Method of accurately setting the flow rate of a variable-flow metering pump, and a metering pump employing the method

A pump in accordance with the invention has moving equipment (32) driven by a reversible synchronous motor (42) whose windings (44, 45) are connected to a power source by means of optocoupled triacs (48, 49) which, under microprocessor control, are used to adjust the flow rate of the pump by acting on its admission period (by powering the windings in one direction) and on the pumping cycle time (by varying the period for which the pump is unpowered in each cycle).

A "metering" pump delivers an accurately determined quantity of pumped 
fluid, and a variable-flow metering pump may have its flow rate varied 
under manual control, either directly or indirectly via a servomotor, or 
else by applying a suitable electric or pneumatic control signal thereto. 
BACKGROUND OF THE INVENTION 
In most prior systems the flow rate is varied either by acting on the 
operating speed (i.e. the number of pump cycles per unit time) while 
keeping the fluid flow rate per pump cycle constant, or by varying the 
fluid flow rate per cycle while keeping the operating speed constant. More 
rarely, the flow rate is adjusted by acting in combined manner both on the 
operating speed and on the flow rate per cycle, as is the case for 
electromagnetic pumps in which timing circuits act both on the frequency 
of electromagnet excitation periods and also on the stroke of the 
electromagnet. Such systems require two data inputs to the pump, i.e. two 
manual controls or two distinct signal inputs. 
The invention is directed more particularly to such known pumps in which a 
fluid is caused to flow as the result of reciprocating motion of a piston 
or of cyclical deformation of a mechanically-actuated membrane. 
When the pumping member is a piston, the volumetric displacement of the 
pump is exactly proportional to the stroke of the piston and the resulting 
flow rate is proportional to said stroke and to the pumping speed. When 
the pumping member is a mechanically-actuated membrane, a non-linear 
relationship exists between the volume swept by the membrane and the 
displacement of the membrane-actuating member which is connected to the 
center of the membrane. This relationship is complex and depends, in 
particular, on the manner in which the actuator member is connected to the 
membrane, on the physical nature of the membrane, on the shape of the 
membrane seating surfaces, and also on the operating speed. The 
relationship is generally determined experimentally. 
In pumps of the type to which the invention applies, the means for 
adjusting the volumetric displacement are generally situated in the 
members for transmitting motor drive to the piston or the membrane 
actuator, and such means considerably complicate the transmission and make 
it difficult to transmit forces properly. In addition, if such means are 
operated automatically it is necessary to supply a non-negligible quantity 
of power to operate them, and as a result the equipment is expensive and 
its flexibility in use is reduced relative to devices which can be 
controlled by small currents using cheap components. 
The present invention is the result of researoh into means for making it 
possible to construct a simple variable-flow metering pump using 
commonly-available components as much as possible, while still obtaining 
flow rates which can be adjusted with an error of less than one percent. 
The present invention provides a method of setting a determined flow rate 
which enables the metering pump to be driven by a synchronous motor which 
is itself powered by AC at a fixed frequency, which includes an extremely 
simple transmission mechanism without any need for a drive-varying device, 
and which uses an electronic control circuit associated with a 
microprocessor to provide highly flexible operation at low cost, where all 
of the above factors are highly advantageous in providing such a variable 
flow metering pump at low cost. The invention also provides a pump which 
implements the method. 
SUMMARY OF THE INVENTION 
In a first aspect, the present invention provides a method of accurately 
setting the flow rate of a variable-flow metering pump, said setting being 
expressed as a fraction of the pump's maximum flow rate, and said pump 
including a pumping member coupled to a transmission element driven with a 
rectilinear reciprocating motion and creating a direct and positive link 
between said pumping member and a reversible synchronous electric motor 
which is powered by AC at fixed frequency. 
According to one of the main characteristics of the invention the method 
consists in using a microprocessor to control the power supply to the 
drive motor for each pump cycle in accordance with the following steps: 
a first step in which the power is applied to the motor to rotate it in a 
first direction and in which said power is maintained for a first period 
of time determined from a fixed predetermined origin position of the 
moving equipment in the pump; 
a second step in which the power applied to the motor at the end of said 
first period of time is switched to reverse the direction in which the 
motor rotates, and in which said reverse power is maintained for a period 
of time equal to that required by the moving equipment of the pump to 
return to its origin position; 
a third step in which the power supply to the motor is turned off at the 
end of said second period of time and in which the power is maintained off 
for a third period of time corresponding to the difference between the sum 
of said first and second periods of time and a pumping cycle time which is 
not less than said sum; 
said first period of time and said cycle time being selected by the 
microprocessor as a function of the desired fraction of the maximum flow 
rate as indicated to the microprocessor, and being selected from a 
plurality of time values stored in the microprocessor memory and each 
equal to an integer multiple of one half of the period of the AC power 
supply. 
