Pump unit for sampling air

An improved pump unit for sampling air that operates at a constant air flow rate in the range of 5-5000 cc/min. having a filter for removing particles or vapors from the air stream, an air accumulator, a variable drive pump optionally with a bypass, an electric motor for driving the pump, an optional air reservoir, an orifice which creates a pressure drop in the air stream, an optional bypass for the orifice and a pressure switch connected in parallel to the orifice which monitors a change in the air pressure drop; the improvement in the pump is the use of a digital circuit electrically connected to the pressure switch and a closed loop control means electrically connected to the digital circuit and motor; a digital signal determines the open or closed position of the switch and the control means and allows current to flow or not flow to the motor driving the air pump to provide a constant flow of air through the unit; the pump unit is worn by a worker or is placed in a work area and at the termination of a period of time, such as a work day, the filter is removed and the contents collected are analyzed by conventional techniques such as gas chromatography to determine a level of exposure of the individual or the level of exposure of people working in that area.

BACKGROUND OF THE INVENTION 
This invention is related to an air sampling pump unit and in particular to 
an improved air sampling pump unit designed for use by an individual. 
Air sampling pump units having a constant air flow rate are used to monitor 
air to which workers are exposed. These pump units are well known in the 
art. Typical examples of such pump units are shown in Baker et al. U.S. 
Pat. No. 4,063,824 issued Dec. 20, 1977, Baker et al. U.S. Pat. No. 
4,123,932 issued Nov. 7, 1978, Wells U.S. Pat. No. 4,257,746 issued Mar. 
24, 1981 and Baker U.S. Pat. No. 4,269,059 issued May 26, 1981. These pump 
units are excellent for the particular use for which the pump units were 
designed. However, there is a need for a versatile pump which can 
accurately pump air at very low flow rates such as 5 cc/min. to very high 
flow rates such as 5000 cc/min. Also, it would be desirable to have the 
following features on the pump unit: user programmable starting time, 
running time, and tolerated restricted air flow time; measure air 
temperature; have memory retention of program; have memory retention after 
the unit is turned off; have a liquid crystal display which shows values 
of flow, time and temperature and a computer interface which will allow 
loading of data into a computer and allow loading of operational data into 
the pump unit. 
In the operation of the aforementioned prior art air sampling pump units, 
the pump is controlled through a pressure switch positioned in parallel to 
an orifice which monitors pressure drop across the orifice caused by a 
change in air flow. For example, if there is a blockage of air flow, the 
pressure switch closes and through an integrator circuit and an amplifier 
circuit provides an increased voltage to the motor driving the pump and 
thereby increases the air flow. When the pressure drop across the orifice 
returns to normal, the pressure switch opens and the pump operates under 
usual conditions. The pressure switch also constantly opens and closes 
with pulsations in air flow caused by pulsations of the pump. Under 
extremely severe operating conditions such as opening and closing of the 
switch several hundred times a minute, the electrodes of the pressure 
switch rapidly deteriorate because of electrical arcing across the 
electrodes. An improvement that is required would determine whether or not 
the switch was open or closed without applying a constant voltage across 
the switch to generate an electrical signal. 
The improved air sampling pump unit of this invention pump units air 
accurately at very low and high rates, is user programmable to start and 
run for a period of time, measures temperatures, has a memory retention, a 
liquid crystal display and measures the on-off position of the pressure 
switch without applying a constant voltage across the switch and controls 
the air pump via a computer and allows for loading and unloading of data. 
