An electric servo-drive for adjusting a control element between an open position and a closed position. The tightness of the control element in the closed position can be assured without any additional tightening device. Starting signals are issued to the drive motor by a control unit after the control element reaches the closed position, thus causing the control element to be driven further in the direction of the closed position by the drive motor.

BACKGROUND INFORMATION 
The present invention is directed to an electric servo-drive, and in 
particular to an electric servo-drive that includes an adjustable control 
element that can be switched by a drive motor between an open position and 
a closed position. 
When coupled to pipeline fittings such servo-drives are operable to cut off 
the proper pipeline when the fitting is in the closed position. For 
example, there are applications where the pipeline fitting is closed when 
a temperature exceeds a predetermined threshold. Servo-drives are usually 
designed so that for the closing operation, the drive motor of the 
servo-drive is stopped at a torque sufficient to close the fitting 
tightly. After a fitting which is hot is closed, the fitting subsequently 
cools down. As the fitting cools, the connecting parts between the 
servo-drive and the fitting shrink, thus reducing the tightness of the 
fitting achieved in the high temperature state. As a result of this 
cooling, the fitting may develop a leak. 
To maintain a continuous pressure when the valve is closed, European Patent 
A 327,676 discloses a design with a spring pack which is put under tension 
by the drive motor when approaching the closed position of the valve. The 
spring pack maintains the pressure for the valve after the motor is shut 
down. Such a spring pack also requires a great deal of space. 
Thus, there is a need to improve on an electric servo-drive to ensure that 
such a fitting will not leak during or after cooling, without requiring 
additional fixtures. 
SUMMARY OF THE INVENTION 
An electric servo-drive according to a representative embodiment of the 
present invention provides starting signals to the drive motor at 
intervals by a control program after the control element reaches the 
closed position, thus further driving the control element by the drive 
motor in the direction of the closed position. The starting signals 
generated at such intervals cause the fitting to be repeatedly driven by 
the drive motor into the closed position, so that when the tightness of 
the fitting declines due to cooling, the fitting can be repeatedly 
tightened to its closed position again. 
In the simplest type of control, the intervals between the control signals 
are preset by the control program. 
In order to control the drive motor in accordance with actual conditions, 
at least one sensor element is provided to detect a change in position 
occurring in the control element in response to a starting signal that has 
been provided to the drive motor. The interval for the next starting 
signal is varied as a function of this change in position. Thus, for 
example, with a relatively great change in position measured on the 
control element, the next starting signal is delivered after a relatively 
short interval, and as the changes in position of the control element 
become smaller, the interval between starting signals becomes 
progressively longer until finally no starting signals are issued when no 
change in position can be detected. 
When the drive motor is controlled in accordance with the thermal time 
constant of the fitting, the starting signals are issued at short 
intervals at the start of the cooling phase when the temperature drops 
steeply. As the steepness of the temperature curve declines, the intervals 
become longer accordingly until finally the starting signals cease 
entirely.

DETAILED DESCRIPTION 
Fitting 1 includes a control element, such as a valve disk 2, that can be 
raised and lowered by a driving element, such as actuating spindle 3. 
Actuating spindle 3 is operated by drive motor 5 via gear 4. Drive motor 5 
is associated with control unit 6, which issues starting signals to the 
drive motor 5 according to a certain control program. 
Sensor element 7 can be provided for drive motor 5 to detect the angle of 
rotation of the rotor or the shaft, which is a measure of the change in 
position of valve disk 2. Sensor element 7 then transmits signals to 
control unit 6 in accordance with the change in position of valve disk 2. 
These signals control the intervals between the starting signals delivered 
by control unit 6 to drive motor 5 so that the interval between starting 
signals is small when the change in position is relatively great, and as 
the change in position becomes smaller the interval increases until no 
starting signals at all are delivered when there is a "zero" change in 
position. 
When a fitting is closed under high temperature conditions, the fitting 
cools according to an exponential function, i.e., the cooling curve is 
steepest immediately after the fitting closes. Due to this steep cooling, 
the pressure on valve disk 2 also drops greatly, so there is a relatively 
great change in position of valve disk 2 when it is retightened by drive 
motor 5. Thus, the next starting signal is issued in a very short interval 
and the valve disk is tightened again. This assures tight closing of valve 
disk 2 even during the steep cooling phase. 
Instead of detecting the change in position, the temperature of the fitting 
can also be detected by a temperature sensor, and the control program can 
be designed on the basis of the temperature. For example, the information 
provided by the temperature sensor can be used to determine the thermal 
time constant of the control element, and the control program can be 
designed on the basis of this thermal time constant. 
If drive motor 5 is supplied with power via a converter, it is possible to 
adjust the stalling torque of the drive motor by regulating the voltage 
level and the frequency of the voltage supply such that the stalling 
torque corresponds to the tightening torque of valve disk 2. Then drive 
motor 5 will come to a standstill again on reaching the required 
tightening torque. In addition, there is no danger of overtightening valve 
disk 2.