Control circuit for a servo-motor

The invention relates to a control circuit for a servomotor of the type having a piston displaceable to selectable positions in one direction by an expansible substance heatable by an electric heating element and in the other direction by a return spring. The circuit includes a pulsating power supply and a control switch and a thermostatic unit for generating a temperature responsive control voltage. A potentiometer for generating voltages corresponding to different positions of the piston is operably connected to the piston. A comparator for comparing the control voltage with the potentiometer voltage generates a go-no go signal which is relayed to the heating element control switch. Upper and lower limit switches are associated with the selectable positions of the piston. The limit switches selectively override the potentiometer to effect operation of the heating element control switch. Light emitting diodes are provided which give visual indications of the heating and cooling cycles.

The invention relates to a control circuit for a servo-motor comprising a 
piston displaceable into selectable positions in the one direction by an 
expansible substance heatable by an electric heating element and in the 
other direction by a return spring, in which a control switch in series 
with the heating element is actuatable according to the piston position. 
In a known control circuit of this kind, the control switch responsive to 
the piston position consists of a stationary contact and a movable contact 
which is mounted on a stationary mounted lever connected to the piston 
through a friction clutch. The pivotal motion of the lever is limited by 
two stationary abutments. This control switch permits the piston to be 
held in a selected position with a very small amplitude of oscillation 
because the control switch is closed upon cooling-off movement and opened 
upon heating movement. To change the selectable position, there is a 
first, normally open, position switch parallel to the control switch and a 
second, normally closed, position switch in series with the entire 
arrangement. By closing the first position switch or by opening the second 
position switch, the piston can be brought into a new position, the piston 
being moved by the lever that is being held by one of the stationary 
abutments. 
In another known control circuit, two limit switches are provided of which 
one switches off the energy supply to the heating element as soon as a 
predetermined limiting position has been exceeded during the heating 
movement and of which the other switches on the energy supply to the 
heating element as soon as a predetermined limiting position has been 
passed during the cooling-off movement. In particular, the limiting 
positions can be prescribed by fixed abutments and actuation of the limit 
switches can be effected in that two piston rod portions interconnected by 
prestressed springs are moved relatively to one another. 
The invention is based on the problem of providing a control circuit of the 
aforementioned kind, with the aid of which the number of required switches 
can be reduced and a very simple displacement of the piston effected. 
This problem is solved according to the invention in that the selectable 
position is predetermined by a control voltage, that the piston is 
connected to the movable part of a potentiometer to signal the position of 
the piston, that a comparator compares the voltage at the potentiometer 
tapping with a voltage depending on the control voltage, and that the 
control switch is controllable according to the result of the comparison. 
With this circuit it is sufficient to have a single control switch lying in 
series with the heating element because this switch can be so actuated 
that the piston stands still or moves in the one or other direction. By 
comparing the voltage at the potentiometer tapping with the voltage 
depending on the control voltage, one not only ensures that the piston is 
displaced up to the selected position, this automatically resulting in the 
function of the two known position switches, but also that, without making 
any additional provisions, as much heating energy is supplied by switching 
the control switch off and on that the piston remains in the selected 
position. Since each value of the control voltage corresponds to a certain 
piston position, a servo-motor equipped in this manner is ideally suited 
to automatic control circuits in which a control signal that represents a 
measure of the piston position is obtained from any measuring or guiding 
parameter. For example, such a servo-motor can be used to displace valves 
that are provided in a heating, cooling or air-conditioning installation 
and are controlled in response to a temperature senser. The voltage 
depending on the control voltage can in special cases also be the control 
voltage itself. 
It is of particular advantage if the control voltage is an adjustable but 
constant D.C. voltage and the potentiometer is fed with a pulsating D.C. 
voltage. Since the voltage at the potentiometer tapping will then likewise 
pulsate, signals for opening and closing the switch will alternately occur 
at the comparator output in the position of equilibrium. In this way the 
heating element is supplied with just enough energy to ensure that the 
piston remains still in the elevated position, i.e. does not oscillate 
about a selected mean position. By reason of the inertia of the thermally 
operating parts, a really small frequency of pulsation is adequate, 
certainly the frequency of 50 Hz of industrial alternating currents. 
It is also desirable to provide two limit switches which are each 
actuatable in one limiting position of the piston and with which the 
voltage at the potentiometer tapping can be switched to a value below the 
lower limiting value of above the upper limiting value of the voltage that 
depends on the control voltage. Since these limit switches are not 
disposed in the circuit of the heating element but need merely influence 
relatively small currents in the control circuit, they can be of a very 
simple and cheap construction. 
A very simple circuit is obtained if one side of the tapping an end section 
of the potentiometer is in parallel with the one, normally open, limit 
switch and the parallel circuit is in series with the other, normally 
closed, limit switch. 
With particular advantage, an electric control switch is used of which the 
control electrode is connected to the output of an amplifier controlled by 
the comparator. Such an electrode control switch not only creates little 
noise but also operates relatively rapidly, as is the case when the 
pulsating D.C. voltage is derived from the normal A.C. voltage at the 
potentiometer tapping. In particular, the electronic control switch may be 
a thyristor. Thyristors can conduct comparatively large currents and are 
easy to control. 
