the letter to find the desired dimension. NEMA categorizes standard frame sizes as either fractional or integral. Fractional frame sizes are designated as 45 and 56, and mainly include horsepower ratings of < 1.0. Integral (or medium) horsepower motors are designated by frame sizes that range from 143T to 445T. A "T" in the motor frame size designation of integral horsepower motors indicates that the motor is built to current NEMA frame standards. Motors built before 1966 have a "U" in the motor frame size designation, indicating that they were built to previous NEMA standards. The frame-size
designation is a code to help identify key dimensions. For example, the first two digits are used to determine the shaft height. The shaft height is the distance from the center of the shaft to the mounting surface, given in inches. To calculate the shaft height, divide the first two digits of the frame size by four. For example, a 143T frame size motor has a shaft height of 3.5 in. (14 ÷ 4). The third digit in the integral "T" frame-size number is the NEMA code for the distance between the center lines of the mounting bolt-holes. The dimension is determined by matching the third digit in the frame number with a table in NEMA MG-1.