Instruction manual

16
Get/Set Position. If you enter a Zero in the field (or hit refresh), RoboFocus will respond with the current
position. You can always send a position and command the RoboFocus to accept the new position — in
effect, you are recalibrating the RoboFocus. This is useful if the RoboFocus loses its position (e.g.,
because of equipment removal and replacement), but you know approximately where it is. You can also
use this command to set a convenient reference point (e.g., 5000) and do not care about losing the absolute
calibration that you got from completing the training process. Limit is 64,000 steps.
Get/Set Max Travel. You can send a distance (# steps) that you want for the maximum total travel. If you
enter a Zero in the field, RoboFocus will respond with its current setting. Usually this setting is derived
from the full travel calibration and you would not normally reset this value, but you can if you wish to do
so. Assuming you have the Max Travel from your calibration session, if you are using a R&P focuser you
can divide the measured movement (say 4 inches) by the number of steps (say 2000) to get the distance per
step (0.002 inch). Limit is 64,000 steps.
Duty Cycle. When the stepper is actually moving, the RoboFocus sends full current to it so that full
turning power is available. When not moving, a large “holding current” will fully lock the stepper rotor so
that external forces cannot make the motor turn and lose calibration. However, the motor we supply has a
large gear reduction, so the need for this is minimal. The “worst case” is with a rack and pinion. Our
measurements show that even with over 15 lb exerted on a 3/8in diameter pinion, the stepper will not budge
even at zero holding current (and of course the stepper easily moves the weight when required). In S-C
applications, there is of course, no force on the focuser knob except when moving it.
Why use a lower holding current? A lower current reduces current needs (useful in battery applications),
reduces stepper heating (especially important if the motor is in the optical path as it is in some
applications), and to reduce heating inside the controller that will cause a temperature offset for the internal
sensor. RoboFocus allows you to set the holding current roughly equal to a fraction of the full current by
this menu item from 0% to 100%. The new setting will not take effect until you leave the item. Duty
settings other than 0 or 100% will usually cause the stepper to “sing”—this is normal. Duty cycle set
through the configuration screen will override the settings done by the manual menu. In any case, the
settings are stored in the RoboFocus processor.
On what basis should you set the duty cycle? In general, we recommend using zero (0%) and this is the
factory default. In general, if you do need a higher setting, you will likely find that 10-20% is more than
sufficient. FYI, using 10% (instead of zero) will cause an increase in the internal controller temperature of
about 8F, and much more for higher duty settings. Duty cycle values over 50% will make the motor run
hot, but they are built for this. Values other than 0% or 100% will also make the motor “sing”—this is
normal. Finally, note that the ability to MOVE a given load is not dependent on the selected duty cycle
because the actual move is always done at 100% power.
MicrostepPause. The RoboFocus processor pauses between its microstep moves for a chosen number of
milliseconds. Thus, the longer the microstep pause, the slower the stepper will move. If the pause is set
too small, the pulses may be too fast for the stepper to follow and movement will cease or be erratic. A
stepper period of about 4 is a good choice for most motors. You may want a longer period (slower
stepping) if you want to perform diagnostics on the system.
StepSize (Microsteps/Step). The stepping motor goes through four microsteps before it completes one full
electronic/magnetic cycle. One full cycle is small fraction of rotation of the output shaft (depends on the
gearing used in the particular motor). The RoboFocus controller counts microsteps internally, but you can
set how many microsteps equals one control program count. This configuration item allows you to set the
number of microsteps (1-255) that will be made when you tell the RoboFocus to move one step (count).
By changing the microsteps per count setting, you can change the “fineness” of the focus positioning.
If you are using a Rack and Pinion where the focus changes quickly as the shaft turns, you would might use
1-10 microsteps per count, while a much slower focus movement on a S-C scope would benefit from 16-32
or more microsteps per count. With a count maximum of 64,000, a larger microsteps/count also allows a