Spotlight and control system therefor

A multifunction spotlight and control system therefor, the spotlight functions each being adjustable by a bipolar stepper motor, a control means such as a dimmer unit for enabling analogue control of the rotational positions of the motors, and a second control means such as a microprocessor which enables a multichannel serial datastream to be supplied for selectively controlling the motors.

FIELD OF THE INVENTION 
This invention concerns remotely controlled multifunction spotlights and 
control systems therefor by which one or more such spotlights can be 
controlled from a remote console. 
BACKGROUND TO THE INVENTION 
Remotely controlled multifunction spotlights and a control system for such 
lights is described in U.S. Pat. No. 4,392,187, by which the colour and 
position of the beam from each of a number of spotlights in a system, can 
be controlled from a single control console. In common with other lighting 
control systems such as that described in earlier U.S. Pat. No. 4,095,139, 
the system in 4,392,187 uses a single transmission path between the 
control console and the spotlights in the system, and serial data streams, 
each having a precursor which is only recognised by one of a plurality of 
data receivers associated with the spotlights, so that serial data for 
controlling the different spotlights can be routed automatically to the 
appropriate spotlight, via its associated receiver, without causing any 
response in the other spotlights. 
Such systems have the advantage that only a single conductor pair is needed 
around the entire system to convey the digital control signals to the 
various control devices within the different spotlights. There is thus a 
considerable saving in the amount of cabling required to set up and 
interconnect the lights--but the spotlights concerned are not suitable for 
control by analogue control signals such as may be obtained from a mixing 
desk or console, a number of potentiometers, usually in the form of linear 
potentiometers, each having a sliding control for adjusting the output 
voltage/current from a minimum to a maximum value, as required. 
In an analogue system, if six functions are to be controlled in each light, 
the control signals from six potentiometers will need to be supplied to 
each light by way of for example a six core cable with a common return 
path which may double up as one of the conductive paths providing 
operating current for the lamp. 
There are many existing installations which can produce analogue signals as 
a large number of separate channels. Complete with control console and 
wiring. However it is often desirable to be able to update or replace some 
of the spots, to enable special effects to be produced or to extend such 
systems by the addition of new spotlights, and it is one object of the 
present invention to provide an improved multifunction spotlight which can 
be controlled from either a multichannel analogue control system or a 
single wire serial data digital control system. In this way new spotlights 
can be added to an exisiting system in the full knowledge that if and when 
the multichannel console is replaced by a digital console, these new 
spotlights can still be employed in the new system. 
It is a further object of the invention to provide an improved 
multifunction spotlight for use with either type of control system, by 
which N facilities can be controlled using only n channels of control 
signals (N being greater than n)--either analogue or digital. 
It is a still further object of the invention to provide an improved 
multifunction spotlight having controllable parameters such as beam 
position and colour, which can be controlled from a conventional 
multi-channel lighting control console of the type previously only 
designed to provide one analogue control signal to an appropriate fader to 
vary the light intensity of each of a plurality of spotlights. 
It is a further object to provide a control system for the currents to a 
bipolar stepper motor to enable the transistions between stable postions 
of the armature to be achieved smoothly. 
SUMMARY OF THE INVENTION 
According to one aspect of the invention in a spotlight having n adjustable 
parameters (such as beam position and colour), each of which is adjustable 
by a biopolar electric stepper motor mounted within the spotlight housing, 
there is provided a first control signal input for the spotlight by which 
n different analogue electrical signals may be used to control the 
rotational position of the n different stepper motors, to allow remote 
control of the n different parameters, and a second control signal input 
by which a single transmission path carrying control signals as a serial 
datastream can be connected to the spotlight, and receiver means is 
provided associated with the second signal input adapted only to respond 
to one particular data stream precursor, so that the spotlight may be 
uniquely addressed from a remote control, which in known manner generates 
the appropriate precursor and transmits thereafter the requisite data as a 
serial data stream to alter one or another or all of the n controllable 
parameters of the spotlight, by using n different channels within the 
transmitted data. 
Where the parameter controlling drives in the spotlight required analogue 
signals for their control, the interface typically includes digital to 
analogue converter circuit means for generating appropriate analogue 
signal values from the incoming digital data. 
Where digital signals are required to control the parameter drives within 
the spotlight, analogue to digital converter circuit means may be provided 
to convert the incoming analogue signals on the n different control 
channels into digital signals. 
