Material handling system and method of operating the same

A method of controlling a material handling system that lifts a load, the method comprising:

• providing a material handling system having a motor, a brake, an inverter, and a model of the motor;
• generating a signal in the inverter;
• providing the signal to the motor;
• sensing a current of the signal, the sensing occurring within the drive;
• determining a modeled torque producing current based in part on the sensed current;
• determining a modeled motor speed based in part on the sensed current;
• determining an applied torque producing current;
• determining an actual motor speed;
• calculating a first difference value between the applied torque producing current and the modeled torque producing current;
• comparing the first difference value with a first deviation amount;
• setting the brake when the first difference value is greater than the first deviation amount;
• calculating a second difference between the actual motor speed and the modeled motor speed;
• comparing the second difference with a second deviation amount;
• and
• setting the brake when the second difference is greater than the second deviation amount.

A method of sensing an unstable state of a material handling system that lifts a load, the system including a motor, a brake, and a drive, the method comprising:

• providing a material handling system having a motor, a brake, an inverter, and a model of the motor;
• generating a signal in the inverter, the signal including a voltage and a frequency;
• providing the signal to the motor;
• sensing a current of the signal, the sensing occurring within the drive;
• when one of the voltage and the frequency is less than a first percentage of a maximum voltage and a maximum frequency, respectively,
• determining a modeled current based in part on the sensed current,
• determining a magnetizing current based in part on the sensed current,
• determining the difference between the modeled current and the magnetizing current,
• determining the difference between the sensed current and the magnetizing current,
• calculating a first difference value between the applied torque producing current and the modeled torque producing current;
• comparing the first difference value with a first deviation amount;
• setting the brake when the first difference value is greater than the first deviation amount;
• when one of the voltage and the frequency is less than a second percentage of the maximum voltage and the maximum frequency, respectively,
• determining a modeled motor speed based in part on the sensed current;
• determining an actual motor speed based in part on the sensed current;
• calculating a second difference value between the actual motor speed and the modeled motor speed;
• comparing the second difference value with a second deviation amount;
• and
• setting the brake when the second difference value is greater than the second deviation amount.

A material handling system comprising:

• a lifting apparatus being connectable to a load, the lifting apparatus including a brake, and a motor;
• an inverter electrically connected to the motor and being operable to generate an inverter signal that drives the motor;
• a current sensor being operable to sense a current of the inverter signal and to generate a current signal having a relationship to the sensed current;
• and
• a controller being operable to
• receive the current signal,
• determine a modeled value of the motor based in part on the current signal,
• compare an actual value to the modeled value for determining whether the load is stable, and
• generate an output that sets the brake when the load is potentially unstable.