Motor driver

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Revision as of 11:40, 19 March 2010 by DavidCary (talk | contribs) (two more open-source stepper drivers)
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kinds of motor drivers

There are many kinds of motor drivers, each one specialized to drive its own type of motors:

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

In all cases, we have an electric motor that has wires coming out of it. At any one instant, the motor controller connects each wire to either the Hi voltage on the + side of the battery, or to the Lo voltage on the - side of the battery, or neither. When we tell the motor controller to make the motor go "forwards" or "backwards" or "fast" or "slow", the motor controller changes which wire is connected to which end of the battery (or not connected at all). Some motor controllers switch the connections thousands of times per second in some modes.

A DC motor controller that is 'reversible' 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 utilized 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 achieved either using a sawtooth waveform compared against a sine wave reference, or is done using a lookup table in a microcontroller.

Push Pull Transistor Circuit: one half-bridge. (Fixme: show the flyback diodes, and convert to the more common MOSFET drive transistors ... also replace the resistive "load" with a (M) motor symbol.)

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 in a sine-wave-like manner from adjacent full-steps and half-steps.

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

noise control

Many motors make sparks when the brushes make or break contact. This causes causes lots of electrical noise ("brush noise"). Your TV-watching neighbors won't be happy if you allow this noise to leak out.

"Sparks emit RF energy from DC to daylight as I was once told by an EMC expert." -- HydraRaptor: "DC to daylight". More details: HydraRaptor: "GM3 motor suppressor"


current sense

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

See current sense for several different techniques.

tolerance against software bugs

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?


FET driver

What do you put between the CPU output pins and the 4 FETs of the H bridge?

The simplest solution is to use 2 lo-side nFETs and 2 hi-side pFETs, and use a power supply for the motor that has the same voltage as the CPU power supply, and drive the 4 FETs directly using 2 CPU output pins. One of those output pins connects to the gates of the left side and controls whether the left leg of the motor is Hi or Lo. The other output pin connects to the gates of the right side and controls whether the right leg of the motor is Hi or Lo.

But alas, that circuit won't work for any of the following situations:

  • you want to run the motor off a much higher voltage -- say 12 V.
  • you want to use 4 nFETs (because they are slightly cheaper, and it's simpler to stock one kind of FET rather than 2 kinds) rather than 2 nFETs and 2 pFETs. You need a "nFET high side driver". There are several clever circuits for generating a "Vpp" voltage that is higher than your motor power supply voltage; "Vpp" is needed to turn high-side nFETs completely on.
  • you want more isolation between the "noisy" motor power supply and the "quiet" CPU power supply.
  • you are driving a large FET with high gate capacitance, and your CPU output pins can't source or sink enough current to turn the FET on and off fast enough.
  • You want a hardware-enforced blanking time as alluded to earlier.


external links

A random collection of semi-related links in no particular order (please prune out the irrelevant ones):


A3977

Using the A3977 microstepping driver chip from Allegro:

astronomy

astronomy telescopes use motor drivers:

robots

Robots use motor drivers.

self-balancing personal transportation systems

Main Article: vehicle

Self-balancing personal transportation systems use motor drivers:


generator

(This doesn't have much to do with motor drivers -- is there a better page for electric power generation tips?)

further reading


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