In accordance with a further aspect of the invention selection circuit 
means is provided so that if signals are received at both the analogue and 
serial data inputs, for a given parameter drive, the higher value is 
always selected and employed to determine the rotational position of the 
particular parameter drive. 
Typically the analogue signal input range required to obtain the complete 
range of values of each of the controllable parameters, is a voltage 
change from between 0 V and 10 V. In this way each parameter drive can be 
controlled by a 0-10 V analogue output from a multichannel control console 
of the type commonly found in theatres for controlling the intensity of a 
corresponding number of conventional spotlights via a bank of dimmers, 
there being one dimmer for each lamp and one control on the console 
associated with one of the dimmers--and thereby the associated lamp. 
According to a further aspect of the invention, in a spotlight adapted to 
be remotely controlled from a console by the transmission of electrical 
control signals thereto for controlling, inter alia, the movement of a 
parameter controlling element in the spotlight, from a first position to a 
second position, the drive for the element is actuated to move the element 
into the first position at values near to and at one extreme of the 
possible range of values of the control signal on the channel appropriate 
to that parameter, and into its second position by values at or near the 
other extreme of the possible range of values of the control signal for 
that channel, and circuit means is provided within the spotlight to detect 
intermediate values of the incoming signal and the rate of change of the 
signal values, thereby to generate a supplementary control signal if 
during said intermediate range of values the rate of change of value falls 
below a preselected rate for a preselected period of time, and a drive 
means in the light is adpated to be responsive to the intermediate values 
of the incoming control signal only when the said supplementaey control 
signal is generated, whereby that drive means is caused to perform a 
further controllable function within the spotlight whilst the incoming 
signal dwells in the said intermediate range of values, depending on the 
actual intermediate value which is received. 
The drive means which is rendered responsive to the intermediate signal 
values may be the self same drive to which the channel relates, the 
generation of the supplementary control signal causing the said drive to 
perform in a different way. 
Alternatively the drive controlled by the said intermediate values may be a 
totally separate drive, for another function, only energised when the 
supplementary control signal is generated. 
In a particularly preferred embodiment the element is a shutter which is 
movable into and out of the path of the beam and in the presence of the 
supplementary control signal, the shutter drive is caused to oscillate the 
shutter between its two extreme positions at a speed determined by the 
actual intermediate value of the control signal. In this way the spotlight 
may be made to strobe at a variable strobe frequency determined by the 
selected value in the intermediate range of values. 
According to a further aspect of the invention the transistion between two 
adjacent positions of the armature of a bipolar stepper motor, may be 
rendered more smooth than hitherto, by gating the changing one of the two 
currents at a high frequency such that there are M gated periods between 
the first position and the second position, and successively increasing 
the proportion of each gated period during which the changing current is 
at its new value and thereby decreasing the proportion of each interval 
during which the changing current is at its initial value, so that if the 
motor could respond, the armature would perform a series of M oscillations 
between the first and second positions, with the dwell time at and just 
after the beginning of the sequence tending towards the first position and 
the dwell time towards the end of the sequence tending more and more 
towards the second position.

DETAILED DESCRIPTION OF DRAWINGS 
The internal mechanisim of a spotlamp having various motorised facilities 
is shown in FIG. 1. The majority of the components are mounted on a 
baseplate 10 or on upstanding bulkheads which themselves are secured to 
and extend vertically from the baseplate 10 such as 12, 14 and 16. 
From left to right the assembly comprises a back plate 18 on which is 
mounted on stand offs a printed circuit board 20. A choke 22 is provided 
for a gas discharge tube 24 supported between two pairs of sprung 
conductors at each end one pair being shown at 26 which themselves are 
mounted on an insulating bridge 28. The latter is supported on a platform 
30 which extends from the first bulkhead 12. This additionally provides 
support for a concave mirror 32, typically a parabolic mirror, for 
focussing light from the lamp 24 towards a pair of condensing lenses 34 
and 36 and supported by an extension of the platform 30, which at its 
front end is carried by a bridge 38 shortly in advance of the bulkhead 14. 
A focusing lens 40 is carried in an adjustable mount 42, and between the 
lens 40 and the lens 36, mounted on the two bulkheads 14 and 16 are four 
drive motors for rotating different elements which each affect the beam of 
light which is to be transmitted from the spotlight. 