SUMMARY OF THE INVENTION 
An improved pump unit for sampling air and having a constant air flow rate 
through the unit; the unit having an air intake, a filter means for 
removing particles or vapors from the air tubularly connected to the air 
intake, a variable drive air pump connected to the filter means which pump 
units an air stream through the unit, a variable electric speed motor 
connected to the pump, a power source for the motor, an orifice positioned 
in a tubular connection coupled to the pump and causes an air pressure 
drop in the air stream being pumped through the pump unit, a differential 
pressure switch with an open and a closed position tubularly placed in 
parallel relationship to the orifice and is activated by a change in air 
pressure drop of the airstream caused by a change in air flow through the 
unit, an exhaust port tubularly connected to the orifice and the 
differential pressure switch; the improvement that is used therewith is as 
follows: 
digital circuit electrically connected to the pressure switch and a closed 
loop control means electrically connected to the digital circuit, the 
power source and the motor; wherein the digital circuit sends a digital 
low duty cycle pulse signal over a set time interval to determine the 
predominant open or closed position of the switch during the time 
interval; in the event the switch is in the predominant open position, the 
control means gradually increases voltage from the power source to the 
motor thereby driving the motor at an increasing speed which in turn 
drives the pump at an increasing speed and increases air flow through the 
unit; in the event the switch is in the predominant closed position, the 
control means gradually decreases voltage from the power source to the 
motor thereby driving the motor at a decreasing speed which in turn drives 
the pump at a decreasing speed and decrease air flow through the unit; 
whereby the air flow through the unit is maintained at a constant flow 
rate.

DETAILED DESCRIPTION OF THE INVENTION 
The air sampling pump unit is of a compact size and is designed for 
individual use. The unit measures about 5.7.times.10.2.times.12.7 cm. and 
weights about 800-1100 gms depending on the weight of the battery pack 
used in the unit. 
The air sampling pump unit with its constant flow feature and excellent 
accuracy can be used to monitor air which may contain environmental 
hazards to which individuals may be exposed. For example, vinyl chloride 
vapors can be monitored in a work place, toxic radon gas and related gas 
in mines can be monitored, coal dust in mines and pits can be monitored. 
The filter of the air sampling pump unit is analyzed for the substance 
being monitored at the end of a work period, such as an eight hour work 
day, and results are recorded in a workers file. If a worker is exposed to 
a hazard over a specified amount, he is reassigned to another job. 
Referring to the block diagram of FIG. 1, air is pulled into the intake 1 
and through the filter 2 and preferably into an accumulator 3 by the 
variable drive air pump 4 driven by a variable speed motor 5. The pump can 
operate without a bypass and a bypass valve but for low flow rates a 
bypass and valve are preferred. A bypass valve 7 is positioned in parallel 
to the pump 4 in tube 6. The bypass valve is usually an adjustable needle 
valve and is adjusted to provide the desired air flow rate. By opening the 
valve, more air is recycled thereby reducing the air flow rate through the 
unit. By closing the valve, more air flows through the unit. The pump can 
operate smoothly at a normal speed providing low air flow rates with the 
bypass open rather than operating slowly and sticking and binding in the 
event the bypass was not used for low air flow rates. 
An air reservoir need not be used but is highly preferred. The air 
reservoir 8 is positioned in the tube or channel connecting the pump 4 to 
the orifice 11. The air reservoir helps reduce pulsations caused by the 
pump and in general helps provide a smooth flow of air through the unit. 
An orifice 11, usually an adjustable needle valve or a fixed orifice, is 
positioned between the air reservoir 8 and the exhaust 17. The orifice 
causes an air pressure drop which is monitored by pressure switch 12 which 
opens and closes with a change in the pressure drop. Preferably, in 
parallel with the orifice is a bypass which comprises a tube or a channel 
9 in which a valve 10 such as an adjustable needle valve is positioned. 
Valves 10 and 11 are similar in function. Valve 11, the orifice, functions 
as a course control and valve 10, the by-pass valve, functions as a fine 
control. 
In channel or tube 18 in which the pressure switch 12 is positioned, a 
filter 19 usually of a rubber foam is positioned in the channel. This 
filter reduces and modulates pump pulsations to the pressure switch. 
Changes in the air flow through the unit which can be caused by a blockage, 
either partial or complete, of the air intake or air filter or by 
pulsation caused by operation of the pump causes changes in the air 
pressure drop and causes the pressure switch to open and close. 
Electrically coupled to the pressure switch is a closed loop control means 
13 which preferably is a computer and motor driven circuit. 
