# Development of a Conveyor Cart with Magnetic Levitation Mechanism Based on Multi Control Strategies
## 4 Magnetic Levitation Unit Controller Strategies
### Experimental Test of Magnetic Levitation Unit with PID Controller
#### 4.3.2 Performance of Magnetic Levitation Unit with PID Controller  
In order to measure the dynamic characteristics of the experimental device, one magnetic levitation unit experiment was conducted. One by one, the test shows the real dynamic characteristics of the actually fabricated experimental apparatus to obtain an accurate measurement and eliminate the influence from other disturbances. The chosen position is 4.0 mm, which is set as the reference point for the levitation balance position. It is the displacement of the air gap between the rail and the coil. The position of 4 mm represents the displacement 0 mm. Displacement change means that the magnetic levitation unit was displaced in the direction of the narrowing of the air gap. In the single magnetic levitation unit experiment, the other three units are fixed by inserting spacers which remain located in the displacement at 0 mm and without any current input. After the PID controller open, the displacement curve is near 0 mm and the current is on +0.1 A. Figure 8 shows the experimental results of one magnetic levitation unit. The magnetic levitation unit was displaced by approximately 0.3 mm due to the load disturbance, and it returned to its original levitation position in approximately 1 s. The rise time was about 24 ms, the overshoot was about 40%, and the settling time was about 0.64 s. The quick response was fast enough for the application to a magnetic levitation conveyor cart. The displacement curve of the magnetic levitation unit has a consistent change in the target levitation position. Figure 7: Plant controlled by the PID controller; input reference is displacement (mm); PID controller output is the current signal; the plant output is displacement; the feedback of the PID controller is the displacement error \(e(s)\). Then, we measured the response when a load was applied to the magnetic levitation unit. The levitation target value was fixed at 0 mm, a load of 8.0 kg was placed on the upper part of the bracket, and the displacement and control current were measured. The current axis in Figure 9 shows the disturbance load added on the magnetic levitation unit. The support force obtained from the magnetic levitation unit can support the load. The magnetic levitation generates a 0.3 mm displacement response to the disturbance, and it returned to the original levitation position in about 1 s. The control current increases from 0 A to 0.5 A. And, the movement of the displacement is very small. The device is supported by four magnetic levitation units which should support a load of 32 kg under the same conditions as in this experiment. Since the maximum weight of baggage that can be checked in at an airport is generally about 30 kg, this device is considered to have practical support force and dynamic characteristics. Each magnetic levitation unit that works with the PID controller has to meet the design requirements and can keep the magnetic levitation unit stable after the disturbance is input. Because the magnetic levitation conveyor cart is a synchronous system comprising four magnetic levitation units, when the four magnetic levitation units turn on, the magnetic levitation units will influence each other. Figure 10 shows the displacement movement of  
Figure 8: The response to step changes in the lifting position by the PID controller; the blue curve shows the actual displacement change and the black line is the target value of displacement; the red line is the current curve. Figure 9: The response to disturbance in the lifting position by the PID controller: the blue curve shows the actual displacement change and the black line is the target value of displacement; the red line is the current curve. The disturbance is input at 8 s.  
each magnetic unit. In the system working with the same PID controllers, the four magnetic levitation units have a better collaborative performance. Although there are differences in the coils and the different working conditions, the PID controller has an excellently stable performance.