Motor control system for achieving peak torque
View Patent ↗In a motor controller, a modified interpolation technique uses an extrapolated torque command for the upper table to improve torque linearity under certain conditions. When the torque command input is greater than the max torque limit of a first look-up table, but less than the maximum torque limit of a second, adjacent look-up table, the desired current command is computed by interpolating between the maximum torque limit of the first table and a revised torque value for the second look-up table, wherein the revised torque value is determined by extrapolating from the maximum torque limit of the first look-up table through a torque value based on the torque command input.
1. A motor control system comprising:
an electric motor,
a control system coupled to the electric motor and configured to receive a torque command input, a motor speed input, and a voltage input;
a plurality of look-up tables, each corresponding to adjacent values of the voltage input, each having an associated maximum torque limit, and each including a two-dimensional array of current commands indexed by values of the motor speed input and the torque command input; wherein the control system is configured to control the electric motor in accordance with the current commands;
wherein the control system is configured to determine an applicable operating condition from a set of operating conditions and determine a corresponding desired current command;
wherein one of the operating conditions corresponds to a first condition wherein the torque command input is greater than the max torque limit of a first look-up table, but less than the maximum torque limit of a second, adjacent look-up table, and wherein the corresponding desired current command is computed by interpolating between the maximum torque limit of the first table and a revised torque value for the second look-up table;
wherein the revised torque value is determined by extrapolating from the maximum torque limit of the first table through a torque value based on the torque command input.
2. The motor control system of claim 1 , wherein the set of operating conditions further includes a second condition wherein the torque command input is less than the maximum torque limit of the first look-up table, but less than the maximum torque limit of the second look-up table.
3. The motor control system of claim 2 , wherein, after determining that the applicable operating condition is the second operating condition, the control system is configured to compute the desired current command by interpolating between the first and second look-up tables.
4. The motor control system of claim 2 , wherein the set of operating conditions further includes a third condition wherein the torque command input is greater than the max torque limit of the first look-up table, and greater than the maximum torque limit of the second look-up table.
5. The motor control system of claim 4 , wherein, after determining that the applicable operating condition is the third operating condition, the control system is configured to compute the desired current command by interpolating between the maximum torque limit of the first look-up table and the maximum torque limit of the second look-up table.
6. The motor control system of claim 3 , wherein the control system is configured to compute the desired current command using the relation:
T
(
px
)
=
(
V
High
-
V
Low
)
·
(
T
cmd
-
T
(
p
1
)
V
dc
-
V
Low
)
+
T
(
p
1
)
x
(
V
dc
)
=
(
V
dc
-
V
Low
)
·
(
x
(
px
)
-
x
(
p
1
)
V
High
-
V
Low
)
+
x
(
p
1
)
wherein V Low corresponds to the first look-up table, V High corresponds to the second look-up table, p 1 is the operating point at V Low and maximum torque limit for that voltage, px is the operating point to be used for indexing the second look-up table at V High , T(p 1 ) is the torque value at p 1 , and T(px) is the torque value at px.
7. A method for controlling an electrical motor, comprising:
receiving a torque command input, a motor speed input, and a voltage input;
accessing a plurality of look-up tables, each corresponding to adjacent values of the voltage input, each having an associated maximum torque limit, and each including a two-dimensional array of current commands indexed by values of the motor speed input and the torque command input;
determining an applicable operating condition from a set of operating conditions, wherein the applicable operating condition corresponds to a first condition wherein the torque command input is greater than the max torque limit of a first look-up table, but less than the maximum torque limit of a second, adjacent look-up table,
computing the desired current command by interpolating between the maximum torque limit of the first table and a revised torque value for the second look-up table, wherein the revised torque value is determined by extrapolating from the maximum torque limit of the first look-up table through a torque value based on the torque command input; and
controlling the electric motor in accordance with the desired current command.
8. The method of claim 7 , wherein the set of operating conditions further includes a second condition wherein the torque command input is less than the maximum torque limit of the first look-up table, but less than the maximum torque limit of the second look-up table.
9. The method of claim 8 , wherein, after determining that the applicable operating condition is the second operating condition, the control system is configured to compute the desired current command by interpolating between the first and second look-up tables.
10. The method of claim 8 , wherein the set of operating conditions further includes a third condition wherein the torque command input is greater than the max torque limit of the first look-up table, and greater than the maximum torque limit of the second look-up table.
11. The method of claim 10 , wherein, after determining that the applicable operating condition is the third operating condition, the control system is configured to compute the desired current command by interpolating between the maximum torque limit of the first look-up table and the maximum torque limit of the second look-up table.
12. The method of claim 7 , the desired current command is computed using the relation:
T
(
px
)
=
(
V
High
-
V
Low
)
·
(
T
cmd
-
T
(
p
1
)
V
dc
-
V
Low
)
+
T
(
p
1
)
x
(
V
dc
)
=
(
V
dc
-
V
Low
)
·
(
x
(
px
)
-
x
(
p
1
)
V
High
-
V
Low
)
+
x
(
p
1
)
wherein V Low corresponds to the first look-up table, V High corresponds to the second look-up table, p 1 is the operating point at V Low and maximum torque limit for that voltage, px is the operating point to be used for indexing the second look-up table at V High , T(p 1 ) is the torque value at p 1 , and T(px) is the torque value at px.