The computer sends a digital low duty cycle pulse signal over a set time 
interval to the pressure switch to determine the predominant open or 
closed position of the switch during the time interval. In the event the 
switch is in the predominant open position, the control means of the 
computer gradually increases voltage from the power source which through 
the motor driver circuit drives the motor at an increase speed which in 
turn drives the pump at an increased speed and increases air flow through 
the unit. In the event the pressure switch is in a predominantly closed 
position, the control means gradually decreases the voltage from the power 
source which through the motor driver circuit decreases the speed of the 
motor and pump and decreases air flow through the unit. 
A constant voltage is not directly applied across the electrodes of the 
pressure switch 12 as was done with prior art pump units. In these prior 
art pump units excessive opening and closing of the pressure switch caused 
the switch to wear out very rapidly since arcing across the electrodes 
took place on opening and closing of the switch. In a relatively short 
time, the electrodes were burned and pitted. In the present invention, a 
voltage is not applied across the pressure switch. The digital signal from 
the computer determines whether or not the switch is predominantly closed 
or open. The signal voltage is applied for a very short time and 
eliminates virtually all of the arcing across the electrodes of the 
switch. 
Preferably, an LCD (liquid crystal display) driver unit is electrically 
coupled to the computer and feeds a signal to a LCD (liquid crystal 
display) which displays data that has been accumulated by computer of the 
unit. 
The filter 2 of the pump unit can be adapted to entrap almost any type of 
substance such as gases, liquids of solids. If mechanical filtration is 
only required, for example, to collect dust particles to which a worker is 
exposed, a filter is provided which will entrap particles of 0.01 microns 
or larger. If the filter is to entrap a gas such as sulfur dioxide, a 
chemical filter is used which will entrap this gas. If vapors are to be 
entrapped, then a filter such as a charcoal filter, is used which entraps 
vapors. At the end of the period which an individual is wearing the unit, 
such as an 8 hours work day, the filter is removed and examined for the 
substance or substances to which the individual was exposed. A simple 
count of particles under a microscope may be used or the filter can be 
analyzed, for example, with a gas chromatograph. 
The accumulator 3 is usually an integral part of the frame of the pump unit 
and is milled, molded or cut into the frame with an elastomer sheet 
covering one wall and with appropriate openings. A typical accumulator has 
a volume of about 2 to 20 cc and reduces and moderates surges created by 
the pump and allows a build-up of air on the suction side of the pump. 
A variable drive air pump is used in the dosimeter. Generally, a diaphragm 
type pump is used that pump units from about 5 to 5000 cubic centimeters 
per minute. Other pump units such as piston pump units, rotary pump units 
and centrifugal pump units can also be used. 
The pump is electrically connected to a conventional D.C. motor of about 
0.0001-0.02 horsepower. The motor is a variable speed motor and operates 
from about 80 to 8000 revolutions per minute. Under some circumstances, a 
reducing gear can be used between the motor and the pump. 
The reservoir 8 is usually an integral part of any framework on which the 
various components used in the unit are mounted. Part of the reservoir may 
be enclosed with a thin sheet of an elastomer so that any pulsations of 
the air stream created by the pump can be readily dampened by the 
elastomer absorbing the pulsation. 
The purpose of the reservoir is to smooth any pulsations of the air stream 
created by the strokes of the pump at least to some degree before the air 
stream passes through the orifice. The volume of the reservoir is as small 
as possible but of sufficient volume to reduce the pulsations of the air 
stream usually has a volume similar to the volume of the accumulator. 
An orifice typically as an adjustable needle valve, is positioned in a tube 
connecting the reservoir to the exhaust port. An orifice is used that 
creates a pressure drop of about 0.4-4.0 inches of water. Usually a 
pressure drop of 2.5-3.5 inches of water is used. 
A differential pressure switch of a relatively high level of sensitivity is 
used and is sensitive to a pressure drop change in the air stream of about 
0.1-0.5 inches of water. 
