as it appears from fig1 of the drawing , the main components of the actuator comprises a housing 1 with a reversible dc - motor 2 , which through a transmission in the nature of a worm gear 3 drives a spindle 4 on which is arranged a spindle nut 5 , to which is secured a tubular activation element 6 ( inner tube ) surrounded by an outer tube 7 . for suspension of the actuator in a structure the actuator is further equipped with a front fixture 8 at the front of the activation element 6 and a rear fixture 9 at the rear end of the housing 1 . for determining the end positions of the spindle nut and thereby the activation element 6 , two end stop switches 10 , 11 are arranged in mutual spaced relationship in the actuator along the spindle 5 so that the signals from the switches may be used to calibrate a position determination system for the spindle nut 5 . the end stop switches 10 , 11 are activated by the spindle nut 5 in its desired inner and outer end position . further switches 12 , 13 , 14 are arranged between the end stop switches 10 , 11 , as shown , located in known positions along the spindle 4 , and may thus indicate the positions to the position determination system . as it appears from fig2 , which shows a detailed view of the spindle nut 5 , where this is in a position between an end stop switch 11 and the further switch 14 for determining the position of the spindle nut 5 in another known position . the individual switch is activated in that the spindle nut meets a leaf spring 15 , which with one end is suspended at a holding device 16 and thus activates the switch 10 , 11 , 12 , 13 , 14 . the leaf spring 15 is designed with a longitudinal profile , which determines with how much hysteresis the switch is affected . the length of the piece 17 of the leaf spring 15 which is parallel to the spindle 4 thus indicates the two positions when the switch is closed and when it is opened again . thus two positions are indicated in the longitudinal direction of the spindle 4 at each switch 12 , 13 , 14 . as shown in fig3 the end stop and further switches 10 , 11 , 12 , 13 , 14 are connected in a network where each switch , with the exception of the end stop switches 10 , 11 , is series - connected with an electric resistance 18 , 19 , 20 . this means that the electric resistance of the network is changed by activating of a switch , so that it is possible to determine when a switch is activated and which switch it is . the resistance values may thus be adapted so that they in a voltage divider provide a voltage which is proportional to the position of the spindle nut 5 on the spindle 4 , incidentally in a manner similar as when using a potentiometer for position determination . alternatively , the position determination system is able to , based on the measured resistance value , to determine the position based on programmed table values incorporated into the software thereof . a curve form of a voltage gradient in relation to the activation and deactivation of the switches due to the travel of the spindle nut 5 on the spindle 4 between the two end stop switches 10 , 11 is shown in fig4 , where it appears that an exact position can only be read in the positions where an end stop switch 10 , 11 has just been activated or deactivated . in order to determine the position of the spindle nut 5 when it is not in the activation areas of a switch , the position determination system is equipped with a device which is able to calculate the position based on a mathematical model . fig5 shows a model of an ideal motor equivalent to the motor of the actuator , the ideal motor having an electromotive force emf and a corresponding resistance loss rs . the control and speed regulation of the actuator is performed by means of a microprocessor μp , which impresses a pulse - width modulated voltage to the motor 2 of the actuator . the microprocessor is further equipped with a feedback system which measures a set of parameters for the motor in the shape of the impressed terminal voltage v 1 and the motor current . the motor current is practically calculated based on the voltage v 2 over a small series resistance rm , incorporated into the electrical circuit . the mathematical model which includes the ratio of terminal voltage , current collection and rotational speed of the spindle is calculated based on the formula : the speed at which a spindle nut 5 will move on the spindle 4 is proportional to the rotational speed of the spindle and also depends on the pitch of the spindle . if the rotational speed of the spindle 4 is incorporated over time and an expression of the gearing , the pitch between spindle 4 and spindle nut 5 is known , the position of the spindle nut during its travel on the spindle may be calculated at any time . the model is based on measured parameters for the actuator , where the motor constant and rloss may be calculated from two sets of measurements , where the numbers are inserted into the mathematic model . often a set of parameters from a measurement where the actuator is not loaded and a measurement with a load close to the maximum performance of the actuator will be chosen . for explanation of the invention an example of two numbers from two measurements are stated in the following 2 sets of values : spindle speed 10 mm / sec , 24 v impressed and motor current almost 0 spindle speed 6 mm / sec , 24 v impressed and motor current 4 a spindle ⁢ ⁢ speed = ( motor ⁢ ⁢ terminal ⁢ ⁢ voltage - motor ⁢ ⁢ current * rloss ) * motor ⁢ ⁢ constant ⁢ 10 ⁢ mm sek = ( 24 ⁢ ⁢ v - 0 * rloss ) * motor ⁢ ⁢ constant ⁢ motor ⁢ ⁢ constant = 10 ⁢ ⁢ mm sec 24 ⁢ ⁢ v ⁢ motor ⁢ ⁢ constant = 0 , 417 ⁢ ⁢ mm sec * v if measurement 2 is inserted into the equation relating to the motor constant calculated above : in practice it will not be possible to take a no - load measurement without any power consumption . the no - load current is here fixed at zero in order to simplify the explanation . in practice it would be expedient to consider even a very small current in the calculation in order to , in overall terms , achieve a more accurate calculation of the position of the spindle nut , but the method is incidentally the same . thus it is achieved to calculate a conversion ratio between motor voltage , motor current and rotational speed of the spindle , which may be used in the continuous calculation of the position of the spindle nut . an actuator with a length of stroke of 200 mm , it will , when the actuator is not loaded , take 20 seconds to move the spindle nut over the full length . the integral emf is thus : the position may now at any time be calculated based on an integral emf . if the loaded actuator is operated for 10 seconds while being loaded to a current of 2 [ a ], the integral emf may be calculated : it thus appears that the model can take into account the effect a load has on the speed of the actuator meaning that the calculation of the position is reliable during all loads of the actuator . even when the mechanical parts of the actuator are worn and thus create more mechanical friction , the model can be used , as the friction when taken into the calculation will be regarded by the model as a larger mechanical load of the actuator , as the model does not distinguish between inner and outer load . the position calculation is relative as it is calculated based on a known position and a contribution to the change of position is calculated every millisecond , so that a new and updated position deviation is thus achieved every millisecond . this also means that the variation in the load of the actuator occurring during operation will be included . this particularly applies to conditions during start - up and stop , where the load during e . g . start - up is larger until the actuator has reached full speed . the software manages the position and stores it when the supply voltage is interrupted . to remove possible errors which may mount up , the software calibrates the position every time the spindle nut reaches a known position on the length of the spindle . a known position is as a minimum one of the two end stops , but the use of further switches 12 , 13 , 14 , located in known positions along the length of the spindle , increases the accuracy of the position indication . it is thus preferred that the further switches are located strategically , so that they during normal operation from time to time are activated by the spindle nut . if a further switch e . g . has an activation point positioned 82 mm in on the length of the spindle and a deactivation point 86 mm in on the length of the spindle , depending on in which direction the spindle nut moves , the software should calibrate the calculated position of the spindle nut with these absolute position indications when it passes the absolute switch points . the term “ microprocessor ” used in the description covers any unit , capable of meeting the requirements for data processing performed by the microprocessor mentioned in the description . the mentioned software is portions of program code which in connection with the processing in the microprocessor manages a control and operation function in the control of the actuator .