IMPLEMENTATION OF A PID CONTROLLER EMBEDDED IN A FPGA FOR POSITIONING A DC MOTOR

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Author:  Edgar Rodrigo Mancipe Toloza

 

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ABSTRACT: The main objective of this paper is to present the steps of how to program a PID controller in a FPGA, and in that way to control a DC motor using Pulse Width Modulation. Also we want to give some ideas to people interested in program embedded controllers in hardware.

During the project we will use some tools to help us to visualize the behavior of the controller on the computer screen through rs232 communication and LabView chart to plot, and LCD display for visualize the Set Point, gains and the current value.

 

KEYWORDS: PID, Control, FPGA, DE0-Nano, Embedded.

 

INTRODUCTION

 

Nowadays FPGA's have increased their popularity due the many ways to acquire it, reliability, and low costs. As a consequence of that, we can see more FPGA's applications in control systems.

For many years, microcontrollers used to be on the top of this field due their low-cost, but through years FPGA's have turned into a striking option, thanks to its main property of controlling many systems in parallel in the same embedded system.[1]. 

In this project we want to show in a quick way, how to program a PID control in FPGA using Verilog language, also we will change the values of Gain variables to see the behavior of the controlled value.

This article is not intended to give a definitive PID algorithm, the intention is to introduce and give some ideas for the readers of how to create and implement their own PID equation embedded in FPGA.

 

 MATERIALS 

This section will show the name of the main objects used in the analog and digital interface of the project for signal conditioning and power circuit.

  • DE0-NANO BOARD: The development board selected for this Project is the DE0-Nano, which contains the following characteristics: [2]
    • Altera FPGA Cyclone IV EP4CE22F17C6
    • Analog-to-digital converter ADC128S022
    • Configuration device EPCS16 for non-volatile data storage
    • 4 dip-switch
    • 2 push buttons
    • 8 LED's

      Figure 1. DE0-Nano Board.

  • GEARMOTOR: The gearmotor selected have the next characteristics:

    • Torque 18Kg*cm
    • 80RPM
    • Power supply 12Vdc
      • Quadrature encoder of 8384 pulses per round 3v.




        Figure 2. Gearmotor. [3]



  • DISPLAY HD44780 2x16:  Monochromatic display 2X16 that will be use to visualize de set-point, current variable, proportional, integral and derivative gain

    Figure 3. LCD 2x16. [4]

  • H BRIDGE: the control circuit and power circuit, a L293D H Bridge is the selected option, so in that way the control circuit send digital signals to the chip for select the rotation of the motor. [5]

    Figure 4. L293D Block Diagram.

  • SERIAL-TO-USB CONVERTER:  For visualize the behavior of the variables in the computer screen, we decided to use a serial-to-usb FTDI FT232RL converter. So through a serial RS232 algorithm coming from the embedded we can send the information of the data's. [6]

    Figure 5. Serial-to-USB converter FT232RL.

 

BLOCK  DIAGRAM OF THE CONTROLLER

Before start to program the FPGA, is necessary defining a strategy through a block diagram. In the Figure 6, we can see that the Set Point is selected using a Dip- switch and the feedback signal is coming from the quadrature encoder coupled to the DC motor. The error is the difference between Set Point and feedback (Encoder), and the error sign defines the rotation of the motor through the H Bridge. A Pulse Width Modulation (PWM) is the output signal of the PID algorithm that will change its Duty Cycle depending of the controller result.

 

Figure 6 Block Diagram of the controller.

 

PROGRAMMING

Once defined the strategy for the controller, we proceed to program the algorithm in HDL Verilog, using Quartus II. In this case we divided the entire Project in different sub-modules that will allow us to interpret and design in modular way, so that is going to be an advantage at the moment of write and understand the program lines

 

  • SET POINT:
    For the Set Point there are 5 predefined positions that will be selected through a dip-switch. Due that the encoder coupled to the gearmotor generates 8384 pulses per round is necessary to calculate the value for each position