One useful closed loop control means comprises a digital circuit 
electrically connected to the pressure switch and to an integrator circuit 
that is connected to the power source which integrates the signal from the 
digital circuit and sends this integrated signal to an amplifier circuit. 
The amplifier circuit is connected to the power source and in series to 
the integrator circuit and the electric motor and feeds an amplified 
signal to the motor and drives the motor at the appropriate speed for 
uniform air flow through the pump unit. 
The digital circuit provides pulses in the range of 50-1000 puluses per 
second to the pressure switch. The pulses have a duty cycle of 0.1-5%. 
This limits the range of effective current to the switch to about 0.1-5.0 
milliamps. 
Another closed loop control means for controlling the motor comprises the 
digital circuit electrically connected to the pressure switch a digital 
integrator circuit connected to the digital circuit. The integrator 
circuit receives and integrates the signal received from the digital 
circuit. A digital to analog converter is connected to the integrator 
circuit and converts the signal to an analog signal which is sent to an 
amplifier circuit connected to the motor and drives the motor as 
appropriate speed to achieve uniform air flow through the unit. 
In one preferred closed loop control means, the digital circuit is 
connected to a digital integrator circuit which integrates the digital 
signal received from the pressure switch. This integrated signal is then 
fed to a digital pulse width modulator switching device connected to the 
digital integrator circuit. This switching device is connected to the 
power source and the motor and feeds current to the motor to operate the 
motor and hence, the air pump such that a uniform flow of air is passing 
through the unit. 
Referring to FIG. 2, the above preferred control means is incorporated into 
a computer 13A which is electrically connected to the motor drive circuit 
13B which amplifies the signal from the computer and drives the motor 5. 
The digital circuit and digital integrator are part of the computer 
circuit. The digital pulse signal from the computer 13A is sent to the 
pressure switch. The pulse width modulator switching device is part of the 
motor drive circuit 13B which drives the motor. A keyboard 18 is used to 
enter data and select program limit parameters is connected to the 
computer. Electrically attached to the computer 13A is a LED (light 
emitting diode) which is activated when the pump is in the hold position 
and not operating. A second LED 20 is activated when flow through the pump 
is in control. 
The LCD Driver 15 attached to the computer feeds a signal to the LCD 14 
which displays data which has been accumulated by the computer 13A. 
An I/O device 20 (Input Output device) is a device that permits a data buss 
to be attached to the computer and allows for unloading of data to a 
second computer for recording. The I/O device also allows for loading data 
into the computer such as operating parameters, run time, low flow time 
and the like. This can be accomplished with a programmer unit known in the 
art. This loading technique is particularly useful when a number of pump 
units must be programmed for the same information. For example, a shift of 
50 people working in a mine or plant are each given a pump unit to wear 
during the shift. Programming each unit individually would be time 
consuming. By using the programmer, each unit can be loaded with the 
desired program within 10 seconds or less. 
The unit is activated with an ON/OFF switch 21 electrically attached to the 
battery BATT 22 and the computer 13A. A battery charger circuit CHG 23 
attached to the battery 22 changes the battery but also can be used to 
drive the motor through Motor Drive circuit 13B. I/C shown attached to the 
charge is an outside power source that provides power to the charger. 
A temperature sensor 24 is connected to an analog/digital converter A/D 25 
which in turn is connected to the computer 13A. The A/D converts the 
temperature signal and any other analog signals which may be received for 
other functions to digital signals which can be processed by the computer 
13A. 
The following is a more detailed description of the functioning of the 
components of the pump unit shown in FIG. 2. 
Computer 
The computer 13A typically is an 80C49 CMOS single chip microcomputer and 
is the central controller for the pumps unit. The microcomputer's primary 
function is to provide closed loop control of the pump motor based on the 
state of the pressure switch at the time of periodic samples. Control of 
the motor is via pulse width variation to the motor control circuitry. In 
addition to this the microcomputer performs the house keeping functions 
such as keeping run time, low flow time, display data presentation, local 
control switch monitoring and responses, battery charge timing and 
control, temperature monitoring, and external communication. 