If the pumping member is a piston driven with sinusoidal motion, the values 
stored in memory for said first period of time and said cycle time are 
determined by calculation. 
In contrast, if the flow rate of the pump is a complex function of the 
motion of the drive motor, by virtue of the nature of the drive chain 
connecting the motor to the pumping member and of the nature of the 
pumping member if it is a membrane, the said values stored in the 
microprocessor memory are determined experimentally. 
Finally, it is advantageous for the origin position of the pump's moving 
equipment to be detected and controlled by means of at least one sensor 
whose output signal is made use of by the microprocessor. 
In a second aspect the invention provides a pump applying the above method 
and comprising: a pumping member; moving equipment for driving said 
pumping member; said moving equipment being driven to perform 
reciprocating rectilinear motion by means of a transmission device coupled 
to a motor, said motor being a reversible synchronous motor with each of 
its windings connected to a source of AC power at fixed frequency via a 
triac which is optocoupled to a controlling light emitting diode (LED) 
which is itself connected to the output from a microprocessor for 
controlling the application of AC power to said windings, and a detector 
for detecting when said moving equipment is in its origin position, said 
detector being connected to an input of said microprocessor.

MORE DETAILED DESCRIPTION 
Reference is made initially to FIG. 1 which is a highly diagrammatic 
section through a piston metering pump comprising the following 
components: 
a pump chamber 2 connected to an inlet duct 3 via a non-return valve 4 and 
to an outlet duct 5 via a non-return valve 6; 
a piston 7 which constitutes a moving wall of the chamber 5 and which is 
driven in reciprocating rectilinear motion along a slideway 8 in the body 
of the pump; 
a transmission device coupled to the piston 7 and comprising a frame 9 in 
which a shoe 10 is capable of sliding transversely to the motion of the 
piston 7, the shoe being coupled to an eccentric cam 11 on a gear wheel 12 
which meshes with an endless screw 13; 
a synchronous motor 14 capable of rotating in two opposite directions and 
having an outlet shaft coupled to the endless screw 13; 
an electronic control circuit 15 for switching on, for stopping, and for 
reversing the motor 14; 
a microprocessor 16 for controlling the circuit 15 and including a keypad 
17 which may be used to cause the flow rate which the pump is to deliver 
(expressed as a fraction of the maximum pump flow rate) to be displayed at 
18, and keys 19 for selecting or rejecting particular control sub programs 
used for correcting an overall pump operation control program as a 
function, for example, of the nature of the fluid to be metered; and 
a sensor 20 having its output connected to said microprocessor, said sensor 
detecting the presence of the piston 7 at its forward dead-center position 
which is taken by the microprocessor to be the origin position in which 
the moving equipment is to be found at the beginning of a pumping cycle. 
The maximum pump flow rate is obtained when the stroke of the piston is 
equal to twice the eccentricity and when the pump is operating at maximum 
speed. For example, assume that the synchronous motor 14 has 16 poles and 
is powered by AC at a fixed frequency equal to a mains frequency of 50 Hz. 
Also assume that the transmission ratio between the endless screw 13 and 
the gear wheel 12 is 1/4. Thus, in operation the motor 14 rotates at 375 
revolutions per minute (rpm) and the gear wheel 12 rotates at 93.75 rpm. 
The time taken to sweep through the maximum volumetric displacement is 
thus 640 milliseconds. 
One of the possible ways of varying the pump flow rate is to act on the 
frequency of the AC powering the synchronous motor. Such a solution would 
require means for varying the frequency of an AC power supply and has been 
avoided for reasons of cost and reliability. 
A second way lies in limiting the piston stroke for a given pumping cycle. 
