Abstract
In the earlier chapters, an attempt has been made for stable suspension of a cylindrical rod under single electromagnet controlling single-axis movement. During experimentation it has been observed that the rod gets tilted to one side that exerts more levitating force due to non-uniformity of the distributed field flux. Moreover, for some specific industrial applications (like induction heating, manufacturing industry, active magnetic bearing, precision instrumentation, mechatronics, etc.), it is required to levitate such cylindrical rods with better pitching control where both ends may be controlled independently. Obviously, for controlling the other degrees of freedom movement of the cylindrical rod, at least two electromagnets are necessary.
TopDescription Of Experimental Set-Up
The block diagram of individual unit for the proposed DCALS is shown in Figure 1. In each case the current of the electromagnet is controlled through the DC to DC switch mode chopper circuit utilizing an outer position control loop and an inner current feedback control loop. The parameters of the maglev systems are given in Table 7.2. The photograph of the experimental setup is shown in Figure 2.
When the two electromagnets are simultaneously excited, a net attractive force is generated between the magnet pole-faces and the ferromagnetic rod, as a result of which the magnets try to pull up the complete ferromagnetic rod. The dedicated independent controller used for each magnet tries to control the air-gap between that magnet pole-face and the cylindrical rod by maintaining the required current in the corresponding magnet-coil. With each magnet cum controller unit working satisfactorily, each side of the cylindrical rod gets the desired vertical lift and in the process the whole cylindrical rod is levitated.
The magnet current is controlled by a single switch based DC to DC chopper circuit (Figure 24 in Chapter 8). Since the electromagnetic levitation system is inherently unstable, the selection and design of the controller is important so that the overall closed loop system becomes stable and gives satisfactory performance. A cascade compensator is used with the position control loop for maintaining overall closed loop stability.
Two linear inductive type position sensors have used to measure two air-gaps (gap between the magnet pole-face and cylindrical rod). Output of the position loop (current reference signal) is compared with the actual coil-current signal sensed by an LEM make (LA-55P) Hall-effect current sensor. For better dynamic response and steady state accuracy the current error is processed through a PI controller and its output is used to control the chopper output voltage through PWM control logic. The duty ratio of the MOSFET switches varies as the object moves up and down within the electromagnetic field. When the rod moves upwards (beyond reference gap) the duty ratio of the MOSFET gate pulse is decreased, consequently the magnet current decreases and causes it to go down and vice-versa.
The stable suspension of the prototype is achieved through the control of air-gap between the magnet and the cylindrical rod by controlling the current flowing through the magnet-coil. The two magnets are controlled independently through two identical controllers and stable levitation of the rod is achieved through SISO control of each air-gap. The steady-state position of each end of the rod may be controlled through each reference gap command and has been demonstrated experimentally.
Figure 1. Schematic diagram of individual unit for the proposed DCALS