In a preferred embodiment, it is ensured that the commparator is formed by 
a first transistor to the base of which the voltage at the potentiometer 
tapping is applied and to the emitter of which the voltage depending on 
the control voltage is applied by way of a resistance. Since the control 
switch knows only two switching conditions, it is sufficient to recognize 
at the comparator output whether the voltage at the potentiometer tapping 
is smaller or larger than the control voltage; this is indicated by the 
conducting or non-conducting condition of the first transistor. 
In this case the base and emitter should be connected by a diode poled in 
the opposite sense to the emitter-base path. The blocking voltage of the 
diode ensures that the base-emitter path of the first transistor cannot be 
loaded beyond a voltage of 0.6 V. 
Further, a first fixed voltage divider having its tapping connected to the 
emitter can be in shunt with the potentiometer. The operating point of the 
first transistor is fixed with the aid of this voltage divider. 
In a preferred embodiment, the first transistor is connected by a 
Darlington connection to a second transistor of which the 
collector-emitter path bridges the one resistance of a second fixed 
voltage divider and the control electrode of the electronic control switch 
is connected to the tapping of this voltage divider. In addition, the 
collector-emitter path of the second transistor can be in shunt with a 
condenser. When the second transistor blocks, a voltage determined by the 
voltage divider is applied to the control electrode of the electronic 
switch, whereby the electronic control switch opens. On the other hand, if 
the second transistor is conducting, the control path of the electronic 
control switch is practically short-circuited and this switch therefore 
blocks. The condenser keeps random voltage peaks away from the control 
electrode of the electronic control switch. 
It is favourable if, upon connection to an A.C. voltage source, the 
potentiometer is energized by way of a full-wave rectifier and a 
subsequent partial smoothing circuit, e.g. with a longitudinal diode and 
transverse condenser. In this way one obtains a partially smoothed, 
full-wave rectified voltage which is applied to the potentiometer as a 
pulsating D.C. voltage with a pulsating frequency of 100 Hz. 
It is advisable if the series circuit of the heating element and control 
switch branches off between the full-wave rectifier and partial smoothing 
circuit. In this way existing circuit components are used to ensure that 
the current through the electronic switch becomes zero after each half 
wave. Using a thyristor ensures that it extinguishes again after each half 
wave. 
Further, between the heating element and control switch there may branch 
off an indicating line leading to a point between two series-connected, 
like-poled luminous diodes and the series circuit of the luminous diodes 
is in shunt with the series circuit of heating element and control switch. 
If the one luminous diode lights up, the piston moves in the one direction 
and if the other luminous diode lights up the piston moves in the other 
direction, the piston being at a standstill if both luminous diodes light 
up.

A transformer T is connectible on the primary side to a conventional A.C. 
mains of 220 V and 50 Hz. On the secondary side one obtains an A.C. 
voltage of 24 V. This is rectified by means of a rectifier bridge 
comprising the diodes D1, D2, D3 and D4. A full-wave rectified voltage U1 
therefore exists between the two lines 1 and 2. A longitudinal resistance 
R1, a longitudinal diode D5 and a transverse condenser C1 form a partial 
smoothing circuit so that a pulsating D.C. voltage U2 is provided between 
the line section 5 and the line 2. 
The voltage U2 feeds a potentiometer P consisting of a fixed resistance R2, 
a resistance R3 with adjustable tapping 6 and a fixed resistance R4. 
A comparator 7 comprises a first transistor Tr1 of which the base is 
connected to the tapping 6 and is connected to the line 2 through a 
resistance R5. The emitter is connected to the tapping 8 of a voltage 
divider consisting of the resistances R6 and R7. It is in addition 
connected to the base through a diode D6. Finally, a control voltage U3 
produced by a control device 10 is applied through a line 9 with a 
resistance R8. The control device 10 can be of any desired kind. For 
example, it is influenced by a senser 11 so that the control voltae U3 is 
a function of the measured temperature. This control device 10 can, as 
shown, be energised through the conductors 1 and 2 and may possibly have a 
smoothing circuit. 
The transistor Tr1 is connected to a transistor Tr2 by a Darlington 
connection. Accordingly, the base of the transistor Tr2 is connected to 
the collector of the transistor Tr1 and the emitter is connected to the 
line 2. The collector is connected to a tapping 12 of a voltage divider 
consisting of the resistances R9 and R10, of which the latter is bridged 
by a condenser C2. This voltage divider is energised by the voltage U1 
taking into account the voltage drop at R1. Similarly, the voltage U1 
energises a series circuit consisting of a heating element 13 of a 
servo-motor 14 and a controllable rectifier or thyristor E, e.g. an SCR. 
The control electrode 15 of the thyristor E is connected to the tapping 
12. 
The voltage U1 further energises a series circuit consisting of a luminous 
diode LD1, a resistance R11, a further resistance R12 and a second 
luminous diode LD2. A point 16 between the two resistances is connected to 
a point 17 between the heating element 13 and the thyristor E. 
