Motor driver

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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...))

current sense

Often people want to measure the current going through the motor.

There are 3(?) basic techniques:

  • low-side current shunt
  • high-side current shunt
  • magnetic field sense
  • ... (any others I missed?)

Low-side is (electrically) the simplest.

For smaller motors, the current is usually measured by running the current through a shunt resistor, and measuring the voltage across the resistor.

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.

Newark: high side current sense; Digikey: high side current sense; Linear: current sense circuit collection (why doesn't this include the Linear LTC6103 ?); Texas Instruments: "Current Sensor".

For large motors, the current is measured by running the power wires through a magnetic field sensor -- either

  • directly measuring the magnetic field (often with a Hall effect sensor, for example, the Allegro ACS712), which can measure DC and AC current, or
  • indirectly measuring the magnetic field with a "one-loop current transformer" (which can only measure AC current).

Because magnetic field sensing is inherently non-contact, it works just as well high-side as low-side. ( "Closed-Loop Magnetic Current Sensor". )

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.

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. These circuits are designed so that, no matter what the inputs, it is impossible to get a short circuit through the output drivers. 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).

Guess which type of design I prefer?

A random collection of semi-related links (please prune out the irrelevant ones):

Robots use motor drivers.

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