The synchronous motor 14, together with the electronic control circuit 15 
which is essentially constituted by on/off switches and reversing switches 
in the power supply circuit to the motor, are particularly suitable for 
this purpose. Thus, starting from a maximum flow rate with a cycle time of 
640 milliseconds it can be seen that the admission stroke takes 320 
milliseconds and that the exhaust stroke also takes 320 milliseconds. By 
powering the motor 14 for a shorter period of time than 320 milliseconds 
and then reversing the power supply at the end of said first period of 
time and maintaining the reverse power supply until the sensor 20 detects 
that the piston has returned to its origin position, the volume swept in 
the pump chamber is less than the volume swept by the piston when 
performing a maximum stroke. The motor is then kept switched off until the 
entire cycle time has elapsed, i.e. until the full 640 milliseconds are 
up. 
It is thus possible, in theory, to obtain any desired fraction of the 
maximum pump flow rate by computing the admission stroke time which 
corresponds to the desired flow rate and by switching the motor on in the 
admission direction for that length of time. 
However, when the windings of a reversible synchronous motor are controlled 
by means of triacs, as is described below with reference to FIG. 2, the 
operating characteristics of these components add a degree of inherent 
inaccuracy to the length of time the motor is switched on. The range of 
error is from zero to one-half period of the power supply circuit, i.e. 
anything up to 10 milliseconds for a power supply at 50 Hz. A triac 
becomes conductive as soon as it receives a switch-on signal, but it does 
not cease to conduct immediately after its control signal is switched off, 
but continues to conduct until the next zero crossing of the alternating 
voltage which it is being used to switch. In order to escape from this 
inaccuracy, it is advantageous to detect the voltage zero crossings in 
order either to inform the microprocessor of the moments at which said 
zero crossings occur so as to synchronize the application of control 
signals to the triac with the power supply voltage zero crossings, or else 
to gate the control signal so that the triac is only effectively triggered 
at a zero crossing in the power supply voltage. Thereafter, by ensuring 
that the period for which the power is to be applied is a multipIe of the 
half period of the power supply voltage, the time during which the triac 
is actually conductive is constrained to be exactly equal to the desired 
period (although slightly late if the zero passage detector is used 
directly to gate the application of control signals to the triac). 
Starting from the value 320 milliseconds (which corresponds to the maximum 
piston admission stroke) it follows that the admission time can be varied 
in increments of 10 milliseconds for a power supply frequency of 50 Hz. 
The following table gives all possible admission times t.sub.1 in the range 
320 to 160 milliseconds in increments of 10 milliseconds together with the 
corresponding fraction Q (as a percentage) of the maximum flow rate for 
each of said periods. 
__________________________________________________________________________ 
t.sub.1 
320 310 300 290 280 270 260 250 
Q % 
100 99.8 
99 97.8 
96.2 
94.1 
91.6 
88.7 
t.sub.1 
240 230 220 210 200 190 180 170 160 
Q % 
85.4 
81.7 
77.8 
73.6 
69.1 
64.5 
59.8 
54.9 
50 
__________________________________________________________________________ 
It is recalled that the required accuracy for a metering pump is .+-.1% of 
the indicated flow rate. The above table shows that this accuracy cannot 
be achieved for flow rates of less than 93% of the maximum flow rate. 
Indeed, the theoretical inaccuracy rises to about 10% when the flow rate 
is about 50% of the maximum. This procedure is thus not entirely 
satisfactory and the invention therefore proceeds by applying a correction 
to these values by also varying the duration (T) of the pumping cycle, 
i.e. the sum of the admission period, the exhaust period, and the dead 
period of the moving equipment in the pump. 
As mentioned above, one known way of modifying the flow rate of a pump is 
to modify the pumping speed. When a pumping cycle (T) includes a period 
for which the moving equipment is at rest, as is the case when the stroke 
of the moving equipment is limited in the manner described above, the 
length of time for which the equipment remains at rest can be reduced or 
increased in order to reduce or increase the duration of the pumping cycle 
relative to the cycle duration which corresponds to the maximum piston 
stroke (640 milliseconds), and such modification can take place in 10 
millisecond steps, for the same reasons as those already described above. 
Assuming that the flow rate obtained for a cycle period of 640 milliseconds 
is Q, then the flow rate Q.sub.1 obtained for a cycle period of X 
milliseconds is given by: 
EQU Q.sub.1 =Q (640/X) 
It is undesirable to attempt to achieve a low fraction of the maximum flow 
rate of the pump by excessively lengthening the pumping cycle time. If the 
cycle time is too long, the flow delivered by the metering pump becomes so 
discontinuous as to be ill-adapted to applications which require 
continuous metering and additional means would need to be provided for 
"smoothing" the flow (e.g. tanks, buffers, . . ) and the use of such means 
is not always desirable. Also, the maximum stroke cycle time cannot be 
reduced by an amount which is greater than the rest time which is made 
available when the piston stroke is reduced. 
