DC Motor Controller for Robotics

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DC Motor Controller for Robotics

• Alexander Gray
• Shaurya Mehta
• ECE 445 Senior Design
• Group 6
• 30th November, 2005

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Introduction

• Robotic manipulators and similar systems require
  precise and often relatively powerful drive
  motors and control systems
• We aimed to create an easy-to-use drive module
  integrating the motor, power amplifiers, gearing,
  sensors and basic control functions, which can be
  interfaced to a PC

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Goals Objective

• Fully integrated drive system including Motor,
  Drive Train, Power Electronics, and Control Logic
• User can control Velocity, Position or Torque
• Complete PC interface using HyperTerminal
• Input interface using the PC keyboard
• Sensor data output via HyperTerminal

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Performance specifications

• Approximately 1.5 kW electrical output and 1
  kW peak mechanical output
• Protection circuitry against over-current,
  voltage spikes and noise from the motor

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Performance specifications
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Motor, Shaft Encoder
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PCB
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System Block Diagram
24V Power Supply
Control Logic PC Interface Unit
DC Motor Sensors
Power Electronics Unit
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System Hardware

• MOSFET Driver HIP4081A
• Power MOSFETs STP140NF55 (2 per leg)
• Current Sensor Allegro ACS755-100
• DC Motor DeWalt 18V Drill Motor
• Shaft Encoder US Digital E4P
• Voltage Regulators Micrel LM2575 buck converters

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Power Supply Circuit protection

• Regulated 5 volt power for the logic circuits and
  12 volts for the MOSFET gate driver IC is
  provided by Micrel 2575 buck converter ICs with
  external inductors, diodes and capacitors. Each
  can supply 1A output, staying cool and efficient.
• The power input is filtered by a 3300 uF
  capacitor and a 60 V,1.5 kW transient voltage
  suppressor (TVS) diode.
• Interference and voltage spikes from the motor
  are blocked by two RC snubber and TVS diode sets.

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DC Motor Shaft Encoder

• The DeWalt drill motor and gearbox are rated for
  18V, 450 RPM, and 450 oz-in. However, it is run
  at 24V, giving a max speed of 600 RPM, and
  momentary torque is achievable far over the
  rating without damage.
• The shaft encoder is a compact quadrature type,
  approximately 1 dia x 0.5 high with 256 pulses
  per revolution. It is operated at TTL logic
  levels, and was very easy to integrate.

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Power Amplifier Unit

• The motor is driven by an H-bridge circuit, with
  two MOSFETs (55V, 80A, 8 mohm) per leg and large
  heatsinks (approx. 6ºC/W total) for a total
  current handling ability of at least 70A.
• The logic level PWM signals are converted to the
  necessary gate voltages by a HIP4081 full bridge
  driver IC. The gates are driven through 150 ohm
  resistors and reverse-bypass diodes for insurance
  against shoot-through, and Zener diodes protect
  the gates from voltage spikes.

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System Software - PIC 18F452

• The Brain of the DC Motor Controller
• Communicates with the PC via RS-232 interface
• Outputs 2 PWM signals to the MOSFET driver
• Completes the control loop by receiving signals
  from the
• 1. Current Sensor Torque Control
• 2. Shaft Encoder Position Velocity Control
• Uses PID controller algorithm to drive the motor

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Controller Algorithm

• PID (Proportional, Integral, Derivative)
  Controller

Target Value
Computed Signal u
PID Controller
Motor Sensors
Error
Actual Value
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PID Controller
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PC Interface

• We use the UART module of the PIC to implement
  RS232 serial communications with the PC at 9600
  baud.
• To match the voltage levels we use a MAX-232 chip
  to connect to the PC serial port.

RS232 Serial Communication
MAX 232
PIC 18F452
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Encoder Logic

• The shaft encoder produces two out-of-phase pulse
  trains which are fed to an 8 bit up/down counter.
  The direction of rotation is determined by using
  a D flip-flop to detect the phase difference.
• The 8 bit counter output indicates the Motor
  position, with a resolution of 256
  counts/revolution (1.4 degrees)
• Speed is measured by counting pulses for 100 mS,
  so that a count of 255 corresponds to the motors
  maximum speed of 600 RPM.

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Software Evolution

• We started of with a 8 bit PIC using a very basic
  control algorithm to drive the motor
• Initially the PC interface did not account for
  the conversion of numerical values to ASCII
• Once the basic functions were working, we
  improved the control algorithm and also added the
  ASCII conversion in the code.
• Ultimately we switched to a 16 bit PIC to
  incorporate a fully functional PID controller

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Hardware Evolution

• The original motor required bulky and expensive
  gearing, so it was replaced with a drill motor
  and matched off-the-shelf gearbox.
• The original encoder was replaced with a much
  more compact and higher resolution model, and the
  associated logic went through several designs.
• Two MOSFETs were placed on each heatsink instead
  of one, since only one MOSFET in each pair can be
  on at a given time.

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Challenges

• Due to high current and voltages it was
  impossible to test the circuit on a breadboard
• Blew up a few MOSFETs in the process
• Had to wait for PCB to be ready to do any kind of
  testing, causing considerable down time
• In Torque control mode, we encountered a lot of
  noise from the current sensor
• Initially, the update frequency under torque
  control was too high for the current to stabilize.

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Position Control - 23º
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Position Control - 293º
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Velocity Control 38 RPM
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Velocity Control 413 RPM
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Torque Control 4A
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Torque Control 38A
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Performance Results
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Possible Improvements

• Further tweaking of the PID controller could
  definitely improve the overall performance
• Find ways to eliminate noise in the circuit, such
  as better grounding practices
• Could use a DSP or analog system and a higher
  resolution encoder to allow a higher update rate
  and better accuracy

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Conclusion

• During the Demo we encountered difficulties
  operating the torque mode, but we fixed the
  problem the next day
• In the end we met many of our proposed
  performance levels even though some of the goals
  were chosen near theoretical limits
• The complete circuit on a single PCB makes it a
  convenient, ready to use product

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Questions?
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Thank you

• Thank you all for your time today