Hi Nick,
quite so, the vast majority of steppers we use work that way and quite adequately mostly. Servos do require
a closed loop however and the only practical way to do that is with a servo drive with feedback.
Pablo's question was about a hydraulic motor with an encoder. Mach could quite easily read the encoder and
decide for instance whether the motor is required to rotate CW or CCW or within some deadzone not rotate at
all. The problem is how Mach could communicate to the motor or rather the electrovalves to achieve the rotation.
Mach's native communication is step/direction. Maybe you could use the direction signal to direct the sense of rotation.
That does not however give the motor enuf info as to when to stop so that it matches the commanded move.
Maybe you could write some code which controls the valves so that say one valve remains open until such time
as the encoder ticks equals the step ticks. The hydraulic motor could then achieve position control but not
velocity control. Linearly coordinated moves would not be possible even if two axes eventually ended up at
the right location. Maybe Pablo's application would accept that limitation.
The other way would require some sort of electronic drive which accepts step/direction and compares that internally
to the encoder, ie a feedback servo drive by any other name. I have seen references to hydraulic servos but never
read any of them. Apparently they were popular in the 50's and 60's when CNC was in its infancy. Pressbrakes and
benders must use some hydraulic system.... maybe Pablo can look in that direction for ideas.
Craig