The way in which the cycle time (T) is selected between these two extreme 
values depends essentially on the way in which it is desirable for the 
intended flow rate to be delivered as a function of time, and that is 
generally dictated by the application to which the metering pump is being 
put. 
For example, supposing that it is desired to obtain a flow rate of 75% of 
the maximum flow rate with an accuracy of .+-.1%, i.e. suppose that the 
theoretical flow rate must lie in the range 75.7% and 74.3%. It can be 
seen from the above table that this range of values cannot be achieved 
merely by adjusting the stroke of the piston. Several solutions for 
achieving the desired flow rate are possible: 
1. The admission period is selected to be 160 milliseconds and the above 
table indicates that the flow rate for a nominal cycle time T=640 
milliseconds is equal to 50% of the maximum flow rate. Assuming that the 
exhaust period is equal to the admission period, that leaves a rest period 
of 320 milliseconds. The cycle time can thus be reduced to 430 
milliseconds. The theoretical flow rate obtained in this way is then 74.4% 
of the maximum flow rate. (Due to the technological constraint on 
accurately timing synchronous motor ON periods, the total cycle time, must 
be controlled in increments of 10 milliseconds at a power supply frequency 
of 50 Hz). 
2. The closest value in the above table to the desired value is selected 
(210 milliseconds) and the cycle time is correspondingly corrected (in 
this case it is reduced from 640 milliseconds to 630 milliseconds). The 
resulting flow rate is theoretically 74.8% of the maximum flow rate. 
3. The longest possible admission stroke time (320 milliseconds) is 
selected from the above table which leads to a requirement for the cycle 
time to be 850 milliseconds in order to obtain 75.3% of the maximum flow 
rate (or 860 milliseconds in order to obtain 74.4%). 
4. Numerous intermediate solutions are also possible. 
Numerous tables can therefore be established combining cycle times and 
admission periods for each desired partial flow rate, and the most 
appropriate one of such tables is selected depending on the desired nature 
of the flow, the nature of the fluid being metered, and other factors 
depending on the installation to which the metered fluid is being 
delivered. 
The above-described method is advantageously implemented by an electronic 
circuit for controlling the synchronous motor and itself under the control 
of a microprocessor which includes a program for organizing the various 
switching operations to be performed by the electronic circuit as a 
function firstly of the available cycle time values and admission time 
values as previously determined experimentally or by calculation and as 
stored in the microprocessor memory, and secondly of the desired flow rate 
as expressed by means of a data input device to the microprocessor, e.g. 
under manual control and coupled with a device for displaying said flow 
rate. 
FIG. 2 is a diagram of an embodiment of a metering pump which is a membrane 
pump and whose electronic control circuit satisfying the above criteria 
and is shown in greater detail. 
As in FIG. 1, the pumping chamber is connected via non-return valves to an 
admission duct and to an exhaust duct (not shown). The pumping member is 
constituted by a membrane 31 which is mechanically actuated by moving 
equipment 32 which is driven in reciprocating rectilinear motion by a link 
mechanism comprising a lever 33 which is hinged at one of its ends 33 
about a fixed pin 34, and which is hinged about a pin 35 to the furthest 
end 32a of the moving equipment 32 from the membrane 31, and which is 
hinged at its opposite end 33b about a pin 36 to the end 37a of a drive 
lever 37. The other end 37b of the drive lever 37 is hinged about a pin 38 
on a wheel 39 which is rotated about a fixed shaft 40. The distance 
between the shaft 40 and the pin 38 constitutes the crank arm of the 
crank-lever system constituted by the lever 37 and the wheel 39. The wheel 
39 is rotated by the output shaft 41 of a synchronous motor 42 by means of 
a stepdown gear (not shown) and merely indicated diagrammatically by the 
difference in diameter between the wheel 39 and the shaft 41. 
