MC1151A PMD, MC1151A Datasheet - Page 30

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

Each quadrature channel consists of a square wave offset 90 deg. from
the other. Positive motion consists of the A channel leading the B
channel by 90 deg., and negative motion consists of the A channel
lagging the B channel by 90 deg. For each full phase of one channel,
four resolved quadrature counts will occur, resulting in a 4 to 1
resolution enhancement over the basic channel resolution.
To enhance reliability of the received encoder information the MC1451
provides digital filtering of the quadrature data lines (A and B
quadrature count) as well as the index and home signals.
For all of these signals a valid high or low condition is recognized only
when the input signal has been maintained for 3 clock cycles of 160
nSec each (total required duration of 480 nSec)
Although this digital filtering scheme can increase the overall reliability
of the quadrature data, to achieve the highest possible reliability
additional techniques may be required, such as differential line
drivers/receivers, or analog filtering.
Stall Detection
The MC1451A-E chipset supports two primary operations in connection
with encoder feedback:
Readback of the current encoder position is accomplished using the
GET_ACTL_POS command. This command allows the user to confirm
that the stepper axis has achieved a particular location. The
GET_ACTL_POS command can be used at any time, whether the axis
is in motion or not.
Automatic stall detection allows the chipset to detect when the step
motor has lost steps during a motion. This typically occurs when the
motor encounters an obstruction, or otherwise exceeds its rated torque
specification.
Automatic stall detection operates continuously once it is initiated. The
current desired position (target position) is compared with the actual
position (from the encoder) and if the difference between these two
values exceeds a specified limit a stall condition is detected.
To initiate automatic stall detection the host must specify the number of
encoder counts per output motor step. This is accomplished using the
command SET_STEP_RATIO. The following equation shows how this
value should be set for various values of encoder count resolution and
output step resolution:
Ratio = (N counts /N pulses )*256.
where:
- readback of current axis position
- automatic stall detection.
Ratio is the ratio value specified to the SET_STEP_RATIO
N counts is the number of encoder counts per motor
N pulses is the number of output step pulses per motor
command
rotation.
30
For example if a step motor with a 64 ustep per full step pulse and
directionping amplifier (12,800 total pulses per motor rotation) is used
with an encoder which has 4,000 counts per motor rotation, the ratio
specified in the SET_STEP_RATIO command would be
(4,000/12,800)*256, or 80.
Although the MC1451A-E supports stall detection with encoders that
have a different number of counts then pulses, the ratio provided with
the SET_STEP_RATIO command must be an exact integer. For
example in the above example an encoder with 4,000 counts per
rotation which gives a ratio value of 80 is acceptable however an
encoder with 4,096 which gives a ratio value of 81.92 is not acceptable.
Position Error
The difference between the desired position, also called the target
position, and the actual encoder position is known as the position error,
or the actual position error.
The position error is continuously maintained by the chipset and can be
read by the host at any time. To read the position error the command
GET_ACTL_POS_ERR is used.
To perform the stall detection function the position error is continuously
compared with the maximum allowed position error, which is set using
the command SET_POS_ERR. To read this value back the command
GET_POS_ERR is used. The units of the maximum position error is
encoder counts.
If the maximum position error value is exceeded (stall is detected), then
the axis is said to be in a "motion error" condition. When this occurs the
motion error bit in the axis status word is set, and further pulse
generation may be halted, depending on the state of the automatic
motor shutdown mode (see SET_AUTO_STOP_ON and
SET_AUTO_STOP_OFF host command descriptions).
If the automatic motor stop mode is not set than only the motion error
status bit is set.
Recovering From A Motion Error
To recover from a motion error which results in the pulse output being
halted, the following sequence should be performed:
1)
2)
3)
After the above sequence the axis will be at rest, and the position error
between the target position and the actual encoder position will be set
to zero.
Determine cause of motion error and correct problem (this may
require human intervention).
Synchronize the profile generator's target position with the actual
axis location by executing a SYNCH_PRFL command, and then
an UPDATE command
Turn pulse generator back on using MTR_ON command.
rotation