Resistance determination in an electric motor assembly

An electric motor assembly, comprising:

• an electric motor, comprising brushes and sections of a commutator ring, wherein positions of the sections and the brushes change relative to each other during rotation of the motor;
• a current sensor, coupled to the electric motor to measure an electric current to the electric motor;
• a temperature sensor, thermally coupled to the electric motor;
• and
• a processing circuit with inputs coupled to outputs of the current sensor and the temperature sensor, the processing circuit being configured to determine a motor resistance value by:
• determining predicted values of the motor resistance for a first position and a second position of the sections relative to the brushes, at least one of the brushes contacting different numbers of the sections in the first position and the second position respectively, the predicted values being based on a temperature value measured by the temperature sensor and a predetermined dependence of variation of the motor resistance dependent on the temperature value;
• determining an electric current value measured by the current sensor, when the electric motor is at least substantially at standstill;
• selecting between different ways for determining the motor resistance value using the electric current value, dependent on which of the predicted values of the motor resistance most closely corresponds to the electric current value;
• and
• determining the motor resistance using said electric current value that was selected using the electric current value based on which of the predicted values of the motor resistance most closely corresponds to the electric current value;
• wherein the processing circuit is configured to count ripples in the electric current to the electric motor during rotation of the electric motor and to compute a progressive integral of V−I*R, wherein V is the supply voltage of the electric motor, I is the current to the electric motor and R is the motor resistance value that has been determined, wherein the processing circuit increases the ripple count once for each time interval wherein a value of the progressive integral is between a first threshold and a second threshold, and reduces the value of the progressive integral by a value of the progressive integral at a time point in the time interval where the processing circuit detects a ripple in the current, or by a predetermined value when the processing circuit does not detects a ripple in the current in the time interval.