Since the microcomputer contains the CPU, I/O, data RAM and program memory 
internally a minimum of external circuitry is required to support its 
operation. Typically, a 6 MHz crystal, and two capacitors and establish 
the computer instruction cycle timing time base. One capacitor performs 
the power up reset function. A transistor, typically a VN 2222L, which is 
in parallel with a capacitor is required to allow computer controlled 
turn-off. 
The keyboard 18 and LED indicators 19 and 20 are under direct control of 
the microcomputer. The operator uses the keyboard to affect local control 
and select program limit parameters. In the simplest mode of operation 
where parameters are selected locally, the pump has three states. On 
power-up initiated by depression of the ON/OFF key the computer comes up 
in the HOLD mode. In this mode it is possible to select runtime parameters 
including run time, low flow time, and start delay. This is done by using 
the DISP SCROLL to select the parameter to be changed and pressing the SET 
FUNC key to scroll through the available parameters. Depressing the 
RUN/HOLD key will then force the pump into the RUN state where the motor 
is under control of the computer. Once the computer has been forced into 
the RUN state it may be returned to the HOLD state to stop the pump motor, 
however it is not possible to change the programmed parameters. The last 
state is selected by the ON/OFF key. When this button is pressed the 
computer is placed in the OFF state and a 20 minute timer is started. If 
the pump remains in the OFF state until this timer times out the computer 
generates a shut-down pulse which removes power to itself. In the OFF 
state two successive depressions of the RESET key will force a return to 
the initial power-up HOLD state. 
DISPLAY INTERFACE (LCD) 
The pump unit states and the parameters are presented to the operator via 
an LCD display 15 capable of full numeric character display and limited 
alpha character display. The LCD display, is interfaced to the 
microprocessor via the LCD Driver 14 which typically is a MM5453 serial 
input LCD interface chip. The driver interface requires a data and clock 
input from the computer. These two outputs are also made available via the 
external interface connector. 
In normal operation of the pump unit, the LCD is not visible to the user. 
Two LEDs are provided to give minimum operational information to the user. 
The HOLD LED is ON when the computer is in the HOLD or OFF state to 
indicate that the pump is not actively taking a sample. The FLOW LED, is 
used to indicate flow status. A continuous ON LED indicates that the pump 
is under control of the microcomputer. A flashing LED indicates that a low 
flow condition exists. 
TURN-ON LOGIC AND POWER SUPPLY 
As stated above the pump is activated by depression of the ON/OFF key of 
the keyboard but once the circuitry is powered-up turn-off is under the 
control of the microcomputer. Turn-on is also performed when the battery 
charger 23 is connected. The turn-on logic function is performed by 
discrete digital circuitry in conjunction and with micro computer. 
The shut-down function is controlled by this digital circuitry and 
microprocessor. While the battery charger is connected the power supply, 
latch is forced to the on state and shut-down is precluded. 
MOTOR DRIVE CIRCUITRY 
The motor drive circuitry comprises discrete transistors, resistors and an 
operational amplifier. The motor is controlled by the microcomputer using 
a pulse width technique. The pulse width is variable in 0.1% steps over a 
range of approximately 3% to 97% duty cycle. 
A current limit circuit in conjunction with the motor is key to allowing 
the pump to run continuously from the battery charger as well as 
establishing the minimum voltage to the microcomputer during the time the 
pump is operating. The magnitude of the maximum current was established by 
the normal running current of the motor, the voltage drop across the 
current limiting resistor in the battery, and the current required to 
charge the batteries. Note that the current to the motor is only peak 
limited. When the motor is rotating fast enough to develop a back EMF, the 
peak current is determined by the motor and not the current limiting 
circuitry. At very low flow rates the current limit circuit is functional 
which tends to give better motor response to the pulse width variation of 
the control loop. 
