MC1151A PMD, MC1151A Datasheet - Page 24

• Manufacturer Part Number: MC1151A
• Description: (MC1x51A) Advanced Step Motor Control Chipset
• Manufacturer: PMD

STOP
This sequence will start a homing move which will stop as soon as the
axis encounters the home switch.
As is the case for all of the breakpoint modes, the external breakpoint
can not only be use to stop an ongoing move, but to start or otherwise
modify a move as well. This flexibility makes it well suited for
applications such as cut-on-the-fly where externally-initiated motions
are required.
Disabling Automatic Profile Update
Normally, when a breakpoint condition has been satisfied, it causes the
profile and other double-buffered parameters to be automatically
updated. For certain types of profiles however, it may be desirable to
still use the breakpoint mechanism (to allow it to generate a host
interrupt for example), but not to have the profile update.
Whether the profiles are automatically updated or not for a given axis is
controlled by the commands SET_AUTO_UPDATE_ON and
SET_AUTO_UPDATE_OFF. When auto update is set to on, the
breakpoint/profile mechanism behaves as described above. When set
to off, upon a breakpoint condition, no profile update will occur. When in
this mode the only way to update the profile is to use the UPDATE
command or the MULTI_UPDATE command.
Travel Limit Switches
The MC1451-series chipsets support motion travel limit switches that
can be used to automatically recognize an "end of travel" condition.
The following figure shows a schematic representation of an axis with
travel-limit switches installed, indicating the "legal" motion area and the
over-travel regions.
There are two primary services that the MC1451A provides in
connection with the over-travel limit switch inputs:
1)
over-travel
negative
region
The host can be automatically notified that an axis has entered
an over-travel condition, allowing the host to take appropriate
special action to manage the over-travel condition.
negative limit
switch
Legal travel region
; load (but do not update) a stop command
positive limit
switch
over-travel
positive
region
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To recover from an over-travel condition the corresponding status bits
in the status word should be reset (see the section of this manual on
axis status for details on resetting status word bits). Once this has been
performed the host can command a trajectory move to bring the axis
out of the over-travel region.
The over-travel detector is 're-armed' when the axis exits the over travel
condition.
Only one over-travel signal can be processed at a time. For
example if the negative over travel switch becomes active, the
corresponding status bits must be cleared, and the axis moved
into the legal travel range before a positive over travel switch will
be recognized.
Axis Timing
Each axis of the MC1451-series chipsets receives a "time slice" of the
available computation power of the CP chip. The amount of time
required for the chipset to perform one complete pass of calculations for
all of the axes is known as the chipset cycle time. This chipset cycle
time is important to the host processor because it determines the rate at
which profile trajectories are updated.
The cycle time is the same for all MC1451-series chipsets. The cycle
time value is 330 uSec*. All velocities, accelerations, and jerk values
are related to this cycle time via the various trajectory generator modes
that generate axis motion.
For example, to determine the velocity of a given axis in units of
steps/second, we use the conversion ration 1 sec = 3,030 cycles (3,030
cycles/sec = 1 cycle every 330 uSec). Therefore if the desired
maximum velocity to be provided to the chipset is (for example) 12,345
steps/sec we convert to units of steps/cycle by dividing by 3,030, giving
a value of 4.07425. The value we send to the chipset using the
SET_VEL command (see host command section for details) would be
65,536 times this amount since the velocity parameter uses 1/2 16
scaling. Therefore we would send a value of 267,010 to the chipset.
As an additional example, to determine the acceleration of a given axis
in units of steps/second 2 , we again use the conversion ratio 1 sec =
3,030 cycles, however we must take into account the conversion to
cycles 2 (not cycles). Therefore if the desired acceleration to be
provided to the chipset is (for example) 67,890 steps/sec 2 we convert
to units of steps/cycle 2 by dividing by 3,030 2 (or 9,180,900), giving a
value of .00739469. The value we send to the chipset using the
SET_ACC command (or SET_MAX_ACC command if we are in S-
curve mode) would be 65,536 times this amount since this parameter
2)
* exact cycle time is 327.68 uSec, 330 is an approximation.
Upon entering an over-travel condition, the trajectory generator
will automatically be halted, so that the motor does not travel
further into the over travel region.