Negative pressure filtration device

A negative pressure filtration device is used in conjunction with a containment enclosure for the removal of hazardous material such as asbestos insulation surrounding pipes in habitable buildings. The inventive negative pressure filtration device generates a negative pressure within the containment enclosure, which negative pressure is quantitatively adjustable and capable of being continuously monitored by the user. Advantageously, the negative pressure filtration device is small, lightweight, portable, battery-operated and reliable, for use in a wide variety of hazardous material removal scenarios. The portability and versatility of the negative pressure filtration device is achieved through the use of operational vacuum pressures and air flow volumes much smaller than those of known systems.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to devices for assisting in the removal of 
hazardous material such as asbestos, and filtering the hazardous material 
from the air so that microscopic particles are not released into the 
atmosphere during the removal process. More specifically, the invention 
relates to such removal and filtration devices which employ "negative 
pressure", which as used herein denotes a lower pressure in a containment 
enclosure than ambient atmospheric pressure. 
2. Related Art 
Various methods and devices are known in the art for removal of hazardous 
materials from habitable environments. For example, methods have been 
developed to remove asbestos (believed to be carcinogenic) in insulation 
which encloses pipes and other conduits in buildings. The removal of the 
carcinogenic asbestos must be performed in a safe manner, if microscopic 
asbestos particles are not to be introduced into the atmosphere, thereby 
increasing the danger to building occupants rather than reducing it. 
A common method of removing asbestos insulation from around pipes has been 
to enclose a section of pipe within a containment enclosure, sealing the 
apertures from which the pipe penetrated the bag with duct tape or with 
wire ties After the containment enclosure was secured about the insulated 
pipe, measures were taken to attempt to insure that, during the physical 
removal of the asbestos insulation from the pipe within the containment 
enclosure, any microscopic particle matter was retained within the 
containment bag rather than escaping through any hole or seams 
inadvertently present in the containment bag. 
Typically, known methods involve the use of either no negative pressure, or 
negative pressure created with a HEPA vacuum. The use of HEPA vacuum 
creates a large amount of negative pressure and air flow volume. The large 
amount of negative pressure causes the containment bag to totally collapse 
around the insulated pipe. This collapsing is disadvantageous in that the 
material cannot be removed from the pipe because the arms of the user may 
become immobilized. Also, the plastic bag may be drawn against the vacuum 
hose aperture, causing total cutoff of air flow which puts excess strain 
on the vacuum motor. 
Furthermore, in known systems, there is no way to controllably and 
accurately vary the vacuum pressure and air flow volume. The comparatively 
large vacuum in known systems, typically capable of maintaining a pressure 
of approximately 120 inches of water while moving 100 cubic feet per 
minute (cfm) possesses many disadvantages. Similarly, known systems are 
not pressure-adjustable or air flow volume-adjustable, nor are they 
capable of being delicately controlled or monitored. 
High vacuum pressure in known systems places increased stress on the vacuum 
motor, which may cause burnout of the motor at an earlier time than if 
lower vacuum pressures were employed. Also, the high vacuum placed stress 
on the containment enclosure (typically a plastic bag), either resulting 
in dangerous rupture of weak containment bags or necessitating higher 
costs of stronger containment bags. Furthermore, the use of such a 
powerful vacuum requires 110-volt line voltage, causes the unit to weigh 
too much for true portability, and necessitates the unit to occupy too 
great a space to be conveniently carried into tight work areas. 
Finally, known systems have possessed the disadvantage of unnecessary 
complexity. Certain systems employing high vacuum pressure air flow have 
required two apertures, including a first aperture for inputting clean air 
into the containment bag and a second aperture for allowing the vacuum 
pump to withdraw contaminated air from the interior of the containment bag 
through a filter. 
Various U.S. patents disclose subject matter which is related to this area 
of technology. For example, U.S. Pat. Nos. 4,604,111, 4,613,348, 
4,626,291, and 4,812,700, all to Natale, disclose containment devices 
and/or filter devices. U.S. Pat. Nos. 4,783,129 and 4,842,347, both to 
Jacobson, disclose systems for removal of hazardous waste involving glove 
bags. Finally, U.S. Pat. No. 953,825 (Gekeler), U.S. Pat. No. 2,741,410 
(La Violette), U.S. Pat. No. 4,774,974 (Teter), and U.S. Pat. No. 
