The invention relates to an apparatus for controlling a valve between a number of fixed positions of the type which comprises a magnetic valve which supplies an amount of fluid to a pressure cylinder which activates the valve.

In chemical industry it is usual to activate valves by means of pneumatic or hydraulic cylinders which are controlled by magnetic valves. Within certain industries there is a requirement for monitoring signals to allow determining that the valve has moved mechanically to the desired position so that it is either open or closed or has a desired position between these extremes. These signals are typically generated by microswitches which may be fitted so that they have a well-defined position at closed, open, or the desired position therebetween. In the case that a monitoring signal has to be given for "fully open" and "fully closed" two microswitches are used. Inductive sensors, possibly of the on/off type, may also be used. The purpose of the monitoring is to create a certainty that the valve has indeed been activated to the desired posi¬ tion. The requirement for the precision of the valve position may be in the order of magnitude of 0.1 mm, in particular if the system is also supposed to catch faults in the valve seat gasket or a totally missing valve seat gasket.

The present system is very difficult and time- consuming to adjust, since it consists in moving to or several microswitches upon activation to each position. Wear of the valve and the valve seat gasket require frequent adjustment. The mechanical construction requires instrument quality adjusment facilities. Valves which operate at changing temperatures similarly need different adjustments dependent on temperature, and in ^■ ' particular this problem has been difficult to solve

until now.

It is the purpose of the invention to provide an apparatus which solves the desired tasks without the disadvantages mentioned. This is obtained in an apparatus according to the ivention which is particular in that the valve stem is provided with a linear position indicator which gives a series of readings during the movement of the valve stem, of which the readings relating to the desired fixed position are stored in a data store during a first activation of the valve, whereby the result of subsequent activations of the valve is determined by comparing the instant reading with a stored reading for a desired fixed position. A position indicator may be e.g. of the type difference transformer which is non-contacting, or optical which is based on optically reading a coded rod. Thereby, instead of a manual adjustment of microswitches it is a question of determining position data corresponding to the desired positions and storing them as a future reference. Thus it is possible to let a calculating unit place an uncertainty interval around the position indication to be obtained, and similarly the determination of slow changes in the position indication for the fixed points developed during many activations of the valve in question may enable automatic compensa¬ tion for wear and/or settling of the valve seat gasket. One may actually perform a statistical analysis of the position indication relating to the extreme position obtained, in order that slow drift is compensated or accepted while a sudden change causes an alarm. Furthermore the whole data stream of position indications can be monitored during an activation, in that a certain time is provided for the change from one position to another, and an instant report may be had in case the valve is not activated in a regular fashion. Similarly it becomes possible to compensate by calculation the influence of temperature on the

dimensions of the valve, in case the position monotor is fitted together with a temperature sensor.

In practice the whole monitoring system is cast into an insulating and sealing compound, and in this case one will use a non-contacting system. In this manner a very stable construction is obtained which has a much higher probability of error-free survival in the frequently aggressive environment in which the pressure cylinder activated valves discussed are used. The invention will be described in detail with reference to the drawing which schematically shows a valve 1 which is activated by means of a pressure cylinder 2 which is supplied with air under pressure via a magnetic valve 3. In this case the pressure cylinder is single acting with a return spring, but it could equally well be double acting. The valve stem 4 is shown extended through the pressure cylinder, and it carries a non-contacting position indicator, on the output of which signals appear which represent the position of the valve stem, so that there is e.g. 1000 divisions within the travel of the valve stem. The signals are taken, e.g. via a twin lead field bus 6, to a data logger 7 which in conjunction with a control program in a monitoring and control unit 8 ascertains whether the command which was initiated by activating the magnetic valve 3 has in fact been carried out. In case the desired activation of the valve 1 does not occur correctly, an error signal appears on the output 9 on the monitoring and control unit 8.