Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Mechanical measurement system
Reexamination Certificate
2002-03-27
2004-05-18
Nghiem, Michael (Department: 2863)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Mechanical measurement system
Reexamination Certificate
active
06738718
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to electrical motor control systems and, more specifically, to a circuit that measures torque and flux current in a synchronous motor.
2. Description of the Prior Art
Electric motor drive applications are increasingly used in automotive applications. Recent trends include the use of electric motors in power steering. Since the battery voltage standard is only 12v, the peak motor power is limited to a low horsepower rating, typically on the order of a 1-2 HP motor. In high performance motion control, such as electric power steering, low torque ripple, low cost, small size and high reliability are required. These factors typically lead to the use of a permanent magnet synchronous motor (PMSM) instead of DC motor, a switch reluctance motor or an induction motor. The magnetic field and magneto-motive forces (MMF) in the PMSM are assumed to be sinusoidally distributed in space in order to minimize torque ripple.
In a PMSM drive, motor torque feedback is required to have a precise torque control. A vector control is typically used to achieve a high performance motor drive system. Speed sensor feedback is required and is easily obtained, for example, through a motor shaft encoder at relatively low cost. However, to control motor torque precisely, an absolute rotor position sensor is also required, which is obtained from sensing a motor back electro-motive force (EMF) or from a resolver. A motor torque feedback signal can be obtained from a torque sensor, but including a torque sensor may be a costly solution. A more commonly used method is to sense the motor currents and then derive the motor torque feedback from the sensed currents. However, existing methods of sensing motor currents are difficult, can be costly and can reduce performance.
PMSM Vector Control Background
The steady state electromagnetic torque (T
e
) of PMSM, expressed in the dq model, is:
T
e
=
3
2
P
2
1
ω
e
[
E
1
I
qd
+
(
X
dx
-
X
qs
)
I
ds
I
qs
]
Since there is no damper winding in PMSM, the torque equation is also valid for the instantaneous case.
In an air gap magnet motor, where X
ds
≈X
qs
, the torque expression is reduced to:
T
e
≈
3
2
P
2
1
ω
e
E
I
I
qs
The internal back EMF peak voltage E
I
(volt) is given as:
E
I
=K
v
&ohgr;
e
And the quadrature axis (q-axis) motor current I
qs
(amp) is:
I
qs
=I
qds
cos &ggr;
Hence, the torque can also be expressed as:
T
e
≈K
T
I
qds
cos &ggr;
Where,
P=Number of poles.
&ohgr;
e
=Electrical synchronous motor speed which is also the stator frequency in rad/sec.
X
ds
=Direct axis reactance encountered by the d-axis and q-axis current components in Henry-rad/sec.
X
qs
=Quadrature axis reactance encountered by the d-axis and q-axis current components in Henry-rad/sec.
X
s
=X
ds
=Stator referred synchronous reactance in Henry-rad/sec.
r
s
=
Short
Circuit
Load
Loss
(
Short
Circuit
Armature
Current
)
2
=
effective
stator
res
.
K
v
=EMF constant in V/(rad/sec.).
K
T
=Torque constant in Nm/Amp.
I
qds
=Amplitude motor stator peak current in Amp, hence it is a dc quantity.
V
qds
=Amplitude motor stator peak phase voltage in volt, hence it is a dc quantity.
&ggr;=A space angle measured at the vector position of the current I
qds
with respect to the q-axis (where E
I
is located). Cos (&ggr;) is defined as internal power factor. Angle &ggr; is positive if the current vector I
qds
leads the voltage vector E
I
. It is also referred as torque angle.
&dgr;=A space angle measured at the vector position of the V
qds
with respect to the q-axis (where E
I
is located). It is sometime called as “phase advance”. Angle &dgr; is positive if the voltage vector V
qds
leads the voltage vector E
I
.
&phgr;=A space angle measured at the vector position of the I
qds
with respect to the Vector position of the current V
qds
. Cos (&phgr;) is defined as load power factor. Angle &phgr; is positive if the current vector I
qds
leads the voltage vector V
qds
.
FIG. 1
shows a typical space vector representation
10
of 3-phase PMSM operation. Positive values for angles &ggr;, &dgr; or &phgr; mean the angle is oriented counter clockwise with respect to the corresponding reference q & d axis. I
qds
, V
qds
, E
i
vectors along with the corresponding reference q & d axis, are simultaneously moving in a counter clockwise direction when the speed is positive. This means that the vectors rotate one 360° turn when the motor rotates one electrical turn. In the time domain expression, the phase sequence is A-B-C which corresponds to positive speed rotation. In a positive speed motoring operation, a PMSM typically operates where current vector I
qds
is in the first quadrant, i.e., &ggr;=0 up to base speed operation and 0°<&ggr;<90 beyond base speed operation.
To summarize, the two requirements for vector control in PMSM are measurement of rotor position (absolute rotor position is required) and precise control of the stator current to correctly position the resultant stator MMF in relation to the rotor position. Therefore, it is important to control the magnitude I
qds
and the angle &ggr; independently. The torque response will follow the stator current I
qds
instantaneously so long as the angle &ggr;=0°. If the angle &ggr; attains a value other than zero, there will be a component of the stator current in the field axis (d-axis) and a flux change will take place. Since the flux change is not instantaneous, the torque response will also not be instantaneous if angle &ggr; or I
qds
is changed.
One way to control amplitude and phase of stator current independently is to use a current regulated PWM inverter (CRPWM) in a stationary reference frame. The CRPWM provides a conceptually simple means for implementing torque control with independent q-axis and d-axis current inputs (I
qs
and I
ds
). In essence, all that is required is to use absolute rotor position information to convert the I
qs
* and I
ds
* commands in the rotor reference frame to a stator reference. The stator referred currents, at stator frequency, become the current commands for CRPWM. However, this technique requires instant stator current feedback information obtained through current sensors. In addition, the bandwidth of this regulator must be relatively high including dc in case of a stall mode condition.
Another method to measure q-axis and d-axis current inputs (I
qs
and I
ds
) independently is by using a current regulator in rotor reference frame, i.e., a synchronous reference frame, since the rotor frequency is the same as the stator frequency. In this method, the I
qs
* and I
ds
* commands are dc quantities. Such regulators typically do not require a relatively high bandwidth. I
qs
and I
ds
feedback are required and in steady state they are dc quantities. I
qs
and I
ds
feedback are typically derived from stator phase currents feedback, i
as
, i
bs
, i
cs
, using Park's transformation.
Current Sensing Techniques
In small horsepower drives, current sensors such as Hall Effect devices or current shunts are often placed in series with motor phases. In a three-phase system, two of such sensors are required. Such devices introduce a significant cost relative to the system cost. A lower cost method is to use a current shunt placed in the dc link.
In a closed-loop current mode motor control, one method is to sample dc link current i
dclink
and, knowing the corresponding PWM inverter switching state, to decode the stator phase currents, i
as
, i
bs
, i
cs
. With the rotor position information, the instantaneous motor torque current I
qs
and motor flux current I
ds
can be derived from the stator phase currents. However, this method requires a significant amount of processing and sampling i
dclink
at a
De Larminat Ronan
Kurnia Alexander
Motorola Inc.
Nghiem Michael
Pickens S. Kevin
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