4,809,391 (Soldatovic) disclose systems for removing asbestos, or devices 
for supporting the broader function of removing hazardous materials. All 
documents cited herein are incorporated herein by reference as if 
reproduced in full in their entirety. 
Known systems, taken individually or in combination, have not provided a 
lightweight, portable, inexpensive means of safely removing hazardous 
materials. Furthermore, known systems employing negative pressure to 
prevent escape of hazardous particulate matter have lacked the ability to 
continuously and reliably monitor and control negative pressure air flow 
in a flexible containment enclosure, or automatically compensate vacuum 
pressure by adjusting the speed of the vacuum motor if a leak develops in 
the containment enclosure or some mechanical malfunction occurs. 
Therefore, a need exists for a negative pressure filtration device and 
method which overcomes the limitations of the known systems. 
SUMMARY OF THE INVENTION 
The invention overcomes the limitations of known systems by providing a 
negative pressure filtration device which may be used when removing 
hazardous material while minimizing escape of dangerous particulate matter 
into the atmosphere. 
The invention provides a negative pressure filtration device which 
automatically adjusts vacuum pressure to assure maintenance of 
substantially constant controlled negative pressure of the proper 
magnitude, both to prevent collapse of a containment enclosures which are 
flexible, and to insure that air is drawn inward through any leaks into 
the containment enclosure rather than contaminated air outward into the 
atmosphere. Provision is made for monitoring the magnitude of the negative 
pressure in a continuous manner. A convenient adjustable control allows 
the user to determine the level of negative pressure air flow to be 
applied in a given scenario. 
Finally, the invention provides a negative pressure filtration device which 
achieves all of the above objectives in a small, lightweight, inexpensive 
and portable unit. 
Other features and advantages of the present invention will become apparent 
upon a reading of the accompanying disclosure of the preferred 
embodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In describing preferred embodiments of the present invention illustrated in 
the drawings, specific terminology will be employed for the sake of 
clarity. However, the invention is not intended to be limited to the 
specific so selected, and it is to be understood that each specific 
includes all technical equivalents which operate in a similar matter to 
accomplish a similar purpose. 
FIG. 1A illustrates in exploded perspective view the physical components of 
a preferred embodiment of the negative pressure filtration device 
according to the present invention. Important functional elements of the 
illustrated embodiment include filter 6, which filters contaminated air 
drawn through hose receptacle 3A by blower motor 10. A rechargeable 
battery pack 13 provides power to the system. A controller unit serves to 
control the speed of the blower motor 10. Circuitry within the controller 
14 receives a user-selected setting from a potentiometer 18. Also, 
circuitry within the controller 14 provides to a display (such as LCD 
display 15) a measurement of differential pressure (a "negative pressure" 
between the interior of the containment enclosure and ambient air 
pressure). 
The functioning of these elements, including functions not specific 
mentioned immediately above, are presented below, with respect to FIGS. 
2A, 2B, 2C and 2D. Specific exemplary circuitry within the controller box 
14 is described below, with respect to FIGS. 2D and 3. 
For completeness, auxiliary elements in the preferred embodiment in FIG. 1A 
are now presented. The illustrated elements may be described as in the 
following chart: 
______________________________________ 
Element Number 
Description 
______________________________________ 
1a and 1b Bolt, 10/24 .times. 5" 
2 1/4" washer 
3A vacuum hose receptacle (flange) 
3B sensor hose receptacle 
4 rivet, 3/16" for securing element 
11 
5a and 5b Neoprene filter gasket 
6 HEPA filter 
7 filter/motor holding plate 
8 mounting nuts for element 7 
9a and 9b nut inserts for element 8 
10 blower motor 
10a blower motor output 
10aa aperture for blower motor output 
11 battery hold-down strap 
12 lock nut 10/24" 
13 rechargeable battery pack 
14 controller housing (for electronics 
and sensor) 
15 LCD display 
16 LCD display back plate 
17a and 17b LCD display retaining nuts 
18 on/off potentiometer 
19 potentiometer knob 
20 screw, 10/24 .times. 1/2", for element 12 
21a and 21b screw, 8/24 .times. 21/2", for element 
14a 
22 battery charging jack 
23A and 23b lock nut, 8/24", for elements 
21a, 21b 
24a, b, c rivet, 3/16", secures element 3 
25 housing 
26 vacuum hose swivel 
27 vacuum hose 
28 pre-filter holder 
29 pre-filter 
______________________________________ 
The specific means of interconnection of the various components illustrated 
in FIG. 1A need not be further described, other than by reference to the 
element descriptions immediately above. Alternative methods of physical 
construction lie within the contemplation of the invention and within the 
ability of those skilled in the art. 
