AN1915D Freescale Semiconductor / Motorola, AN1915D Datasheet - Page 19

• Manufacturer Part Number: AN1915D
• Description: 3-Phase BLDC Motor Control with Quadrature Encoder using DSP56F80x
• Manufacturer: Freescale Semiconductor / Motorola

MOTOROLA
5.1.4 Mask and Swap Calculation
This process performs a rotor commutation. As already mentioned, only two phases are powered by a
six-step control. The proper PWM output can be generated by changing the PWM value (duty cycle)
registers only. This has two disadvantages. Firstly the speed controller, which changes the duty cycle,
affects the commutation algorithm (as a result of changing the duty cycle). Secondly, change in the
duty cycle is synchronized with PWM reload, which may give rise to a delay between a proper
commutation moment and the PWM reload. This is especially pronounced at high speeds, when the
commutation period is very short.
The DSP56F80x family has two features dedicated to BLDC motor control: The ability to swap odd
and even PWM generator outputs, and the ability to mask (disable) any PWM generator outputs. Thus
the same PWM pulse can be applied on the selected upper and lower switch of the inverter. These two
features allow the creation of a rotational field without changing the contents of the PWM value
registers (duty cycle). The influence of masking and swapping on the PWM generator outputs is
illustrated in
the Application Note DSP56F80x MC PWM Module in Motor Control Applications [2].
The commutation algorithm bldchsCommHandlerComp calculates the output PWMState , based
on the actual control word (CommutationCounter). The structure of PWMState consists of two
parts (variables). The first part ( PWMState.Swap ) defines the swapping of phases, and the second
part ( PWMState.Mask ) defines the phase masking. These values are written directly to the PWM
generator. The swapping value is written to the PWM Channel Control Register and the masking value
to the PWM Output Control Register. The following paragraph describes in detail the commutation
transition from the one to the next commutation sector as is depicted on
Figure
For example, the even PWM value registers are set to 75% duty cycle. The odd PWM value registers
are set to complementary value 100% - 75% = 25% duty cycle. The rotor in
sector ABC[110]. The state of masking and swapping is as follow (see
As soon as the rotor passes the sector border
After commutation the state of masking and swapping is (see
This commutation process is repeated six times per electrical revolution.
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•
•
•
•
•
•
•
5-2.
Phase A
not masked, not swapped (the Phase A is connected to positive DC-Bus voltage)
Phase B
masked/disabled
Phase C
not masked, swapped (the Phase C is connected to negative DC-Bus voltage. The swap of
Phase B provides the identical pulses on the upper switch of Phase A and the lower switch of
Phase C and conversely on the lower switch of Phase A and the upper switch Phase C)
swap Phase B (unconnected phase)
mask/disable Phase C; unmask/enable Phase B
Phase A
not masked, not swapped (the Phase A is connected to positive DC-Bus voltage)
Phase B
not masked, swapped (the Phase B is connected to negative DC-Bus voltage.)
Phase C
masked/disabled
Figure
5-2. Detailed information about PWM settings for BLDC motors can be found in
3-Phase BLDC Motor Control with Quadrature Encoder
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
(Table
3-8) the commutation is done in two steps:
Table
3-2):
Table
Figure
Figure 3-7
3-2):
3-7,
Software Design
Figure 3-8
is situated in
and
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