Dc Motors


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Dc Motors

  1. 1. Lecture Outline  DC motors  inefficiencies, operating voltage and current, stall voltage and current and torque  current and work of a motor
  2. 2. Definition of Actuator  An actuator is the actual mechanism that enables the effector to execute an action.  E.g, electric motors, hydraulic or pneumatic cylinders, pumps…  Actuators and effectors are not the same thing.  Incorrectly thought of the same; “whatever makes the robot act”
  3. 3. DC Motors  The most common actuator in mobile robotics is the direct current (DC) motor  Advantages: simple, cheap, various sizes and packages.  DC motors convert electrical into mechanical energy  How?
  4. 4. How DC Motors Work  DC motors consist of permanent magnets with loops of wire inside  When current is applied, the wire loops generate a magnetic field, which reacts against the outside field of the static magnets  The interaction of the fields produces the movement of the shaft/armature  => Electromagnetic energy becomes motion
  5. 5. Motor Inefficiency  As any physical system, DC motors are not perfectly efficient.  The energy is not converted perfectly. Some is wasted as heat generated by friction of mechanical parts.  Inefficiencies are minimized in well- designed (more expensive) motors, and their efficiency can be high.  How high?
  6. 6. Level of Efficiency  Good DC motors can be made to be efficient in the 90th percentile.  Cheap DC motors can be as low as 50%.  Other types of effectors, such as miniature electrostatic motors, may have much lower efficiencies still.
  7. 7. Operating Voltage  A motor requires a power source within its operating voltage, i.e., the recommended voltage range for best efficiency of the motor.  Lower voltages will (usually) turn the motor, but will provide less power.  Higher voltages are more tricky; they increase power output at the expense of the operating life of the motor ( the more you rev your car engine, the sooner it will die)
  8. 8. Current and Work  When constant voltage is applied, a DC motor draws current in the amount proportional to the work it is doing.  E.g., if a robot is pushing against a wall, it is drawing more current (and draining more of its batteries) than when it is moving freely in open space.  The reason is the resistance to the motor motion introduced by the wall.
  9. 9. Stall Current  If the resistance is very high (i.e., the wall won't move no matter how hard the robot pushes against it), the motor draws a maximum amount of power, and stalls.  The stall current of the motor is the most current it can draw at its specified voltage.
  10. 10. Torque at the Motor Shaft  Within a motor's operating current range, the more current is used, the more torque or rotational force is produced at the shaft.  The strengths of the magnetic field generated in the wire loops is directly proportional to the applied current and thus the produced torque at the shaft.
  11. 11. Stall Torque  Besides stall current, a motor also has its stall torque.  Stall torque is the amount of rotational force produced when the motor is stalled at its operating voltage.
  12. 12. Power of a Motor  The amount of power a motor generates is the product of the shaft's rotational velocity and its torque.  If there is no load on the shaft, i.e., the motor is spinning freely, then the rotational velocity is the highest  but the torque is 0, since nothing is being driven by the motor.  The output power, then, is also 0.
  13. 13. Free Spinning and Stalling  In contrast, when the motor is stalled, it is producing maximum torque, but the rotational velocity is 0, so the output power is 0 again. Between free spinning and stalling, the motor does useful work, and the produced power has a characteristic parabolic relationship  A motor produces the most power in the middle of its performance range.
  14. 14. Speed and Torque  Most DC motors have unloaded speeds in the range of 3,000 to 9,000 RPM (revolutions per minute), or 50 to 150 RPS (revolutions per second).  This puts DC motors in the high-speed but low-torque category (compared to some other actuators).  How often do you need to drive something very light that rotates very fast (besides a fan)?
  15. 15. Motors and Robots  DC motors are best at high speed and low torque.  In contrast, robots need to pull loads (i.e., move their bodies and manipulators, all of which have significant mass), thus requiring more torque and less speed.  As a result, the performance of a DC motor typically needs to be adjusted.  How?
  16. 16. Control of Motors  Motors require more battery power (i.e., more current) than electronics  E.g., 5 milliamps for the 68HC11 processor v. 100 milliamps - 1 amp for a small DC motor).  Typically, specialized circuitry is required  H-bridges and pulse-width modulation are used
  17. 17. Pulse-Width Modulation  The exact width/length of the pulse is critical, and cannot be sloppy.  Otherwise the motor can jitter or go beyond its mechanical limit and break.  In contrast, the duration between the pulses is not critical at all.  It should be consistent, but there can be noise on the order of milliseconds without any problems for the motor.  Why?
  18. 18. Noise in Modulation  When no pulse arrives, the motor does not move, it simply stops.  As long as the pulse gives the motor sufficient time to turn to the proper position, additional time does not hurt it.  On the other hand, if the duration of the pulse is incorrect, the motor turns by an incorrect amount, so it reaches the wrong position.