As known to those skilled in the art, any construction should have the 
feature that air drawn through hose receptacle 3A through HEPA filter 6 
should follow an air tight path so that any microscopic contaminants in 
the air are in fact filtered by HEPA filter 6 and do not escape, either 
into the interior of the filtration device's housing 25 or into the 
external atmosphere. To this end, for example, gaskets 5a and 5b surround 
HEPA filter 6, and are compressed by the action of bolts 1a and 1b and nut 
inserts 9a and 9b. 
FIG. 1B illustrates a preferred embodiment of the present inventive 
negative pressure filtration device as deployed in conjunction with a 
typical flexible containment enclosure. 
FIGS. 2A, 2B, and 2C are block diagrams illustrating many functional 
components which were illustrated in perspective view in FIG. 1A. 
The present invention comprises control circuitry which performs several 
functions. A primary function is to regulate the pressure difference 
between ambient air pressure and the pressure appearing in the containment 
enclosure. This function, which may be referred to as "negative pressure 
regulation", is a principle purpose of the present invention. 
The invention provides for maintenance of this negative pressure 
substantially independent of variables which may be beyond the continuous 
control of the user. For example, the negative pressure is maintained 
substantially constant, independent of the magnitude of voltage output by 
the device's power source, an advantage which is of special utility in the 
event that rechargeable batteries are employed as the power source. 
Furthermore, the desired negative pressure may be maintained substantially 
constant even if there is clogging or other restriction in the filter, if 
leaks develop in the containment enclosure, or (with appropriate circuitry 
in certain embodiments) variations in the linearity or zero offsets of 
certain electronic components within the controller itself. 
A primary advantage of the present is its capability of being implemented 
in an extremely small and portable package as compared with known systems. 
The portability of embodiments of the present invention is enabled by the 
fact that the present invention need only maintain much lower vacuum 
pressure and flow requirements (on the order of 0.02-5.0 inches of water) 
than known systems (100-120 inches of water). Optimally, it has been found 
that 0.05-0.10" of water pressure fulfill the needs of safety (exceeding 
the 0.02" EPA requirement), while satisfying costs and size constraints. 
Similarly, in terms of volumetric flow rate of air needed to be processed 
through the blower, the present invention provides a maximum of on the 
order of only 40-100 cubic feet per minute (cfm) need be moved (as 
compared to approximately 80-400 cfm in known systems). Even the 40-100 
cfm acceptable to the present invention is a maximum capability, not a 
normal operating parameter The maximum amount of air flow is needed if a 
rip develops in the containment bag (to minimize escape of contaminants 
into the atmosphere), or to evacuate remaining air from bag after use. By 
employing larger motors, higher volume flow rates are possible, although 
not necessary or generally desirable due to cost and portability 
considerations. Generally, however, in accordance with the normal 
operation of the present invention, the much smaller, lightweight feature 
of the negative pressure filtration device derives from its smaller-scale 
vacuum characteristics and simple design. 
The smaller-scale vacuum characteristics derive in turn from a realization 
that known systems unnecessarily introduce air into the containment 
enclosure, only to spend additional energies withdrawing it for 
filtration. As illustrated in FIGS. 2A, 2B, and 2C, a single vacuum hose 
aperture in the containment bag is sufficient to allow operation of the 
inventive negative pressure filtration device, in contrast to many known 
systems. 
Commercially useful implementations of the present invention, meeting EPA 
standards, may weight as little as 7 lbs. This lightweight and small size 
(6.25.times.7.25.times.9.5 inches) allows substantial choice for the user 
in positioning the unit. The unit may be hung on the pipe from which 
hazardous materials are being removed, or it may be placed on scaffolding 
or other mechanical supports in the area, or it may be carried on a 
shoulder strap or back pack by the individual user. 
As described in three exemplary embodiments in FIGS. 2A, 2B, and 2C, the 
negative pressure regulation function may be performed by a feedback 
control loop comprising a blower 10, a pressure sensor 206 or 252, an 
amplifier 216, a motor power converter/controller 220, and a setpoint 
control 18. These elements function as described below to maintain a 
substantially constant negative pressure at a magnitude set by the user 
using setpoint control 18. 
