Today, electromechanical actuator systems are used in a variety of applications, of which some are more safety critical than others. Likewise, they are used in a wide range of applications where the safety of persons often to a higher or lesser extent depends on that the actuators remains safe both during normal use and in the fault conditions, which may occur. By an actuator comprising a DC motor (2) equipped with a worm drive and a spindle (4), the typical wearing components are the bearing, which carries the spindle (4), the spindle nut (5) and the worm wheel (9) in the worm drive. Common to the described defect situations is that they in the time leading up to the occurrence of the defect will show a decrease in the ratio of efficiency. Whether it is the spindle bearing, the worm drive, the spindle nut (5) or for that matter the motor, which is getting worn, the proportion between the current which the motor absorbs and the force the actuator performs will change. The purpose of the present invention is thus to monitor the ratio of efficiency of the actuator. When this begins to change, becomes worse, it is time to replace the actuator before it becomes defective. For this purpose a load cell can for instance be incorporated in the actuator, for measuring the force on the rear mounting. This force is compared with the power consumption and an indicator for the ratio of efficiency can thus be calculated as force/current.