The heating element 13 consists of an expansible substance 18; on expansion 
of the latter, a piston 20 guided in a housing 19 is pressed towards the 
left-hand side against the force of a spring 21. This piston can carry any 
element to be actuated, e.g. the closure member of a valve. The piston 20 
is mechanically connected to the tapping 6. In the inner limiting position 
22, it actuates a first limit switch 23 which is normally open and bridges 
the resistance R2 of the potentiometer P. In the outer limiting position 
24, it actuates a second limit switch 25 which is normally closed and in 
series with the potentiometer P. This circuit operates in the following 
manner: The base voltage U4 of the transistor Tr1 is equal to the voltage 
at the tapping 6 of the potentiometer P and proportional to the voltage 
U2. In the absence of the control voltage U3, the emitter voltage U5 is 
determined solely by the ratio of the voltage divider R6, R7. In the 
presence of the control voltage U3, the emitter voltage U5 varies 
according to the control voltage. If the base voltage U4 is larger than 
the emitter voltage U5, the transistor Tr1 is blocked. As a result, the 
transistor Tr2 is also blocked. A fraction of the voltage U1 as 
predetermined by the voltage divider R9, R10 is therefore applied to the 
tapping 12 and thus to the control electrode 15. The thyristor E ignites 
shortly after commencement of each half wave and remains conductive until 
zero is passed on the next occasion. The servo-motor 14 is therefore 
supplied with heating energy. The piston 20 is displaced to the left and 
takes the tapping 6 downwardly with it. If the base voltage U4 is smaller 
than the emitter voltage U5, the transistors Tr1 and Tr2 are conductive. 
The resistance R10 is practically short-circuited and the control 
electrode 15 practically has the potential of the line 2. The thyristor E 
consequently blocks. Since the heating element 13 receives no energy, the 
expansible substance 18 cools off and the piston 20 is pushed to the right 
under the action of the return spring 21 and thereby takes the tapping 6 
with it upwardly. 
If all the voltages were to be constant D.C. voltages and the electronic 
switch were to be switching transistor which is conductive only whilst the 
transistor Tr2 blocks, the piston 20 would continuously hunt to and from a 
position determined by the control voltage U3. This is because when the 
tapping 6 exceeds a predetermined position during cooling off, the switch 
is switched on. If, during subsequent heating, the tapping 6 falls below 
the said position, it is switched off again. 
In the present circuit, however, the potentiometer P is supplied with a 
pulsating D.C. voltage U2 and the electronic switch is a thyristor. As a 
result, the correspondingly pulsating base voltage U4 will not intersect 
the constant emitter voltage U5 already on commencement of each half wave 
of the voltage U1 but only after a certain time delay. One therefore 
obtains at the thyristor E a phase commencement control by which the 
heating element 13 is supplied with just sufficient heating energy as is 
lost by radiation. The piston 20 therefore remains at a standstill. In 
this respect it is immaterial whether the piston 20 has been brought to 
the selected new position during changing of the control voltage U3 by 
cooling off or by heating. 
If the piston 20 falls below the lower limiting position 22, the limit 
switch 23 is closed. The resistance R2 is thereby made ineffective and the 
base voltage U4 is artificially increased. The transistors Tr1 and Tr2 
therefore block and the thyristor E becomes conductive until the lower 
limiting position 22 is exceeded again and the limit switch 23 opens. If 
the upper limiting position 24 is exceeded, the limit switch 25 opens. The 
base voltage U4 falls to the potential of the line 2. The transistors Tr1 
and Tr2 become conductive and the thyristor E is blocked until, as a 
result of cooling off, the upper limiting position 24 is again passed and 
the limit switch 25 closes. This function can repeat itself in the 
respective limiting positions. 
If the heating coil 13 is heated, the voltage at the point 17 is so low 
that the luminous diode LD2 will not ignite. The luminous diode LD1 
therefore indicates the outward movement of the piston 20. If the 
thyristor E is blocked, only the luminous diode LD2 will light up and 
therefore indicate the inward movement of the piston 20. If the piston 20 
is at a standstill, the thyristor E is blocked in a first portion of each 
half-wave impulse of the voltage U1 and conductive in a second portion. 
The luminous diodes are therefore energised alternately. Since this takes 
place very rapidly, both luminous diodes appear to light up, which 
therefore indicates the standstill of the piston. 
In a practical embodiment, the voltage U1 had an amplitude of 24 V whilst 
the control voltage U3 varied between 4 and 20 V. At an ambient 
temperature of 20.degree. C, both the heating stroke and the cooling-off 
stroke amounting to a total of 25 mm was traversed in 160 seconds. Voltage 
variations and changes in the ambient temperature lead to a change of the 
time required to execute the complete stroke but do not effect the basic 
functionability. In particular, the servo-motor is useful in a range of 
-10.degree. C to +55.degree. C of the ambient temperature. Voltage 
variations of +10% and -15% can also be readily accepted. 
The limit switches 23 and 25 can also be power switches which are actuated 
when, on the piston 20 striking a terminal abutment, a spring is 
compressed on further heating or expanded on further cooling off. Such a 
limit switch does not have to be specially set for each application, e.g. 
actuation of valves having different strokes. The limit switches can also 
be in series or shunt with the electronic switch.