Customizing Pepper Open-Source FPGA Hardware as a DC Drive Controller

Pepper, an FPGA-based open-source hardware platform, offers exceptional flexibility and performance for industrial applications. One of the most impactful customizations is converting Pepper into a DC drive controller for managing DC motors in applications like robotics, industrial automation, and electric vehicles. This blog provides a technical deep dive into the customization process, including system architecture, hardware implementation, and a detailed block diagram.

What is a DC Drive Controller?

A DC drive controller enables precise operation of DC motors by regulating speed, direction, and torque. Essential features include:

• PWM-based speed control
• Direction reversal
• Current and voltage monitoring
• Closed-loop feedback using encoders

The FPGA’s ability to handle real-time control, high-speed signal processing, and parallel computations makes it ideal for this purpose.

System Requirements

To convert Pepper into a DC drive controller, the following hardware and functional blocks are required:

Input Modules

• Voltage and current sensors for motor monitoring.
• Encoder interface for position and speed feedback.
• User input through GPIOs or external communication (e.g., UART).

Processing Modules

• Pulse Width Modulation (PWM) generation.
• Closed-loop feedback control (e.g., PID).
• Safety features like overcurrent protection and emergency stop.

Output Modules

• PWM signals for motor control.
• Control signals for an external H-bridge circuit.

External Interfaces

• Ethernet or UART for configuration and monitoring.

Implementation Architecture

Block Diagram

Below is the high-level architecture of the DC drive controller implemented on Pepper FPGA hardware:

FPGA Hardware Division: PL and PS Sides

The FPGA hardware is divided into the Programmable Logic (PL) and Processor Side (PS):

PL Side (Real-Time Processing)

• PWM Generator: Produces high-frequency PWM signals for motor control.
• PID Controller: Implements closed-loop speed and position control.
• Quadrature Decoder: Decodes encoder signals for feedback.
• Safety Logic: Detects overcurrent and stops motor operation.

PS Side (Configuration and Monitoring)

• UART Interface: Allows external devices to configure motor parameters.
• Ethernet Interface: Enables remote monitoring and control.
• Control Application: Provides a user interface for system control.

Detailed Implementation

1. PWM Generation

The PWM module generates pulses with a variable duty cycle to control the motor speed. A 100 kHz PWM frequency is typical for industrial DC motors.

Verilog Implementation:

mmodule pwm_generator (

input wire clk, // System clock

input wire [7:0] duty, // Duty cycle (0-255)

output reg pwm_out // PWM signal

);

reg [7:0] counter;

always @(posedge clk) begin

counter <= counter + 1;

pwm_out <= (counter < duty) ? 1 : 0;

end

endmodule odule pwm_generator (

input wire clk, // System clock

input wire [7:0] duty, // Duty cycle (0-255)

output reg pwm_out // PWM signal

);

reg [7:0] counter;

always @(posedge clk) begin

counter <= counter + 1;

pwm_out <= (counter < duty) ? 1 : 0;

end

endmodule

2. Quadrature Decoder for Encoder Feedback

This module reads encoder signals and calculates speed and direction.

Features:

• Pulse counting for position feedback.
• Direction detection using phase difference of encoder signals.

3. PID Control

The PID controller ensures precise motor control by adjusting the PWM duty cycle based on feedback.

Pseudocode:

ererror = desired_speed - actual_speed;

integral = integral + error;

derivative = error - previous_error;

output = Kp error + Ki integral + Kd * derivative;

previous_error = error; ror = desired_speed - actual_speed;

integral = integral + error;

derivative = error - previous_error;

output = Kp error + Ki integral + Kd * derivative;

previous_error = error;

4. Safety and Protection Logic

Implement safety features such as:

• Overcurrent Detection: Monitors current sensors and triggers a stop signal.
• Emergency Stop: Processes input signals to immediately disable the PWM.

5. Communication Interfaces

• UART: For local control and configuration.
• Ethernet: For remote monitoring and integration with industrial systems.

External Hardware Requirements

H-Bridge Circuit

• MOSFET-based circuit for motor direction and speed control.

Sensors

• Voltage and current sensors for monitoring.
• Quadrature encoders for feedback.

Power Supply

• Ensure adequate current handling for motor operation.

Testing and Validation

Motor Speed Testing

• Validate the PWM generation by measuring duty cycle variations.
• Test motor response to varying speed commands.

Direction Control Testing

• Confirm the correct operation of the H-bridge control signals.

Feedback Loop Testing

• Use an oscilloscope to verify the encoder signals.
• Check PID output stability and accuracy.

Safety Feature Testing

• Simulate overcurrent conditions to validate protection logic.
• Test emergency stop functionality.

Pepper’s FPGA platform is a versatile and powerful choice for building a DC drive controller. Its flexibility in real-time processing and high-speed computation enables precise motor control and integration into industrial systems. With this customization, Pepper can serve as a cost-effective solution for diverse applications in robotics, industrial automation, and more.

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