DC Motors, Stepper Motors, H-bridges

Transcription

1 DC Motors, Stepper Motors, H-bridges DC Motors Stepper Motors Motor Control Circuits Relays H-bridges

2 AC versus DC Motors AC motors are not very flexible w/o a transmission They can only turn in one direction The speed is controlled by the design of the motor and the frequency of the AC current source (60Hz) DC Motors can be operated more flexibly They can turn in either direction based on the polarity of the applied voltage The speed is controlled by the magnitude of the voltage

3 DC Motors A DC motor has coils on a rotating electromagnetic armature in a fixed or variable magnetic field A commutator connects the DC power source in sequence to the coils in the armature as it turns Commutator end view: Fixed /Variable Field Magnet Brushes + - Commutator - + Electromagnetic Armature Coil (1 of Several) Fixed/Variable Field Magnet

4 DC Motors A continuous voltage across the brushes will keep the motor turning in either one direction or the other depending on the polarity A higher voltage across the brushes will make the motor turn faster A lower voltage across the brushes will make the motor turn slower The commutator sparks as it turns creating EMI or possible explosion hazard

5 Stepper Motors A stepper motor is a DC motor that has fixed magnets on the armature It does not use a commutator to automatically energize/de-energize different magnetic coils The ends of the coil windings in the field are alternately energized and de-energized by an external control circuit in a desired sequence The order and dwell time of the voltage to each coil controls the direction and the speed

6 Stepper Motors A stepper motor can be held in a fixed position by pausing the sequence and keeping one coil energized for the duration of the hold time Hence, a stepper motor can be used in similar applications as a servomotor (studied in CS341) It can be moved to and held in a desired position It can be rotated continuously at a controlled speed The control is all externally implemented

7 Types of Stepper Motors Many possible geometric arrangements of the: Fixed magnets on the armature Electromagnetic coils in the field around the armature References: Our Mercury Motor is a Bipolar Stepper Motor

8 Stepper Motors - Heat Stepper motors are designed to operate at a high temperature and can get hot (~ 80 deg C) If this causes a problem, try the following: Lower the voltage (But I found the Mercury motor will not step/hold reliably at 9 VDC versus 12 VDC) Turn off the power to the coils when not in use (Expect that the motor will slip out of position) Mount the motor on a heat sink to dissipate heat Use a fan to create air flow over motor / heat sink

9 Motor Control Circuits A motor control circuit: Provides power to the motor coils in either polarity Allows the external logic to control direction/speed There are two basic types of motor controllers Electromechanical relay based controllers Semiconductor based controllers (H-bridges) Each type has its advantages and disadvantages We ll use our elevator motors as an example

10 Relay Motor Control Circuit Two relays can be used to provide on/off and up/down control for our elevator DC motor To Arduino Output Pins & Ground To Motor Power Supply + 9 VDC Ground NO NC Red Black Up Down - + NC NO Yellow + - Black Safety switch: Stops up when the car hits switch

11 Relay Motor Control Circuit Advantages Uses simple, robust electromechanical devices Can handle high voltages and large currents Disadvantages Requires periodic maintenance (cleaning contacts) Relatively slow due to inertia in mechanical parts Contact make/break/bounce sparks creating EMI or possible explosion hazard Example: Used under the hood in cars

12 Relay Motor Control Circuit A DC motor such as the drive motor for our elevator system requires one circuit per motor The direction is determined by one relay or the other being turned on - closing normal open (NO) contact A bipolar stepper motor such as the Mercury Motor requires two circuits per motor Each coil is turned on and off in a selected direction in sequence to change the position continuously One coil can be left on in a selected direction to hold in a desired position

13 Bipolar Motor - Relay Shield Wiring Arduino Uno Don t Connect Positive Sides NO Stepper Motor Coil A NO NO Stepper Motor Coil B Black Brown Orange Yellow NC NC NC NC + 12 VDC NO Connect Grounds Clockwise Counter CW Pin 7 Pin 6 Pin 5 Pin 4 D0 D1 D2 D3 Relay Coil 0 Relay Coil 1 Relay Coil 2 Relay Coil 3

14 H-bridge Motor Control Circuit An electronic version of previous relay circuit Can be implemented with individual components Can use an L298 H-bridge component Two independent H-bridge circuits per component Needs external snubber or flyback diodes

15 H-bridge Motor Control Circuit Half of an L298 H-bridge can be used to provide on/off and up/down control for a DC motor +5 VDC Arduino Uno Vss Input 1 Enable A Input 2 Connect Grounds Pin 9 Pin 5 Pin 6 Pin 7 Pin 8 Vs Pin 4 Pin 2 L298 H-bridge (Side A) Pin 1 Pin 3 Snubber Diodes Yellow Snubber Diodes Safety Switch +9 or 12 VDC Red Black Black Up Down

16 H-bridge Motor Control Circuit Advantages No periodic maintenance is required (no contacts) Fast switching speed available (limited by motor) No EMI or possible explosion hazard due to sparks Disadvantages: Limited voltage and current handling capacity Vulnerable to EMI/EMP or damage due to electrical transients (may need metal shielded container and snubber or flyback diodes)

17 H-bridge Motor Control Circuit A DC motor such as our elevator system drive motor requires one H-bridge per motor The direction is determined by the inputs (1 or 2) The motor can be turned on and off by the Enable A A bipolar stepper motor such as the Mercury Motor requires two H-Bridge circuits per motor The direction is determined by the ordering of the logic signals to the inputs (1, 2, 3, or 4) The speed is controlled by the speed of sequencing

18 H-bridge Motor Control Circuit The Motor Shield provides uses both halves of an L298 component one to control each coil of the stepper motor It is an electronic version of the Relay Shield diagram shown earlier It includes the snubber diodes on the board