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7.1 Brushless Motor Overview
The purpose of this chapter is to provide deterministic procedures
for the user that will allow them to interface brushless motors from
a variety of manufactures to PMD’s products. The brushless motors
addressed in this analysis fall under the category of motors that can
be commutated using Hall Effect sensors by PMD’s products assuming
that the correct Hall Effect sensor connection configuration is found.
The important property of all motors that fall into this category is
the relationship between the three motor phases and the three Hall Effect
sensors. The same relationship is required when using sinusoidal commutation
and Hall-based phase initialization.
There are several standards for labeling the three motor phases: A,B,C
or R,S,T or U,V,W or W1, W2, W3. For the purpose of this analysis, the
A, B, C labeling scheme will always be employed even though the motor
under investigation may employ another labeling method.
Likewise the Hall Effect sensor connections will always be labeled
Hall A, Hall B, and Hall C, even though the manufacturer may label them
Hall 1, 2, 3 or Sensor 1, 2, 3.
The relationship between the motor phases and Hall Effect sensors is
the property that defines the proper configuration and allows PMD products
to successfully commutate the motor. The manner in which the user will
go about determining this relationship will depend greatly on the information
provided to them by the motor manufacturer. Industry research has determined
that the content and form of the information provided by the manufacturer
varies. As a result, explanations for interpreting the various information
formats will be provided.
The procedure developed in this chapter assumes that the amplifier
in use will do nothing more than amplify the phase signals (voltage)
from the PMD controller. Some amplifiers may invert the phase signal,
in which case the results of this procedure will not yield the expected
rotation.
There are three independent methods for determining the Hall configuration.
The selection of which method to use will depend on the information
provided.
- Hall Based Commutation Sequence Provided
- Back EMF Diagrams
- Trial and Error
7.2 Method #1: Hall Based Commutation Sequence Provided
This method is the most straightforward and requires the least amount
of effort on the part of the user. This information is usually provided
in the form of a diagram or table and may have different titles such
as “Block Commutation” or “Brushless DC Motor Timing
Diagram”. Some of these diagrams represent motor phase voltage
during trapezoidal (six-step) commutation. Other tables may represent
the state of the high-side and low-side MOSFETs of the half-bridge amplifiers
for all three phases during trapezoidal commutation. Either method conveys
adequate information about driving the motor phases based on Hall Effect
sensor states.
The relationship between the Hall Effect sensors themselves is always
consistent. In other words the Hall Effect sensor sequence seen in Figure
7.1 can be found in all motors with 120-degree Hall Effect sensors when
the motor rotates. However, the direction of rotation, CW or CCW, necessary
to produce this relationship can vary across different motors.
Very often the binary state of the three Hall Effect sensors will
be combined to create a 3-bit binary word. The mapping between the Hall
states and the three-bit word is also shown in Figure 7.1. Below the
binary word representation in Figure 7.1 is a table that represent the
states of the MOSFETs of the half-bridges. Every Hall state has a unique
half-bridge state defined as follows:
A+ = Phase A high side MOSFET closed
A- = Phase A low side MOSFET closed
B+ = Phase B high side MOSFET closed
B- = Phase B low side MOSFET closed
C+ = Phase C high side MOSFET closed
C- = Phase C low side MOSFET closed
If the state of a MOSFET for a particular Hall state is not defined
then it is assumed to be open. For example during Hall state 1-0-1,
MOSFETs A-, B+, C+ and C- are all open.
Below the table of MOSFET states in Figure 7.1 is a diagram of the
relative voltages through each motor phase based on the Hall states
(and subsequent MOSFET states). For instance in Hall state 1- 0-1, the
path of the current begins at the voltage source, flows through the
high side MOSFET of phase A, through motor winding A, through motor
winding B, through the low side MOSFET of phase B, and finally to the
ground plane.
If the user finds a table in the motor datasheet with the same relationship
then the motor Hall Effect sensor connections between the motor and
the PMD product should be A to A, B to B, and C to C. If the relationship
seen in the motor datasheet is not identical then the user will need
to determine which Hall Effect sensor corresponds to the given MOSFET
state sequence or motor winding excitation. The tables will indicate
a direction of rotation, CW or CCW. When properly configured, this will
be the direction of rotation during a positive motor command.
7.3 Method #2: Back EMF Diagrams
The motor datasheet may provide a back EMF diagram instead of or in
addition to a Block Commutation diagram. This diagram is created by
observing the back EMF voltage produced by various motor phases while
the motor is being manually driven (motor becomes a generator).
Caution: Motor should be electrically “floating” when
measuring back EMF. Pay attention to oscilloscope ground connections when
measuring multiple phases simultaneously.
Figure 7.2 shows the three motor phases, A, B, and C.