The first rotatable element is a circular disc 44 driven by a motor 46 
having a number of circular apertures circularly arranged therearound one 
of which is a simple circular aperture and the others of which contain 
different obstructions in the form of patterns to enable the beam to be 
interrupted and an image of the pattern formed in the final spot of light. 
Such a wheel is referred to as a Gobo wheel. 
Mounted on the same bulkhead 14 is a second motor 48 which drives through a 
pinion 50 the toothed outer periphery 52 of a rotatable iris diaphram, 
which can be rotated in one direction to open and in the other direction 
to generally close a circular aperture defined by the diaphram. 
As with all iris diaphrams, it is impossible to shut off the light 
completely and a further optical element is provided in the form of a disc 
or part disc 54 carried on the second bulkhead 16 and driveable by a third 
motor 56. This element includes a circular aperture through which light 
can pass in an unobstructed manner but also includes a complete shutter 
section which if rotated into the path of the beam will obscure the beam 
completely thereby preventing any light from leaving the spotlight. The 
last rotatable element is a further disc 58 similar to the Gobo wheel 44 
having a number of circular apertures therearound one of which is clear 
but the others of which contain coloured glass or plastics material for 
producing different colour beams of light. A fourth motor 60 is mounted on 
the bulkhead 16 so as to drive the colour filter wheel 58. 
Since the discharge lamp will produce a considerable amount of heat as may 
also some of the other components, a cooling fan 62 and drive motor 64 
therefor, is mounted to the rear of the housing above the choke. 
A typical gas discharge lamp is a Wotan Metallogen type HMI 575 W/GS. 
FIG. 2 shows the apparatus in FIG. 1 from above and enables a transformer 
66 and starter component 68, positioned behind the choke in FIG. 1, to be 
seen. Additionally a second printed circuit board 70 can be seen on which 
is mounted a microprocessor 72 and various other components including 
capacitors such as 74. The same reference numerals have been used in FIG. 
2 to identify the components common to the two figures. In this connection 
the other pair of sprung conductive clips (of which only one pair of 26 
can be seen in FIG. 1) are denoted by reference numeral 27 in FIG. 2. 
For simplicity the focusing lens mounting 42 and lens 40 are not shown in 
FIG. 2. 
Reference is made to FIG. 3 for the remainder of the floodlight detail. 
FIG. 3 shows a housing 76 which is adapted to be mounted at the front of 
the housing container the base plate 10 with the lens housing 42 
protruding into the housing 76. 
Within the latter is mounted a plane mirror 78 which centrally at its rear 
is mounted on one edge face of a generally rectangular motor housing 80, 
the spindle of which is non-rotatably secured to parallel limbs 82 and 84 
of a support, the base of which is attached to a rotatable shaft 86 which 
protrudes from a bearing assembly 88 carried by the opposite end of the 
housing from that through which the lens housing 42 extends. 
The shaft 86 comprises the output drive shaft of a motor 90 which itself is 
non-rotatably externally mounted on the end wall of the housing 76. 
Rotation of the motor 90 thus causes rotation of the shaft 86 and in turn 
rotation of the base or bridge 92 from which the limbs 82 and 84 extend. 
This produces rotation of the mirror (whatever its inclination) about the 
axis of the shaft 86. 
If the motor 80 is powered instead of (or in addition to) motor 90, the 
mirror is then rotated about the axis of the output shaft of the motor 80, 
which as will be seen from the drawings, will always be at right angles to 
the axis of the shaft 86. In this way the inclination of the mirror about 
the axis of the motor 80 can be controlled and the beam of light emanating 
from the lens 40 can thereby be reflected to fall on any one of a large 
number of points as determined by the rotation of the motors 80 and 90. 
FIG. 4 illustrates the shutter disc 54 of FIG. 1. An aperture 96 allows 
light to pass through in an unrestricted manner provided the aperture has 
been rotated into the path of the beam between the lens 36 and the lens 
40. All other rotational positions of the shutter 54 caused the beam to be 
cut off. 
FIG. 5 illustrates the construction of the Gobo wheel and the colour filter 
wheel. Here each wheel includes a circular array of circular apertures, of 
which one is denoted by reference numeral 98, and in the case of the Gobo 
wheel the aperture is partly obscured with opaque material defining a 
pattern such as a star or cross or pattern of small windows through which 
light can pass. In the case of the filter wheel, each aperture forms a 
window for a sheet of coloured glass or plastics or other coloured 
transparent material, so as to colour the beam of light leaving the 
spotlight. In both cases one of the apertures is left completely open so 
as to enable the wide spectrum white light from the lamp 24 to pass in an 
unrestricted manner to the lens 40. 
