Motor control device, walking assist device, and motor control method

1. A motor control device comprising:

a coil temperature detection unit which detects a coil temperature of a motor;

an upper limit current determination unit which determines an upper limit value of a coil current in which a coil temperature after a lapse of a first predetermined time from a present time is maintained at an upper limit temperature or lower every time a second predetermined time shorter than the first predetermined time elapses by using the coil temperature detected by the coil temperature detection unit; and

a coil current control unit which controls the coil current so as to be equal to or lower than the upper limit value,

wherein the upper limit current determination unit is configured to determine the upper limit value of the coil current by using a present coil temperature as the coil temperature and using the upper limit temperature and the first predetermined time.

2. The motor control device according to claim 1 , wherein:

the motor is a three-phase motor;

the coil temperature detection unit is configured to detect an individual-phase coil temperature for each phase of the three-phase motor;

the upper limit current determination unit is configured to determine an individual-phase upper limit value for each phase on the basis of the individual-phase coil temperature; and

the coil current control unit is configured to control each individual-phase coil current so as to be equal to or lower than each individual-phase upper limit value.

3. The motor control device according to claim 2 , wherein the coil current control unit:

is configured to set a circle having a radius corresponding to a minimum individual-phase upper limit value among three individual-phase upper limit values with an origin as a center in a d-q coordinate system in which three individual-phase coil currents are represented by one vector with respect to the three individual-phase upper limit values determined for each phase by the upper limit current determination unit;

with respect to a target vector which has the length corresponding to a required torque to the motor and rotates around the origin in synchronization with the rotation of the motor, is configured to correct the length of the target vector so that a tip thereof is contained within the circle in the case where the tip is located outside the circle; and

is configured to control the respective individual-phase coil currents on the basis of the target vector whose length is corrected.

4. The motor control device according to claim 2 , wherein the coil current control unit:

is configured to set a polygon in which opposite sides are in parallel to each other and a distance between the opposite sides corresponds to a length double or less than the corresponding individual-phase upper limit value in a d-q coordinate system in which three individual-phase coil currents are represented by one vector with respect to the three individual-phase upper limit values determined for each phase by the upper limit current determination unit;

with respect to a target vector which has a length corresponding to a required torque to the motor and rotates around an origin in synchronization with the rotation of the motor, is configured to correct the length of the target vector so that a tip thereof is contained within the polygon in the case where the tip is located outside the polygon; and

is configured to control the respective individual-phase coil currents on the basis of the target vector whose length is corrected.

5. The motor control device according to claim 2 , wherein the coil current control unit:

is configured to set a circle having a radius corresponding to a minimum individual-phase upper limit value among three individual-phase upper limit values with an origin as a center and a polygon in which opposite sides are in parallel to each other and a distance between the opposite sides corresponds to a length double or less than an absolute value of the corresponding individual-phase upper limit value, in a d-q coordinate system in which three individual-phase coil currents are represented by one vector with respect to the three individual-phase upper limit values determined for each phase by the upper limit current determination unit;

with respect to a target vector which has a length corresponding to a required torque to the motor and rotates around the origin in synchronization with the rotation of the motor, is configured to correct the length of the target vector so that a tip thereof is contained within the circle in the case where the tip of the target vector is located outside the circle during a period in which the rotation speed of the three-phase motor is equal to or greater than a predetermined value and is configured to correct the length of the target vector so that the tip thereof is contained within the polygon in the case where the tip of the target vector is located outside the polygon during a period in which the rotation speed of the three-phase motor is less than the predetermined value; and

is configured to control the respective individual-phase coil currents on the basis of the target vector which has been corrected.

6. A walking assist device comprising:

a motor control device including a coil temperature detection unit which detects a coil temperature of a motor, an upper limit current determination unit which determines an upper limit value of a coil current in which a coil temperature after a lapse of a first predetermined time from a present time is maintained at an upper limit temperature or lower every time a second predetermined time shorter than the first predetermined time elapses by using a coil temperature detected by the coil temperature detection unit, and a coil current control unit which controls the coil current so as to be equal to or lower than the upper limit value, wherein the upper limit current determination unit is configured to determine the upper limit value of the coil current by using a present coil temperature as the coil temperature and using the upper limit temperature and the first predetermined time;

a joint which pivotally supports a thigh frame and a crus frame; and

an actuator which contains a motor controlled by the motor control device, drives a joint by using a driving force of the motor, and controls a mutual angle between the thigh frame and the crus frame.

7. A motor control method comprising:

a coil temperature detection step of detecting a coil temperature of a motor;

an upper limit current determination step of determining an upper limit value of a coil current in which a coil temperature after a lapse of a first predetermined time from a present time is maintained at an upper limit temperature or lower every time a second predetermined time shorter than the first predetermined time elapses by using a coil temperature detected in the coil temperature detection step, wherein the upper limit value of the coil current is determined using a present coil temperature as the coil temperature and using the upper limit temperature and the first predetermined time; and

a coil current control step of controlling the coil current so as to be equal to or lower than the upper limit value.