An oscilloscope is placed across phase A and B with the positive probe
on phase A and the negative on phase B. When the motor is back driven,
the voltage seen on the voltmeter will become an approximate sinusoid
with time. If the motor RPM is high enough the top and bottom of the
sinusoid may become “clipped” but the phase relationship
will be maintained.
In addition to observing the back EMF of the phase, the
Hall Effect sensor signal while the motor is being back driven is also
observed. During rotation there is always one Hall Effect sensor signal
that, when graphed along side the phase A-B back EMF values, will result
in the relationship shown in Figure 7.3. Figure 7.3 refers to this Hall
Effect sensor as Hall “X”. The actual label of Hall “X”
depends on the specific motor being used. The important step is to determine
which of the Hall Effect sensors will produce the relationship in Figure
7.3.
The details of creating such a diagram have been provided
here because the user may not have possession of this information, either
because the customer does not have the motor datasheet or because the
motor datasheet does not provide this information.
(If the motor datasheet provides “Block Commutation”
information or something similar then Method #1 should be used.)
Note that direction of rotation has not been defined
in terms of Clockwise or Counter Clockwise in Figure 7.3. This is because
the rotation direction that achieves this relationship is not consistent
across different motors. Some motors may have to be rotated Clockwise
to achieve this relationship and some may have to be rotated Counter
Clockwise. The important concept is that whichever direction achieves
the relationship seen in Figure 7.3 will be the direction of rotation
when the PMD controller is creating a positive motor command signal.
When the motor is rotated in the opposite direction the relationship
between phase A-B and Hall “X” will be 180 degrees out of
phase as seen in Figure 7.4. This will be the direction of rotation
during a negative motor command.
Click to Enlarge
Click to Enlarge
The determination of Hall “X” is achieved
by visual inspection of the relationship between the back EMF of a particular
phase and the candidate Hall Effect sensor. For comparison sake, Figure
7.5 has been provided to demonstrate one of the possible relationships
seen when one of the two incorrect Hall Effect sensors is being considered.
Click to Enlarge
Note the phase relationship shown in Figure 7.5 is different
then the relationship seen in Figures 7.3 and 7.4. The user will know
the correct Hall Effect sensor has been found when the peaks of the
back EMF waveform occur exactly half between the edges of the Hall signal.
The Hall signal seen in Figure 7.5 is not Hall “X” because
the peaks of the back EMF waveform of phase A-B are skewed toward the
Hall signal edges.
Just as the back EMF voltage of phase A-B correlates to
Hall “X”, the other phases B-C and C-A will also have corresponding
Hall Effect sensors (Hall “Y” and Hall “Z”)
that create the exact same relationship. There is always a one-to-one
relationship between corresponding motor phases and Hall Effect sensors.
Furthermore there is one direction of rotation that satisfies the relationships
for all phases.
7.3.1 Connecting to a PMD product
The reason such emphasis was placed on determining
which motor phases correspond to which Hall Effect sensors is because
that relationship will determine how the phases and sensors are connected
to a PMD product. PMD products that are designed to commutate brushless
motors will have Hall Effect sensor inputs labeled Hall A, Hall B,
and Hall C. Once all of the motor phase versus Hall Effect sensor
relationships have been determined, Hall “X” will be connected
to the Hall B input, Hall “Y” will be connected to the
Hall C input and Hall “Z” will be connected to the Hall
A input. The same PMD products will also have motor phase outputs
labeled Phase A, Phase B, and Phase C. These should be connected to
the corresponding motor phases A, B, and C. (It was mentioned before
that the labeling on the motor phases may actually be R,S,T or U,V,W
or W1, W2, W3). Figure 7.6 demonstrates the connection configuration.
Once the relationships have been determined and the
connections established, the system is ready for Hall based commutation.
With the addition of an encoder for position feedback, the system
will be ready for closed loop Hall-based commutation.
PMD products allow the user to invert the interpretation
of the Hall Effect sensor, meaning that the PMD product will see a
Hall Effect sensor “high” state as being at a “low”
state. However, it is never necessary to use the Hall Effect sensor
inversion feature for the purpose of proper configuration.
It was mentioned that the rotation direction necessary
for producing the relationship seen in Figure 7.2 could be Clockwise
or Counter Clockwise depending on the specific motor. Motors that
require a Clockwise rotation to produce this relationship will be
referred to as CW motors. Likewise motors that require Counter Clockwise
rotation will be referred to as CCW motors. If connections are made
with regard to the defined relationship then a positive motor signal
(SetMotorCommand <positive number>) will produce a Clockwise
torque on CW motors and the same positive motor signal will produce
Counter Clockwise torque on CCW motors.
