// PID parameters
    int const Kp = (int)( 10.0000 * (1 << 16) ) ;
    int const Ki = (int)(  1.0000 * (1 << 16) ) ;
    int const Kd = (int)(  0.0000 * (1 << 16) ) ;

    uint32_t const reference_position = *position_var; // initial reading from decoder

    int control_signal = 0;
    int prev_error = 0;
    int prev_error_change = 0;
    unsigned signal_index = 0;

    while (true) {
        // wait for and receive load cell measurement
        int16_t force = receive_measurement();

        // sample motor position decoder
        uint32_t position = *position_var;
        int relative_position = position - reference_position;

        // get target position
        int input_signal = shmem.signal[ signal_index++ ];
        if( signal_index == sizeof(shmem.signal)/sizeof(shmem.signal[0]) )
            signal_index = 0;

        // PID control loop
        int error = input_signal - relative_position;
        int error_change = error - prev_error;
        int error_change_change = error_change - prev_error_change;
        control_signal += Ki * error + Kp * error_change + Kd * error_change_change;
        prev_error = error;
        prev_error_change = error_change;

        // update motor output
        if (control_signal < 0) {
            // place lower bound on control signal
            if( control_signal < -(4000 << 16) )
                control_signal = -(4000 << 16);
            EPWM1A = (-control_signal) >> 16;
            gpio_set_one_low( &GPIO0, 31 );
        } else {
            // place upper bound on control signal
            if (control_signal > (4000 << 16))
                control_signal = (4000 << 16);
            EPWM1A = control_signal >> 16;
            gpio_set_one_high(&GPIO0,31);
        }

        // report to python
        send_message( ++id, force, position, input_signal, control_signal );
    }