It may be observed that this diagram shows the moving equipment in its 
forward dead-center position, i.e. at the end of an exhaust stroke or at 
the beginning of an admission stroke. The connection between the moving 
equipment 32 and the link mechanism is made in such a manner (as shown in 
the drawing) that the crank 38, 40 is in a position so that the force 
transmitted by the pin 38 to the lever 37 is perpendicular to the lever. 
This ensures that, ignoring friction, the opposing torque on starting is 
zero. 
Also, the admission stroke of the equipment 32 takes place against the 
effect of a return spring 43 which is compressed during the admission 
stroke and which thus accumulates energy for assisting in the provision of 
force for the exhaust stroke, which is advantageous for a reason explained 
below. 
44 and 45 represent two windings of the reversible synchronous motor 42 and 
their common point is connected to a first terminal 46 of an alternating 
power supply. Each of these windings is also connected to the other 
terminal 47 of the power supply via a respective triac 48 or 49 which is 
optocoupled to a respective light-emitting diode (LED) 50, 51 which 
constitutes the triac-controlling member. A capacitor 52 is connected in 
conventional manner between the two windings 44 and 45. When the triac 48 
is switched on under such conditions, both windings are powered, with the 
winding 44 being powered by the power supply voltage and with the other 
winding 45 being powered by a phase-shifted voltage by virtue of the 
capacitor, thereby causing the shaft 41 to rotate in direction A and the 
wheel 39 to rotate in direction B. The direction of rotation is reversed 
by switching the other triac 49 on while the first triac 48 is switched 
off. 
The LEDs 50 and 51 are connected to the microprocessor 60 which transmits 
control signals to them corresponding to the different power supply stages 
described above. 
Finally, the wheel 39 may be in the form of a disk having a slot 53 which 
co-operates with an optical detector 54 whose function is identical to the 
function of the detector 20 in FIG. 1. The detector 54 is connected to the 
microprocessor 60 to provide the microprocessor with information 
concerning the presence or otherwise of the moving equipment in its origin 
position (forward dead center in the example shown), as indicated by the 
presence or otherwise of the slot 53 on the light path between an LED 55 
and a light sensor 56. 
The front of the microprocessor 50 may include an on/off switch 61, a first 
button 62 for selecting a so-called "integrated" mode of flow rate 
adjustment, a second button 53 for selecting a so-called "selective" mode 
of adjusting the flow rate by separate action on the pump's speed and its 
volumetric displacement, a third selection button 64 for use within the 
"selective" mode of adjustment to set the speed or the volumetric 
displacement, a display device 65 for displaying the selected value(s) and 
buttons 66 and 67 for determining the selected values and for varying 
them. 
It is shown above that at least one table of values determined by the 
manufacturer may be stored in the microprocessor. Such tables of values 
may be determined by calculation when the volumetric displacement is a 
simple function of the rotation of the synchronous motor. 
However, some pumps, such as mechanically-actuated membrane pumps as 
illustrated in FIG. 2, have a flow rate which is a much more complex 
function both of the stroke of the drive mechanism and of the pump speed, 
and such functions can be determined solely by experiment. Naturally, 
combination tables of the type described above may be established for this 
type of pump experimentally by measuring the flow rate effectively 
delivered by the pump (or by a master pump representative of the type of 
pump used) as a function of the displayed admission and cycle times. 
When operating in integrated mode, the microprocessor stores a table of 
pairs of optimum values of the first period and for the cycle time 
corresponding to each desired value for the fraction of the flow rate (for 
example in steps of 2% to 100% at 1% intervals). 
In this case, once the button 62 has been pressed, the buttons 66 and 67 
are pressed until the desired fraction of the maximum flow rate is 
displayed. The microprocessor then determines the pumping cycle time and 
the first period (the admission period) which corresponds to the desired 
flow rate as a function of the data input. With the pump starting from its 
position shown in FIG. 2, the LED is excited for said first period to 
cause the wheel 39 to rotate through a certain angle. It may be observed 
in this respect that the motor latches onto its synchronous speed very 
rapidly since the opposing torque on starting is nil, given that the motor 
is small and therefore of low inertia. This disposition is important since 
it enables satisfactory accuracy to be obtained for the admission stroke, 
thereby enabling good accuracy to be obtained overall for the metering 
operation. If the synchronous motor were to latch randomly by virtue of 
un-controlled slip due to a non-zero starting torque of varying value, 
different volumes would be swept from one pump cycle to the next in spite 
of the first period being identical in all of the cycles. 