It is still possible to configure a CCW motor to rotate
CW with a positive motor signal and likewise it is possible to configure
a CW motor to rotate CCW with a positive motor signal. This can be
done without altering the connection scheme shown in Figure 7.3. One
way of accomplishing this task is to use SetSignalSense to invert
all Hall Effect sensor inputs. The other way is to use SetSignalSense
to invert the Motor Output.
7.4 Method #3: Connection by Trial and Error
A “trial and error” method also exists for
determining the relationship. The trial and error method involves permutation
through all possible Hall Effect sensor configurations until the optimal
configuration is found. Due to its iterative nature, this method is
only advisable if the other methods cannot be followed. This may be
the case if the motor manufacturer does not provide sufficient information
or if the equipment necessary for back driving a motor or capturing
motor signals is not available.
7.4.1 Definition of Optimal Configuration
The optimal Hall Effect sensor configuration results
in the maximum motor torque for a given current supply2.
This means that some configurations will produce a minimal torque
and other configurations will not produce any torque, but there is
only one configuration that will produce a maximum torque.
Creating a setup that allows the Hall Effect sensor
connections between the PMD product and a brushless motor to be easily
swapped assists in proving this concept. Every permutation is tested
individually by applying a positive open loop motor command to the
system for a specific amount of time. The permutation that results
in the maximum amount of motor displacement corresponds to the permutation
that results in the maximum amount of torque for the give motor command.
This permutation is the optimal configuration. It is the linear relationship
between MotorCommand and current at steady state as well as the linear
relationship between motor torque and motor displacement that allows
this conclusion to be drawn.
The given equation states that the torque as a function of current
of a system in the optimal configuration will be greater than the
torque as a function of the same current of any system that is not
in the optimal configuration.
The equation to be proved states that the velocity
as a function of the MotorCommand of a system in the optimal configuration
will be greater than the velocity as a function of the same MotorCommand
of any system that is not in the optimal configuration at steady state.
A formal proof of equation 7.2 exists in Appendix D. The validation
of equation 7.2 implies that the results of the iterative procedure
described above will yield the optimal Hall Effect sensor configuration.
7.4.2 Procedural Instructions
- Connect PMD motor signals (PWM or DAC) to the amplifier with PMD
phase A to the amplifier input labeled phase A (or equivalent). PMD
phase B is connected to the input labeled phase B (or equivalent).
Likewise for phase C (except when using DAC output).
- Arbitrarily connect motor Hall wires to PMD Hall inputs.
- Power on amplifier.
- Initialize PMD controller for Hall-based commutation and apply an
open loop motor command.
- SetCommutationMode 1
- SetMotorMode 0
- SetMotorCommand <user_selected value>
- Update
- Send GetActualPosition command to PMD controller at regular intervals
and note response.
- Power off amplifier and arbitrarily select another Hall wire connection
scheme and repeat steps 3 to 5.
- Repeat step 6 until all six Hall wire connection permutations have
been tried.
- Determine which permutation produced the largest difference in the
responses to the sequence of GetActualPosition commands in step 5.
- Use this permutation to run the motor open loop as described in
step 4 except select the negative of the value used in SetMotorCommand.
- If the responses to GetActualPosition contain approximately the
same difference as seen with the positive motor command then this
permutation is the optimal configuration. If not then repeat step
9 with the permutation yielding the second largest difference from
step 8.
7.4.3 Trial and Error Test Results
Based on the definition of the optimal configuration,
all configuration permutations were tested and the permutation corresponding
to the optimal configuration was noted. As predicted there was one
optimal Hall configuration that resulted in a steady state velocity
larger than the velocity produced by any other configuration. See
Appendix C for the results of this test.
7.5 Conclusion
This chapter has introduced several methods the user
can follow in order to determine the proper Hall configuration. The
need for a procedure exists because there is no one standard relationship
between Hall Effect sensor wiring and motor phases that is consistent
across manufacturers. The need for different methods exist because,
in addition to different Hall/motor phase relationships, the format
and extent of information provided by manufactures is not consistent.
Experimental results have shown that three methods exist
that will generate the exact same Hall Effect sensor wiring configuration.
The details of the test motors and experimental results are summarized
in Appendix C.
PMD recommends following Method #1 if possible. If the
back EMF versus Hall Effect sensor relationship is provided or if the
user has the ability to derive this relationship on their own, then
Method #2 can be used. As a last resort the user can follow Method #3
by permutation through the six wiring configurations.
The information presented in this chapter is specific
to determination of Hall Effect sensor configuration for brushless motors.
Other initialization steps not covered here will be necessary if sinusoidal
commutation is used. The user should also reference PMD’s “Step
by Step Guide to Phase Initialization” which can be found on the
PMD Application Notes Web Page.
2 Parker Hannifin® -Compumotor Division, OEM770X
User Guide, page 101.
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