FIG. 6 shows a conventional slider control panel. The housing is denoted by 
reference numeral 98 and seven linear potentiometers are housed within the 
housing each having its own independent control 100, 102, 104, 106, 108, 
110 and 112 respectively. Each slider is associated with a printed display 
of numbers ranging from 0 to 10 (or some such similar range) and each of 
the first six potentiometers 100 to 110, includes a reset button (of which 
one is denoted by reference numeral 114) by which the current selected 
value for the potentiometer can be overriden and the full value for that 
potentiometer transmitted, by merely pressing the appropriate button. 
The seventh potentiometer and control 114 is in the form of a master 
control which will simply increase the signal on all channels from the 
lowest possible value up to whatever the maximum value is set by the 
slider associated with each channel, by simply increasing the master 
potentiometer controller from the 0 setting to the highest setting. 
The potentiometers are normally arranged to control the output of six 
analogue signal outputs which in turn control the value of six fader units 
which themselves control the supply of power to the lamps in six 
spotlights. In this mode the dimmer panel shown in FIG. 6 can control the 
brightness level from six spotlights, but it will be seen provides no 
signals for controlling any other parameter such as colour or beam 
position, etc. However large numbers of such dimmer control panels are 
currently installed in theatres and places of entertainment, and in 
accordance with the invention it is an object to provide floodlights which 
can be controlled by such multichannel fader controllers so as to 
eliminate the need for expensive additional control equipment to be 
purchased. The difference is that in the case of a lamp such as that shown 
in FIGS. 1 to 3, the six potentiometers 100 to 110 would in fact control 
the six functions associated with only one floodlight (assuming the latter 
has six drives associated with it as shown in FIGS. 1 to 3). Thus in a 
multichannel control system in which there may for example be sixty linear 
potentiometers of the type shown in FIG. 6, instead of controlling the 
intensity of sixty spotlights, the sixty channel control panel can now 
only control ten spotlights but in addition to controlling the intensity 
of the light, the control panel will also enable the colour of each beam, 
the position of each spot, and other parameters. 
The invention is not limited to a control producing only analogue signals, 
one for each channel, but is also capable of responding to a more modern 
control centre which produces a serial train of data, each packet of 
information in the train being preceded by a precursor or flag by which 
the packet can be identified and which will uniquely identify the 
spotlight which the packet of information is to control. Thus the control 
panel of FIG. 6 may in fact produce six digital information signals to be 
transmitted in succession after an appropriate precursor for controlling 
one spotlight using a single conductor for carrying the data to the 
spotlight, provided the spotlight includes an appropriate interface for 
first of all identifying data which is intended for it, and decoding the 
digital information so as to produce appropriate control signals for the 
motors and/or other apparatus on board the spotlight. 
FIG. 7 is an over view block circuit diagram of a complete control system 
suitable for receiving either analogue signals on six channels or digital 
information in serial form. For simplicity only one motorised drive is 
shown with control signals being generated for only that one motor drive. 
In practice six motor drives and motor output terminal blocks will be 
required. 
The control system includes six DIN socket to receive six analogue signals 
on six different channels. The sockets are denoted by reference numeral 
100. 
The outputs from the sockets are filtered by six filter circuits 102 (see 
FIG. 12), and the filtered outputs are applied to the inputs of an input 
channel selector. Details of this circuit are also shown on FIG. 12 and 
will be described later. The channel selector device is denoted by 
reference numeral 104 and the selected analogue output signal is supplied 
to an analogue to digital convertor 106 details of which are shown in FIG. 
9. The actual function of the analogue to digital convertor involves the 
use of a counter on board the particular microprocessor integrated circuit 
selected for this equipment, and as shown in FIG. 7, information is shown 
transferring from, as well as to, the microprocessor 72. 
The latter is programmed by means of a computer programme contained in an 
EPROM 108 so as to select the data on each of the six lines in sequence so 
as to control each of six motors and to route the digital information in 
the form of a series of control pulses for driving a motor via a data 
latch 110 to the appropriate motor driver 112, and then to the motor 
output terminal block 114. The microprocessor additionally selects the 
latches via the data line 116. 
The selected microprocessor is controlled by an external crystal oscillator 
118 and a voltage source for resetting the microprocessor on turn-on is 
also provided at 120. Details of both circuits 118 and 120 can be found in 
FIG. 10. 
