- Both optical and magnetic (similar to
magnetic tape) encoders produce a quadrature wave form and and index
pulse that lets the receiving electronics keep track of the position of a
rotating shaft or linear position (linear position most often has an
index pulse every so often and may have a home position output as well.
An improved version uses gray-coded encoding (see below).
A Rotary encoder's quadrature waveform output
A schematic of a Rotary encoder's quadrature encoding disc
A Rotary encoder's quadrature encoding disc
- Gray coded encoders
- Gray codes instead of having a
two bit output as in a quadrature encoder, have instead a binary word
encoded with 3, 8, 16 or even more bits printed on the optical plate.
Instead of using normal binary counting, the code increments with a
system so that only one bit changes at a time when moving over each
position. With normal binary counting several bits can change at once
(i.e. 01111 + 1 = 10000), but in the real world there one bit will
always lag the others creating transient erroneous codes. Gray coded
encoders still repeat (there is often an index bit at the full cycle)
but the possibility of over running a count, as sometimes happens on
quadrature encoders is eliminated. Gray coded encoders are not often
used due to their higher costs.
A gray code encoding disc
8 bit gray encoder
A system that uses a transformer with three windings - A reference
winding that rotates (rotor) and sine and cosine stator windings that
are 90° from each other on the circumference of the housing. A sine-wave
is fed in to the rotating reference winding and the amplitude and
polarity of the outputs reveals the quadrant and position with in the
quadrant. The sub quadrant position is based on the ratio of the sine vs
cosine voltages. SIN θ / COS θ = TAN θ, where θ = shaft angle
- Similar to the resolver (see above) but with three stators 120° apart.
Hi-speed motion for laser cutting and engraving
There are a number of inexpensive Chinese CO2 cutting machines that
have a proprietary interface. I am interested in converting one to EMC
and gathering some information. One source told me that it probably had
to move too fast for engraving etc and that EMC with a PC's latency
would not be able to keep up.
It turns out that there are cards that can output a chain of pulses at a particular frequency - called Universal Stepper Controllers or "rate generators" Which produce constant-width pulses of varying frequency.
There is also somewhat different type of card called a Universal PWM Controller
Instead of generating constant-width pulses of varying frequency, it
makes varying-width pulses of constant frequency, to directly drive a
PWM power output stage. With this system there are no steps anywhere in
the system. EMC2 sends out a
PWM output command, and reads encoder position from the board's
counters. With this, I can drive both DC brush motors and brushless
motors, using the appropriate drive units.
There are also at least three EMC compatible cards that will provide analog velocity mode control - g
- There is no problem as far as pulse train and maximum
velocity is concerned. The motors as I recall have a max velocity of
2500 RPM at 200 full steps per revolution, and set to 32 micro steps per
step, which works out to a maximum step rate of just under 270,000
pulses per second. The 7i43 or 5i20 is clocked at either 33 or 100Mhz,
which is over two orders of magnitude above the max step rate.
- The update rate of the 7i43 parallel port is limited to about
400 microseconds however, which affects the rate at which the position
(velocity actually) can be changed. With the 5i20, 200 microseconds is
easily achievable. I should also add that I have created a special smp
kernel for the Atom 330 processors I am using, which pins the real time
tasks to one core and everything else to the other. With this kernel, I
am able to set the servo period to 200 microseconds or faster, excepting
the limitation on the 7i43 mentioned above.
- A further limitation is in the EMC kinematics which has a
maximum rate at which position data can change. This is not a problem
for me, because with the materials I am using and the power of the
lasers, to get any depth when doing 3D engraving it is not practical to
run faster than 200 fpm or 40 ips, and often less than that.
- In effect the EMC kinematics will slow the head in
complex areas where Z changes very frequently and speeds up to the set
feed rate in areas where it does not, so I have added a component which
adjusts the laser power in proportion to the instantaneous velocity
defined by X and Y.
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