Medical injection device with electric motor drive control

1. A controller for controlling operation of an injection device for delivering a medicament, the controller comprising:

a drive signal generator that generates an input drive signal for driving a motor, the input drive signal for driving the motor comprising stepper pulses; and

an encoder that generates an encoder output signal indicative of an output drive of the motor the encoder output signal being a pulsed signal having a timing characteristic that corresponds to the output drive of the motor,

wherein the controller varies the operational control of the device based, at least in part, on a comparison between the input drive signal and the encoder output signal,

wherein a comparison between counts of the encoder output signal with counts of the stepper pulses of the input drive signal provides an indication of a quantum of motor slip,

wherein the motor is deliberately stalled against a predetermined reference point when the quantum of slip exceeds a predetermined threshold value, and

wherein the controller calculates a deficit in the medicament administered that is a dose remaining to be administered and performs administration of the dose remaining to be administered.

2. A controller according to claim 1 , further comprising user interface buttons.

3. A controller according to claim 1 , further comprising dose dial buttons.

4. A method of delivering a medicament to a patient using an injection device comprising a housing; a piston rod for driving a bung of a replaceable medicament container, the piston rod retractable to a backstop position; a drive mechanism including a motor for providing an output drive to the piston rod for delivering a user settable dose of the medicament and retracting the piston rod to the backstop position; and a controller that controls operation of the injection device, the controller including a drive signal generator that generates an input drive signal for driving the motor and an encoder that generates an encoder output signal indicative of the output drive of the motor, wherein the input drive signal for driving the motor comprises stepper pulses, wherein the encoder output signal is a pulsed signal having a timing characteristic that corresponds to the output drive of the motor, wherein the method comprises:

starting administration of a dose of the medicament to a patient;

determining, by the controller, a number of motor pulses and a number of encoder pulses;

varying the operational control of the device based, at least in part, on a comparison between the input drive signal and the encoder output signal including determining, by the controller, if counts of stepper pulses of the input drive signal and counts of encoder pulses differ from one another by more than a predetermined amount, wherein a comparison between counts of the encoder output signal with the counts of the stepper pulses of the input drive signal provides an indication of a quantum of motor slip;

stalling, by the controller, the motor against a predetermined reference point when the quantum of slip exceeds a predetermined threshold value;

calculating, by the controller, a deficit in the medicament administered that is a dose of medicament remaining to be administered; and

performing administration of the dose of medicament remaining to be administered.