The programme from the EPROM 108 determines the data select information 
transmitted via the data highway 122 either to the one of eight selector 
104 or the one of ten selector 124, which is manually programmable using a 
DIL switch 126. The setting of the latter determines the channel 
information supplied along line 122 to the microprocessor 72. 
If instead of analogue signals, the spotlight is to be controlled by serial 
data, the alternative signals are supplied to one of two XLR sockets 130, 
(see also FIG. 12). The other similar parallel connected socket 132 serves 
as a connection for the common serial data bus, to feed the input socket 
(equivalent to socket 130) on the next spotlight in the chain. 
The digital signals are amplified by a receiver and pulse shaping circuit 
134, before being supplied to the serial data input port of the processor 
72. 
The programme stored in EPROM 108 causes the processor 72 to respond to and 
transmit pulses which follow a precursor set up by the DIL switch 126 and 
1 of 10 selector 124. All other incoming serial data is ignored. The six 
data signals following the identified precursor are decoded by the 
processor acting in accordance with the EPROM 108 programme, and are 
transmitted as Motor Data on data highway 122, to the latch 110, and 
thereafter the motor drivers 112. Any change in the data for any 
particular motor from the preceding packet of data thus immediately 
appears as a control signal for the motor concerned to adjust the motor 
position to that determined by the new data. 
The EPROM 108 is addressed in a conventional manner using an address latch 
136. 
A power supply comprises a mains transformer 66 (see FIG. 2 and FIG. 8) a 
rectifying and smoothing and regulating circuit 138, to supply the 
different dc operating voltages for the various circuits making up the 
control system. 
Reference will now be made to the more detailed circuits in FIG. 8 onwards. 
In FIG. 8 the transformer 66 feeds a full wave bridge rectifying circuit 
which includes rectifying diodes D1-D4, a discharge resistor R10 and a 
series voltage dropping resistor R13, smoothing capacitor C1 and C3 and 
voltage regulator REG1 with feed back capacitor C2. 
FIG. 9 must be read in conjunction with FIG. 10, since the A/D converter 
circuit relies on a counter which produces an overflow signal after a 
given count value has been reached which is located in the processor 72. 
The analogue input signal (from the selected incoming line) is supplied to 
the emitter of P1. 
If the incoming analogue signal is at 0 V, the emitter of P1 will be held 
at 5 V (see the connection via the Input Channel Select IC2 and the d.c. 
amplifier P2, N2). If the incoming signal is +10 V, the effect of the 
input attenuators (see FIG. 12) will be to hold the emitter of P1 at 2.5 
V. 
The value of P1 emitter voltage will determine the time taken from the 
capacitor C4-C6 to change up and therefore the time interval between the 
start A/D signal and the stop A/D signal and therefore the number of 
pulses counted by the counter. The number of pulses will therefore be a 
measure of the value of the incoming analogue voltage. This is 
interpretted by the processor and a data stream appropriate to the 
specific function of that motor is sent to the appropriate motor driver 
circuit 1/2 via its data latch 110. 
FIG. 10 shows the connections required to the processor IC1, the address 
latch IC2 and the EPROM IC3. Also shown is the turn-on reset pulse 
generating of C7, R5 and the crystal X1 and associated capacitors C8, C9 
which determine the processor clock frequency. The selected processor is 
type 80C31 with the latching device by 74HC373. The EPROM may be a 27C16 
or 27C64. 
FIG. 11 is exemplory of 6 similar circuits each of which is connected to 
the 01, 02, 03, 05, 06 and 07 output pins of the processor IC1 via the D0, 
D1, D2, D3, D4 and D5 pins of Data Latch IC4A, which in turn provide the 
address links 01, 02, 03 and 05 from the switch selectors IC3 and IC4 of 
FIG. 12 and the 01, 02 and 03 inputs to input channel selector IC2 of FIG. 
12. 
The Data Latch IC4A is a type 40174 and latched digital data provides to 
controlling input signal to windings drivers A and B (IC5A and IC6A) each 
of which is a device type 3717. The currents to the A and B windings of a 
bipolar stepper motor (not shown) are derived from pins 1 and 15 of each 
driver device and are connected to the windings via a terminal block. 
Each of the motor winding driver circuits such as FIG. 11 includes a large 
decoupling capacitor 74 typically of 1000 micro farads. One of these six 
capacitors 74 is identified on the p.c.b. 70 in FIG. 2. 