ANALOG TO DIGITAL INTERFACE 
The analog to digital interface circuitry, and A/D converter 25 on FIG. 2., 
is included to allow monitoring of the battery voltage and measurement of 
the temperature. This function is performed by a temperature sensor under 
control of the microprocessor. The A/D converter 25 typically is is a four 
channel, 8 bit, serial analog-to-digital converter. Channels 0 and 1 of U6 
are used in a differential mode to measure temperature while channel 2 is 
used to monitor the battery voltage. Channel 3 has been implemented as a 
test input. 
All analog-to-digital conversions are keyed to the pulse width control of 
the pump motor and take place at the end of the cycle when the pulse is 
off. This minimizes the current drain of the analog-to-digital circuits 
and also minimizes the self heating in the temperature sensor which can 
produce errors in the temperature reading. 
The temperature sensor, typically is a LM335 band gap reference with an 
output voltage of 10 millivolts/degree Kelvin. Temperature can thus be 
quantized to 0.5 degrees over a range of -28 to +99.5 degrees Celsius. 
When the pump is first powered up and until the pump motor is started the 
temperature is available to the operator as the ambient temperature. Once 
the program has started (motor started) the average temperature over the 
run is computed and available for display. 
The battery voltage is also measured during the off time of the motor. 
Since the motor can not be on while the battery voltage is measured, a 
close estimate of the battery open circuit potential is available to the 
microcomputer. This measurement is used by the microcomputer to determine 
two critical voltage. At 5.5 VDC the pump circuit is automatically 
disabled to save the memory in the microcomputer. This state is indicated 
to the user by flashing decimal points in the LCD display. At 5.0 VDC the 
microcomputer shuts down to prevent reversal of a cell in the battery 
pack. 
EXTERNAL DATA INTERFACE 
An external data interface is provided, shown on FIG. 2 as I/O 20. The 
primary purpose of this interface is to allow data to be read by an 
external computer device and program parameters such as start time, run 
time, low flow time and time of day to be down loaded to pump by a higher 
level device. When programmed externally the pump has features not 
available from the front panel controls. These features are activated by 
turning the pump on and connecting the programmer device and down loading 
the desired program parameters. When programmed in this manner the pump 
has the capability to start at a specified time of day, a run time 
specified in hours and minutes, a low flow time specified in minutes and 
seconds, and the ability to provide intermittent grab samples. In the 
latter feature the pump can be programmed to run a given number of minutes 
out of each hour. 
The external interface also provides as means of paralleling remote 
displays, monitoring critical voltages and powering the pump from sources 
other than the battery. 
BATTERY CHARGING CIRCUIT 
Battery charging, shown as CHG. 23 FIG. 2, is under the direct control of 
the microcomputer. When the external battery charging source is connected 
to the unit the pump is automatically forced on and into the OFF state. 
The microcomputer is able to sense the presence of the external charger 
voltage via external circuitry connected between the charging jack and the 
microprocessor. Charging is at the 10 hour rate of the battery, and over 
charge of the battery is precluded by timers in the microcomputer. At the 
end of a 14 hour charge cycle the microcomputer reverts to a trickle 
charge mode. The microcomputer has the ability to charge two different 
battery pack configurations. An input to the microcomputer indicates what 
type of battery pack, either a sub-C or 1/2 D pack, is connected to the 
unit. 
Charging of the battery packs is done using a constant current DC-to-DC 
converter which is duty cycle controlled by the microcomputer to establish 
the proper charging currents for the two types of battery packs. The 
DC-to-DC converter is a self-oscillating design configured around an 
operational amplifier, typically a CA3130. The output of this operational 
amplifier gates on a transistor at the required repetition rate necessary 
to ensure continuous current flow in an inductor. The pulse rate is 
variable in the range of 100 to 150 Hz. In normal operation, the current 
level is established by adjusting a potentiometer which establishes the 
reference voltage to the inverting input of the operational amplifier. A 
diode performs the fly-back function to allow current to flow continuously 
through the inductor during the off time of the pulses. Two other diodes 
prevent discharge of the battery pack when the charger is not connected, 
and a zener diode clamps the voltage across the battery terminals to allow 
operation from the charger without the battery connected. 