At the end of the first period, the connection of the winding 44, 45 is 
reversed by exciting the LED 51 and by ceasing to excite the LED 50. In 
practice, this switchover is not instantaneous and allowance is made for a 
certain period of time between the motor ceasing to rotate in one 
direction and starting to rotate in the opposite direction. This time may 
be as much as 40 milliseconds and is taken into account when determining 
the stepdown ratio between the motor and the wheel of the crank-lever 
system. Thus, in the above numerical example, and retaining a cycle time 
of 640 milliseconds for the maximum pump capacity, the admission stroke 
will last for 280 milliseoonds as will the exhaust stroke, but the 
transmission ratio will be 0.285 in order to obtain a speed of rotation of 
107 rpm, thus retaining the same maximum flow rate. 
When the motor begins to rotate in the reverse direction the opposing 
torque is not zero. It therefore takes longer for the motor to latch onto 
its synchronous speed, but this state of affairs is unimportant since the 
exhaust period is not fixed a priori, but is determined by the slot 53 
returning to the optical sensor 54. However, it is advantageous for the 
starting torque to be as low as possible, and the return spring 43 urging 
the moving equipment towards its forward dead-center position serves to 
reduce said starting torque. 
When the optical detector 54 sends a signal to the microprocessor, the 
microprocessor turns off the LEDs 50 and 51 and the motor remains 
unpowered until the full cycle time has elapsed. The same sequence of 
operations is then repeated for the next cycle, and so on. 
When operating in "selective" mode, the microprocessor stores a first table 
of values of the first period (the admission period) in multiples of the 
half period of the power supply for various possible desired flow rate 
fractions, for example in 5% intervals, and a second table likewise 
established as a function of the desired flow rate fraction, giving the 
closest value for the cycle time in half period multiples. Thus, by 
pressing the button 63, the microprocessor is placed in a situation where 
it uses one or other of said tables. By pressing the button 64 the 
microprocessor is instructed to act on the flow rate solely by acting on 
the admission stroke period. The buttons 66 and 67 are used to select the 
appropriate 5% range, thereby causing the microprocessor to use the 
corresponding value from the first table so that the installation operates 
as described above with the cycle time being kept constant to the cycle 
time which corresponds to its maximum capacity. Otherwise, if the button 4 
is not held down, the flow rate is modified by adjusting the cycle time 
(i.e. the pump speed). The appropriate cycle time is then taken from the 
second table as a function of the displayed fraction of maximum flow rate. 
Although less accurate than integrated adjustment mode, there are some 
applications for which selective adjustment mode may suffice. 
A metering pump equipped in this manner may be provided in a version which 
gives integrated adjustment only, or in a version which gives selective 
adjustment only. All three possible versions differ only in the 
electronics of the microprocessor, in other words the differences lie in 
the memories and the circuits for selecting them. This pump design is 
therefore suitable for highly standardized manufacture. A fourth version 
(not shown) consists in a pump for which manual adjustment of the desired 
flow rate fraction is replaced by a signal which servocontrols said flow 
rate on an external parameter (for example the flow rate of the fluid into 
which the metered liquid is to be injected). Here too, the basic 
components of the fourth version remain standard with the other versions. 
The only change lies in the way in which the desired flow rate signal is 
applied to the microprocessor. 
Finally, the microprocessor program may also use one or more 
pre-established subprograms for making corrections to the stored values 
(whether they be experimental or calculated) as a function of special 
conditions applicable to pump operation (viscosity of the admitted fluid, 
pressure conditions on admission and on exhaust, inertia of the motor and 
of the moving equipment, changes in membrane behavior, changes in the type 
of pump controlled, supposing the microprocessor is designed to control a 
battery of different pumps either simultaneously or in succession, . . . 
). 
The facade of the microprocessor 60 as described with reference to FIG. 2 
may be made in various ways. By way of an example which is not shown, 
mention may be made of a version in which the buttons 62, 63, and 64 for 
selecting the mode of adjustment are brought together in the form of a 
single button which co-operates with internal circuits for performing the 
selection. Thus, for example, one stroke on the single button puts the 
microprocessor into "integrated" mode, a second stroke puts it into "cycle 
time" mode, a third stroke puts it into "stroke" mode, and a fourth stroke 
may return to "integrated" mode. In such an embodiment it would be 
advantageous for the display device 65 to include space for displaying a 
mark identifying the currently selected mode.