Referring now to FIG. 1B, a first embodiment of the negative pressure 
filtration device 200 is illustrated. The negative pressure filtration 
device is connected by a vacuum hose 202 and a sensing hose 204 to a 
containment enclosure 299. As described above in the Background of the 
Invention, the containment enclosure 299 may comprise a plastic bag which 
surrounds a volume in which hazardous material is to be removed. For 
example, the containment enclosure 299 may comprise a plastic bag hung 
from and surrounding a pipe which is covered with asbestos insulation. 
In order to practice the present invention, one or apertures must be 
present in the containment enclosure to allow gas communication through 
vacuum hose 202 and sensing hose 204. In contrast to certain known 
systems, only one aperture is needed for creation of the negative pressure 
within the containment enclosure; these known systems require two 
apertures (one for receiving clean air into the containment enclosure, and 
a second, corresponding to 202, for withdrawing contaminated air into a 
cleaning or filtration device). 
Contaminants present in the air filtered through vacuum hose 202 are 
filtered by filter 6. Air is drawn through filter 6 by blower motor 10, 
with the filtered air being exhausted to the environment through blower 
output 10a. 
Meanwhile, sensing hose 204 is also in communication with the interior of 
containment enclosure 299. As is known to those skilled in art, it is 
generally considered advantageous to dispose the sensing hose 204 at a 
position distant from vacuum 202. This placement is designed to minimize 
undesired variations in sensed vacuum pressure caused by variations in air 
flow through juncture 298 (between vacuum hose 202 and containment 
enclosure 299). 
Sensing hose 204 is connected to a first port 208 of a differential 
pressure sensor 206. A second port 210 of the differential pressure sensor 
206 is connected to ambient air via pathway 212. Connected in this manner, 
differential pressure sensor 206 outputs a signal along pathway 214. The 
signal indicates the difference between ambient air at 212 and the 
interior of the containment enclosure 299. 
Because the air pressure within containment enclosure 299 is caused to be 
lower than ambient air pressure (through the action of blower 10), the 
differential pressure measured by differential pressure sensor 206 should 
be negative. As used in the present specification, the term "negative 
pressure" denotes a pressure within a containment enclosure 299 which is 
lower than that of ambient air, so that if any leaks develop in 
containment enclosure 299, contaminants within containment enclosure 299 
are substantially prevented from escaping through the leak. 
The differential pressure signal along path 214 is input to an amplifier 
216. Amplifier 216 outputs an amplified differential pressure signal along 
path 218. Amplifier 216 provides for amplification of the magnitude of the 
differential pressure sensor to a magnitude which is sufficient to drive 
indicator 15 and converter/controller 220. The amplified differential 
pressure signal is input to the indicator 15, allowing the user to 
continuously monitor the measured negative pressure within the containment 
enclosure 299. 
The amplified differential pressure measurement on path 218 is also input 
to motor power converter/controller 220. Converter/controller 220 also 
receives an input from setpoint control 18. Setpoint control 18 allows the 
user to specify and control the negative pressure in containment enclosure 
299. The converter/controller 220 controllably varies the voltage 
impressed across the blower's fan 10 in order to set its rotation speed in 
dependence on the setpoint control, so as to regulate the pressure 
difference between ambient air pressure and the pressure within 
containment enclosure 299. 
Voltage regulator 222 serves a primary function of converting a voltage 
level from power source 13 into a controlled voltage on net 224. Net 224 
feeds (directly or indirectly) components such as differential pressure 
sensor 206, amplifier 216, indicator 15, and converter/controller 220. 
The converter/controller 220 and the voltage regulator 222 are contained 
within controller box 14 (FIG. 1). 
Also resident on the circuit board inside the controller box is circuitry 
directed to the performance of a low-voltage disconnect feature. In the 
event that the output of power source 13 (such as rechargeable portable 
batteries) falls below a certain level, the entire unit is shut off 
automatically. The low-voltage disconnect feature provides for 
disconnection of the circuitry and blower fan motor load from the power 
source (battery) 13. This disconnection avoids possible irreversible 
damage to primary cell rechargeable batteries which would otherwise result 
from overdischarge. 