FIG. 12 shows the connections to the pins of the DIL switch 126, the two 
4051 devces which enable a 1 from 10 selection to be made, depending on 
the input of the pins A0, A1, A2 and E. The DIL outputs for supply to the 
DIL-IN terminal of the processor 72 is obtained from the Z pins of both 
switch selectors IC3 and IC4. The serial output, for supply to the serial 
data input of the processor 72, is derived from the output of gate IC1-B 
(one half of a 4093), the strapped inputs of which are connected to the 
collector of transistor P1 in the RS232/42 receiver 14. 
The analogue signal from the A/D converter is derived from the Z output of 
selector 124 having the six input channels connected to the Y1-Y6 input 
pins. Pins Y0 and Y7 are connected to the +5 V and the 0 V lines 
respectively. 
Addressing for the selector 124 is derived from the signals from input pins 
01, 02 and 03 of the processor 72. These signals are supplied to the 
addressing inputs A0, A1 and A2 of selector 124. 
Each incoming analogue voltage is attenuated and filtered by the 
combination of one of the resistors R3-R13, one of the resistors R14-R19 
and one of the capacitors C4-C9, to the extent that the voltage across the 
capacitor C4-C9 is approximately 25% the incoming voltage. 
Each of the motors in the spotlight is a bi-polar stepper motor and 
according to the invention, the normal 200.times.1.8.degree. steps 
obtainable is increased by a factor of 4, to 0.45.degree. steps, by 
further subdividing the currents to the two windings so that between each 
pair of normal positions of the armature, corresponding to (I.sub.1,0) and 
(O,I.sub.2) there can be defined two further positions corresponding to 
(I.sub.1, 1/2I.sub.2) and (1/2I.sub.1, I.sub.2). 
These intermediate current values can be obtained by appropriate signals 
from the processor 72 to the drivers (IC5A, IC6A). This allows smoother 
movement of the driven component to be obtained, and is of benefit to the 
drives for the mirror. 
The 0.45.degree. divisions can if desired be further subdivided by 
generating signals of proportionately decreasing and increasing duration 
over a short period of time (being the time which the component is to take 
to shift through the 0.45.degree. step. Although the component will not be 
capable of responding to the resulting oscillation which should result, 
the net effect is to achieve an apparently smooth transition between each 
pair of 0.45.degree. separated positions. Thus if there are approximately 
100 control current pulses being generated per second, the smooth 
transistion can be achieved during for example 16 successive pulses, if at 
the outset of the 16 pulse periods the drive current IC to the second 
winding is increased momentarily to the value required to obtain the full 
0.45.degree. step only during the last 1/16th of the internal, and during 
the next of the 16 internals of the last 2/16ths of the interval, and so 
on, until during the 16th interval the second winding current is 
maintained at the value required to obtain the full 0.45.degree. step, 
for the whole of the period concerned. 
Two threshold values are provided in the EPROM programme for comparison 
with the setting of one of the potentiometers in the multichannel 
controller so that if the potentiometer slider is within the first 20% of 
its travel the component motor it controls is driven fully clockwise (say) 
and within the last 20% of its travel, the component motor is driven 
immediately to the maximum displacement position in the opposite (anti 
clockwise) direction. By linking this to the motor 56 which controls the 
shutter 54 the remainder of the slider travel (ie between the 20% and 80% 
positions may be used to control an auxilliary circuit for generating 
control signals for another parameter. 
Typically a time delay is built into the programme so that if the slider is 
moved quickly through the range of values between the 20% and 80% 
positions the auxillary circuit is not triggered, whereas if the operator 
slows down the movement of the slider, the dwells for more than a 
predetermined time on a value between the 20% and 80% position so the 
system will trigger the auxillary circuit to generate control signals for 
the motor 56 which will cause the motor to oscillate between its two 
extreme positions, with the rate of the oscillation (complete swings per 
second) being controlled by the precise intermediate value at which the 
slider is positioned. In this way for example, the shutter 54 can be made 
to oscillate between open and closed positions at up to 25 oscillations 
per second thereby giving a strobe effect. 
The criterion as to whether the second mode of control is to be invoked, is 
determinable by measuring the rate of change of input value (voltage or 
digital signal) with time. If this rate of change is slower than a 
predetermined rate the second mode is enabled whereas if it is above the 
predetermined rate, the second mode is inhibited.