Re: servo driver on a chip ?

I was waiting for someone else to answer this, but nobody stepped up.

How homebrew does he want to get?

Generally, Step/Direction is a method to command steppers controllers.
The stepper controller then decides how to energize the windings,
depending on whatever mode it's in -- single step, half step, quarter
step, etc.

It's been expanded, probably out of convenience, as a general command
signal to all kinds of controllers, including DC and AC servos.

When connected to a servo, a controller/driver will drive appropriate
current into the windings to move the shaft to the appropriate
position that the controller reads back with the encoder. Basically,
the controller/driver is faking being an open-loop stepper to the
positioning software.

If he wants to go homebrew, have him buy a Robostix:
http://www.gumstix.com/store/catalog/product_info.php?products_id=139

The Robostix ($50) can function as a motor controller. It has an Atmel
ATMega128 (AVR). It has several peripherals (check the Atmel data
sheet) including an 8 channel A/D converter (useful for monitoring
current), 6 PWM outputs, and many I/O pins.

He should connect power amplifiers to the PWM channels to drive his
motors. Two channels for a DC servo, three channels for an AC servo,
and four I/O channels for a stepper. If he wants to do microstepping,
he'll need four pwm channels for a bipolar and two PWM channels for a
unipolar motor.

Generally speaking, he'll be only able to put one PID loop in each
AVR, so if he's running position feedback, he'll have one micro for
each motor.

Output drivers are cheap to build. Look at International Rectifier
first for MOSFETS and gate drivers. They have a wide variety. Choose
gate drivers with at LEAST 1A output drive to minimize MOSFET
switching time. I'm partial to the IR20124S because it has on-chip
current limiting and dead-time generation, but the IR2110 has 2A
output meaning the MOSFETS will switch faster and dissipate less
power.

For your MOSFET, pick one that matches your power supply and current
needs. Among those that fit, pick the one with the lowest gate
capacitance.

Note on the gate driver output: The higher the current in and out of
the gate while switching, the faster the MOSFET switches. These gate
drivers are intended for DC/DC converter or audio power amps, so they
switch very fast. A Mosfet with a 4V switching threshold, 1300pF gate,
and a driver capable of 1A switches in around 20ns (depending on wire
resistance).

Note that if you have 50V switching in 20ns, you create all kinds of
RF even if your PWM frequency is low so you should use an inductor on
the output (use the ones designed for DC/DC converters).

If he buys a Gumstix as well, he can use it to program the Robostix or
he can use the JTAG port for programming and debugging.

Remember a PID loop chews up an entire AVR, but that one AVR can drive
a dozen steppers, probably more.

On 8/2/06, turbulatordude <dave_mucha@...> wrote:
> Hi all,
>
> A friend is just getting into CNC and wanted me to look for a
> home-brew way to use servo motors.
>
> There are servo drivers on a chip, but the one found is for postioning
> servo's not step and direction use.
>
> Is there a name for the two different technologies ?
>
> both being servos is confusing.
>
> Also, is there a chip, or a circuit or a PIC or AVR method of reading
> the encoders and controlling the motors ?
>
> Dave