1.1 Dc motor control  (Page 3/3)

We used the average of these readings, i.e., (0.13+0.23)/2 = 0.18  and rounded to 0.2 sec. 

Real-time system design

The system design comprises two parts:

1. Simulation, to determine the parameters of the Speed Control Transfer Function
2. Hardware Design of the interface circuit.

Simulation

The speed control is a PID controller. A DC motor model (Figure 14) was created. The complete control loop model is shown in Figure 15. Figure 16 shows the parameters chosen for the PID controller.

Hardware design

The connection between the eZDSP and the Motor Kit requires a dedicated interface circuit, implementing the Low Pass Filter (used for the Digital to Analog conversion) and adapting the voltage levels at the eZDSP (0 to 3V) to those of the DC Motor Kit (0-15V). The various signal types and their voltage range are shown in .

The interface circuit contains two blocks. The first is LPF with an amplifier (shown in Figure 18), connected between the PWM output and the servo amplifier input. The second block is an attenuator (shown in Figure 19) connected between the tachometer output and the Analog to Digital Input of the eZDSP.

Real time implementation

The control loop was implemented in the eZDSP F2812 as shown in Figure 20. The PID controller obtained in the previous chapter was implemented in the DSP. The environment is shown in Figure 21.

The real-time implementation model will be created from the "DC Motor Speed Control via RTDX" SIMULINK demo. In the original demo model the loop is closed by QEP block, we will use a tachometer connected to the Analog to Digital Converter module for speed measurement.

The Model and its subsystems are shown in Figures Figure 22, Figure 23 and Figure 24.

Procedure:

1. Navigate into the following directory:

..\MATLAB\R2006b\toolbox\rtw\targets\tic2000\tic2000demos

1. Copy the following files into your working directory:
   • c2812speedcontrolDC.mdl
   • runc2812speedcontrolDC.m
   • runc2812speedcontrolDC.m
   • Open the c2812speedcontrolDC.mdl model and save it as "DCMotorControlc2812.mdl"(please refer to Figure 21 The model is shown here after deleting the “Info” box. ).

   

2. Double-click the Speed Correction Block, and you will see:

   

3. Configure the C28x PWM block as follows:

   

4. Configure the "PID Controller" block as follows:

   

5. Double-click the Speed Correction Block, and you will see:

   

6. Delete the selected blocks and replace them by the ADC block from Open the ADC block from the C281x Chip support group from the C2000 Target Preferences, and connect is as follows:

   

7. Now the model is ready for real-time, we need however to update the MATLAB script file. Open the “Model Properties” from “File” menu. Change the PostLoadFcn callback to runc2812speedcontrolIDC, as shown:

   

8. The next step is to change the original PWM range (up to 64000) to the desired range (up to 4000). Open the “speddControlIDCLoop.m” file with the MATLAB editor, an change on line 48 the command: cycle = (double(pid).*100./64000);to cycle = (double(pid).*100./4000); Please refer to the following picture:

   

9. Activate the motor
10. Activate CCS.
11. Click the Build/Reload&Run box, the following window should appear:

    

12. You may change the speed of the motor using the slider in the right hand side and the “Apply” button.

References

1. "eZdspTM F2812 Technical Reference", Spectrum Digital, 2003 (External Link)
2. David M. Alter, " Using PWM Output as a Digital-to-Analog Converter on a TMS320F280x", TI Application Report SPRAA88 , September 2008 (External Link)