Microchip Technology DM164130-2 データシート
The BLDC Add-on Board
2012 Microchip Technology Inc.
DS41629A-page 27
2.4.3.5
STALL
A motor that stalls may appear to the control algorithm as if it is still running at high
speed. Blanking, modulation, and intrinsic motor characteristics all play a part in a false
BEMF indication. When this happens, the commutation rate will ramp up to a rate much
higher than the motor is able to operate. This feature is used to detect a stall condition.
Determine an appropriate stall detection time by measuring the commutation period of
the motor at maximum speed. Enter this period minus some margin as the stall period.
If the commutation period is ever shorter than the stall period, then the motor will be
immediately stopped.
speed. Blanking, modulation, and intrinsic motor characteristics all play a part in a false
BEMF indication. When this happens, the commutation rate will ramp up to a rate much
higher than the motor is able to operate. This feature is used to detect a stall condition.
Determine an appropriate stall detection time by measuring the commutation period of
the motor at maximum speed. Enter this period minus some margin as the stall period.
If the commutation period is ever shorter than the stall period, then the motor will be
immediately stopped.
2.4.3.6
ERROR SCALE
The control algorithm compares the actual time from commutation to where the BEMF
crosses the mid-drive level. This is referred to as the zero cross event. The zero cross
event is expected half way through the commutation period. The error between the
expected and actual measured time is scaled down by the error scale factor and added
back into the commutation time, thereby forming the closed-loop operation. The error
scale value determines the loop gain. Less gain will result in a slower, but more stable
response. Conversely, higher gain will result in a faster response, but less stable
operation. The error is scaled by performing right shifts on the error value. More shifts
means less of the error is getting back into the loop, resulting in less loop gain. The
commutation time is a 16-bit integer with a 17
crosses the mid-drive level. This is referred to as the zero cross event. The zero cross
event is expected half way through the commutation period. The error between the
expected and actual measured time is scaled down by the error scale factor and added
back into the commutation time, thereby forming the closed-loop operation. The error
scale value determines the loop gain. Less gain will result in a slower, but more stable
response. Conversely, higher gain will result in a faster response, but less stable
operation. The error is scaled by performing right shifts on the error value. More shifts
means less of the error is getting back into the loop, resulting in less loop gain. The
commutation time is a 16-bit integer with a 17
th
sign bit. Very slow commutation rates
will use all 16 bits. BEMF error for slow motors can be large so a correspondingly large
error scale can be tolerated. However, as the motor speed increases the commutation
time decreases and not all 16 bits are significant. At very high speeds the magnitude
of the error is small relative to the full 16-bit number and large error scales cannot be
tolerated, because the full error value may be shifted to a value of zero. For most
motors, an error scale value of three works well. Again, experiment to see what works
best for your particular motor.
error scale can be tolerated. However, as the motor speed increases the commutation
time decreases and not all 16 bits are significant. At very high speeds the magnitude
of the error is small relative to the full 16-bit number and large error scales cannot be
tolerated, because the full error value may be shifted to a value of zero. For most
motors, an error scale value of three works well. Again, experiment to see what works
best for your particular motor.
2.4.3.7
ADVANCE AND BALANCE CONTROLS
The advance and balance controls should be maintained at zero for normal motor
operation. These controls are only used to observe the effects of manipulating the zero
crossing event. The advance control advances or retards the zero crossing event. The
balance control extends or shortens the commutation time after the zero crossing
event. It is interesting to observe on an oscilloscope how the motor reacts to these
disturbances.
operation. These controls are only used to observe the effects of manipulating the zero
crossing event. The advance control advances or retards the zero crossing event. The
balance control extends or shortens the commutation time after the zero crossing
event. It is interesting to observe on an oscilloscope how the motor reacts to these
disturbances.
2.4.3.8
ZERO-CROSSING ADJUSTMENT
A potentiometer, R21, is included on the BLDC Add-on Board for adjusting the zero
crossing reference voltage. Most motors require a reference voltage of half the applied
motor voltage. However, the winding pattern of some motors results in a nonlinear
BEMF response. Such a motor is the ebm-papst motor supplied with the add-on board.
This motor has an S-shaped BEMF response and operates best when the zero
crossing reference is lower than half the applied motor voltage. If you have difficulty
starting the motor, even after going through all the optimization steps, then check the
position of R21 and try moving the position slightly more and less than the 90 degree
CCW starting position. Once you get the motor operating in closed loop, fine tune the
zero crossing reference with R21. This is accomplished by observing the waveform of
one of the motor terminals. Adjust R21 so that the rising and falling slopes of the
waveform are equal in appearance.
crossing reference voltage. Most motors require a reference voltage of half the applied
motor voltage. However, the winding pattern of some motors results in a nonlinear
BEMF response. Such a motor is the ebm-papst motor supplied with the add-on board.
This motor has an S-shaped BEMF response and operates best when the zero
crossing reference is lower than half the applied motor voltage. If you have difficulty
starting the motor, even after going through all the optimization steps, then check the
position of R21 and try moving the position slightly more and less than the 90 degree
CCW starting position. Once you get the motor operating in closed loop, fine tune the
zero crossing reference with R21. This is accomplished by observing the waveform of
one of the motor terminals. Adjust R21 so that the rising and falling slopes of the
waveform are equal in appearance.