Method and apparatus for estimating system inertia where number of motor rotations are restricted

A method for estimating inertia of an electric motor that drives a load wherein the load has a limited range of load travel, the method comprising the steps of:

• identifying an intermediate position along the range of load travel that is separated from a first end of the range of load travel;
• receiving a sensor signal from a sensor attached to the electric motor;
• with the load separated from a first end of the range of load travel, accelerating the electric motor from a first velocity toward a higher velocity as the load travels toward the first end of the range of load travel;
• acquiring a plurality of torque samples while the electric motor is accelerating, each torque sample representing an amount of torque generated by the electric motor;
• identifying when the load has reached the intermediate position;
• identifying the motor velocity at the intermediate position as a second velocity;
• calculating a first torque average by averaging the plurality of torque samples as the motor velocity changes from the first velocity to the second velocity;
• determining a first rate of velocity change from the first velocity to the second velocity;
• decelerating the electric motor from a third velocity to a fourth velocity;
• acquiring a plurality of torque samples while the electric motor is decelerating, each torque sample representing an amount of torque generated by the electric motor;
• calculating a second torque average by averaging the plurality of torque samples as the motor velocity changes from the third velocity to the fourth velocity;
• determining a second rate of velocity change from the third velocity to the fourth velocity;
• and
• deriving an inertia value as a function of the first and second rates of velocity change and the first and second average amounts of torque.

An apparatus for estimating inertia of an electric motor that drives a load wherein the load has a limited range of load travel, the apparatus comprising:

• a controller that, with the load separated from a first end of the range of load travel, increases a velocity of the electric motor from a first velocity toward a test velocity as the load travels form an initial position toward a first end of the range of load travel;
• an inertia processor programmed to:
• (i) acquire a plurality of torque samples while the electric motor is changing velocity, each torque sample representing an amount of torque generated by the electric motor;
• (ii) determine when the load has reached an intermediate position between the initial position and the first end of the range of load travel;
• (iii) identify the motor velocity at the intermediate position as a second velocity;
• (iv) determine a first rate of velocity change from the first velocity to the second velocity;
• (v) calculate a first torque average by averaging the plurality of torque samples;
• and
• (vi) derive an inertia value as a function of the first rate of velocity change and the first average amount of torque.