During charging the microcomputer gates the charging circuit on by 
controlling external transistor. When charging at the 10 hour rate, the 
sub-C and the 1/2 D batteries are gated at duty cycles of 40% and 80%, 
respectively. Under trickle charge the duty cycles drop to 10% and 20%, 
respectively. Battery charging is only performed when the microcomputer is 
in the OFF state and the full charge time of 14 hours is reinitiated any 
time the microcomputer is returned to that state after the motor has been 
turned on. While connected to the charger, all power to the unit is 
provided by the charger power supply and no energy is taken from the 
battery. When the motor is operating, the charge circuit is gated on in 
synchronism with the motor drive signal. Since the charger current is 
always greater than the motor current, no current can flow out of the 
battery. Discrete external circuitry ensures that the microcomputer and 
logic functions are also powered from the battery charger. 
RECHARGEABLE BATTERY KS 
As stated above the pump can be powered from either a sub-C or 1/2 D 
battery pack. Both packs contain 5 series connected nickel-cadmium cells 
and a 1.5 ohm current limiting resistor as well as a PTC (positive 
temperature coefficient) device. These are included in the battery pack to 
ensure compliance with the intrinsic safety requirements. Three terminals 
are required to the battery, two for the battery potential and the other 
as an indicator of the battery pack type. This third pin is connected to 
the negative terminal of the battery in the case of the 1/2 D battery and 
open in the sub-C battery pack. 
PUMP FLOW CONTROLS 
The pump air flow rate is controlled by the microcomputer using a pulse 
width modulation technique. The flow rate is sensed as function of the 
state of the pressure switch. To prevent arcing of the contacts of the 
pressure switch, voltage is only applied to the switch for 10 microseconds 
at a sample rate of 500 samples per second. This technique takes advantage 
of the inherent design of the 8049 bidirectional output pin circuit. 
During the time when a sample is not being taken, the output pin is held 
at ground potential and is only enabled as necessary to sample the state 
of the switch. During the on time of the sample pulse the current is 
limited to approximately 2 milliamps for less than 2 microseconds and then 
reverts to 50 microamp current level for the remainder of the 10 
microsecond sample interval. 
The control algorithm is a digital implementation of perfect integration 
second order control loop with the added sophistication of variable closed 
loop gain for maximum acquisition of the required pulse width to sustain 
the desired flow rate. In essence the closed loop system averages the on 
and off time of the pressure switch and forces the average to be 50%. 
After acquisition, the loop gain is reduced to minimum to prevent 
"hunting" when under closed loop control. For fast acquisition either 
caused by pump load changes or at power-up, the pump reverts to maximum 
loop gain which effectively increases the rate at which the duty cycle to 
the motor can change. As the motor speed comes closer to control, the gain 
is dropped to intermediate levels until absolute control is established. A 
low flow condition is declared when the duty cycle is forced to 97% and 
loop control is still not detected. 
The pump can be programmed to shut down after a certain period of low flow 
operation. This period is programmable from 1 second-99 minutes and 
typically is 2 minutes. 
One of the features available in an intelligent digital loop is the ability 
to remember the motor state after a run interruption. If at the time of 
turn-off the motor was under control, the duty cycle of the desired flow 
rate is remembered and this duty cycle is used when the pump is turned on 
again. This allows the pump to be restarted with a minimum turn on 
transient. If after a reapplication of power to the motor loop 
stabilization is not attained within a short period of time, the 
microcomputer reverts to the acquisition sequence. 
The duty cycle to the pump motor is variable in steps of 0.1% from 3 to 
97%. Since the pulse repetition rate is 20 Hz, this means that the minimum 
step size is 50 microseconds. The constraints on the control algorithm, in 
addition to this granularity and repetition rate, were that a real time 
base must be kept accurately and that the pressure switch must be sampled 
every 2 milliseconds. These requirements were also dovetailed into the 
limitations of the 8049 time interrupt subsystem.