Motor driver

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== current sense ==
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Hull!
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Dis site gets betta every time I visit it.
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Great work guyz!
  
Often people want to measure the current going through the motor.
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There are 3(?) basic techniques:
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* low-side current shunt
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* high-side current shunt
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* magnetic field sense
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* ... ''(any others I missed?)''
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Low-side is (electrically) the simplest.
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For smaller motors, the current is usually measured by
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running the current through a shunt resistor,
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and measuring the voltage across the resistor.
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In situations where low-side sensing is difficult ( automobile electronics bonded to the "GND" car frame; other systems where it is inconvenient to put a resistor on the "lo" power wire), we turn to high-side sensing.
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[http://www.newark.com/jsp/search/browse.jsp;Ntt=high+side+current+sense Newark: high side current sense]; [http://www.digikey.com/scripts/DkSearch/dksus.dll?KeywordSearch&site=US&keywords=high+side+current+sense Digikey: high side current sense]; [http://www.linear.com/ad/current_sense.jsp Linear: current sense circuit collection] (why doesn't this include the Linear LTC6103 ?); [http://focus.ti.com/analog/docs/gencontent.tsp?familyId=57&genContentId=28020 Texas Instruments: "Current Sensor"].
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For large motors, the current is measured by running the power wires through a magnetic field sensor -- either
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* directly measuring the magnetic field (often with a Hall effect sensor, for example, the Allegro ACS712), which can measure DC and AC current, or
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* indirectly measuring the magnetic field with a "one-loop current transformer" (which can only measure AC current).
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Because magnetic field sensing is inherently non-contact, it works just as well high-side as low-side.
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( [http://focus.ti.com/docs/prod/folders/print/drv401.html "Closed-Loop Magnetic Current Sensor"]. )
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Some motor controller circuits are such that, if the software accidentally sets the "wrong" pins hi or lo, you get a short circuit through the output drivers.  This will generally cause a high current to flow, due to the low on state resistance of the output drivers, which may destroy other electronic components before finally blowing the supply fuse.
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Other motor controller circuits are such that, if the software accidentally sets the "wrong" pins hi or lo, the worst that could happen is the motor spins the wrong way.
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These circuits are designed so that, no matter what the inputs, it is impossible to get a short circuit through the output drivers.
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Between "one branch on" and "the other branch on", there is a minimum "blanking time" which has "both branches off". This guarantees that we never have "both branches on" (short circuit).
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Guess which type of design I prefer?
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A random collection of semi-related links (please prune out the irrelevant ones):
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* [http://www.bobblick.com/techref/projects/hbridge/hbridge.html H-Bridge by Bob Blick]
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* [http://groups.yahoo.com/group/osmc/ the Open Source Motor Controller Project]
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* [http://massmind.org/techref/io/stepper/linistep/ LiniStepper] $30 each; Open Source! Circuit Diagram, PCB (Board) Layout, and PIC Software all available. Nice photos of the LiniStepper at http://www.piclist.com/techref/io/stepper/linistep/lini_bld.htm .
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* [http://roko.ca/articles/hbridge.php H-Bridge Fundamentals] An in-depth article on the design of Mosfet H-Bridges
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* [http://PMinMO.com PMinMO.com] Open Source circuits and information on stepper motor controllers]
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* [http://www.epanorama.net/links/motorcontrol.html ePanorama] ePanorama Motor Control page
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* [http://www.isl.ee.boun.edu.tr/projects/motordriver/ "Electronic Design of DC Motor Drives"] has detailed schematics and PCB layout for a system that has a PC send commands through the serial port to a Microchip PICmicro, which does PWM control of 2 H bridges. Each half-bridge uses a IRF9530N (100V 14A pfet plus flyback diode) and a IRF530 (IRF530NPBF: 100V 17A nfet plus flyback diode), driven by a small transistor inverter based on a BD135 npn, for a total of 12 discrete transistors.
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* [http://openservo.com/ OpenServo wiki] -- developing a digital servo motor that accepts "Go to position X" commands and also more complex curves, and returns actual servo position, speed, voltage and power consumption.
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Robots use motor drivers.
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* [http://glendale.edu/robotics_club/ Glendale robotics club] (Glendale CA)
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* [http://www.robotroom.com/HBridge.html "H bridges" by David Cook at the ROBOT ROOM(TM)]
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[[Category:Projects]]
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Revision as of 03:48, 3 August 2007

There are many kinds of motor drivers:

  • servo motor controller
  • stepper motor controller
  • DC motor controller ("brushed")
  • AC motor controller ("brushless")
  • ... (todo: fill in the other kinds) ...

A DC motor controller that is 'reversable' generally uses an 'H bridge'. This 'H-bridge' uses four output drivers in a configuration that resembles an H where the load is the cross bar in the middle. The lines on either side of the load (the downward strokes in the H) represent a series connection of a pull-up driver and a pull-down driver. This allows each terminal of the load to be connected to either the positive supply rail, or the negative supply rail. This allows a positive, negative or zero voltage difference across the load. This load voltage is then utilised to provide the desired control required of the motor. The various combinations can give a 'forwards' torque on a DC motor, a 'backwards' torque on the same motor, can allow the motor to free-wheel (without any applied torque) or can provide a locking of the motor such that it resists any attempt to rotate it.

A single phase AC motor is generally driven in the same way as a DC motor, however instead of operating the motor drive as a constant DC voltage (in either the 'forward' or 'reverse' direction) the AC motor is driven by an approximation to a sinewave. This approximation is created using the H bridge and driving it with a PWM input such that both the positive and negative voltage periods are the same. This is normally acheived either using a sawtooth waveform compared against a sine wave reference, or is done using a lookup table in a microcontroller.

A similar method is used to drive multiphase (3-phase) AC motors, however instead of just using an H bridge, only a half H bridge is used per phase (3 half-bridges). Each phases half bridge is then driven in the same manner as for the single phase motor, with a phase difference between the phases as appropriate.

Most stepper motor controllers uses 2 independent H bridges (4 half-bridges) for the 2 independent coils of a stepper motor. Each possible state (one bridge driving current one way, the other way, or free-floating) of both bridges gives 4 "full steps", 4 "half-steps" between the full steps. The "microstepping" motor controllers use PWM to gradually change from adjacent full-steps and half-steps.

((fill in more details here...))

Hull! Dis site gets betta every time I visit it. Great work guyz!

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