Also illustrated in FIG. 2A is a storage device for storing a pressure 
history recorded during a particular session of removal of hazardous 
material. The pressure history storage feature allows for generation of 
non-volatile documentation that the desired negative pressure was 
maintained throughout a session. Such documentation may prove useful in 
avoiding liability for illnesses alleged to be related to or caused by 
hazardous waste. If a proper negative pressure history is concretely 
evidenced, the argument that improper introduction of contaminants were 
introduced into the air during the session is substantially disproved. 
In structure, the storage device could comprise any volatile or 
non-volatile electronic storage device, such as a random access memory 
(RAM). The measurements output from amplifier 216 are periodically written 
into the electronic storage device. At the end of a given session, the 
data which had been written into the storage device is down loaded to an 
external non-volatile storage device, or printed in hard copy form. 
The filtration function of the inventive negative pressure filtration 
device may be enhanced through use of a pre-filter disposed in an adapter 
where vacuum hose 202 meets containment bag 299 (FIGS. 2A, 2B, and 2C) at 
298. Briefly, the adaptor may be implemented using a tubular structure 
into which is inserted a cylindrical filter comprising a filtration 
material such as Polyester Part 6, Dinier, and #15 Dinier Mixture, from E. 
R. Carpenter Company, Richmond, Va. The cylindrical filter itself fits 
within the end of the hose's tubular structure in a pre-filter adaptor of 
smaller diameter than the tubular structure. Placement of a pre-filter at 
this point helps to insure that fewer particles, especially macroscopic 
particles, are drawn up the vacuum hose into the negative pressure 
filtration device itself. 
It is advantageous to employ a vacuum hose with a sharp-ended pre-filter 
adaptor, which are initially disposed on opposite sides of the containment 
bag. By pressing the sharp-ended pre-filter adaptor into the containment 
hose through the containment material, thereby piercing the containment 
bag material within the hose, and then inserting the pre-filter itself, a 
sealed aperture is formed. The containment enclosure material which is 
firmly trapped between the vacuum hose and the pre-filter adaptor is held 
in place by the adaptor's insertion into the hose, allowing air to pass 
only through the hole pierced in the interior region of the adaptor. 
Placement of the pre-filter within the pre-filtered adaptor assures that 
all air which passes through the pierced hole has been pre-filtered. 
In a review of the embodiment shown in FIG. 2A, the components may be 
specifically implemented using the following exemplary parts: 
______________________________________ 
Element Implementation 
______________________________________ 
Sensor 206 Honeywell AWM2100V 
Amplifier 216 Suitable operational amplifier(s); 
see also FIG. 3 
Filter 6 HEPA filter (99.97% efficiency at 
0.3 microns) from Cambridge 
Filters, of Rochester, New York 
Indicator 15 LED, LCD, or gauge 
Power Source 13 
Rechargeable batteries (6 V DC) such 
as Panasonic #1CR6V2.4P, E.A.C., 
Raleigh, N.C.; or, less preferably, 110 
VAC 
Blower Motor 10 
Racal Health and Safety, Frederick, 
Maryland 
Set Point Control 18 
Rheostat #31YN401, Mouser Electron- 
ics, Mansfield, Texas 
Vacuum Hose 202 
1.25-inch non-collapsible 
Sensor Hose 204 
clear, flexible PVC tubing, 3/16 I.D.; 
5/16 O.D., such as part # 
206 series from Accuflex of Canton, 
Michigan 
Voltage Regulator 222 
See FIGS. 2D, 3 
Converter/Controller 
See FIGS. 2D, 3 
220 
______________________________________ 
Referring now to FIG. 2B, a second embodiment of the negative pressure 
filtration device is illustrated. Most of the components shown in FIG. 2B 
may be chosen identical to those shown in FIG. 2A. However, certain new 
components and connections are illustrated in what may be considered in 
certain respects an enhancement of the embodiment shown in FIG. 2A. 
Valve 240, autozero subsystem 254, and zero offset subtractor 264, 
alternative storage device 266, and linearizer 268 are structures which 
were not illustrated in FIG. 2A. Briefly, the enhancement offered by FIG. 
2B is the presence of an autozero subsystem which dynamically compensates 
for (among other things) offset inaccuracy of the differential pressure 
sensor 252. 
Valve 240, preferably an electrically-actuated valve or solenoid-controlled 
valve, has its two inputs connected respectively to ambient air via port 
244 or to sensing hose 204 via port 242. The output of valve 240 is input 
to a first port 248 of differential pressure sensor 252. In this manner, 
the switchable valve 240 passes either ambient air pressure via 244 or 
containment enclosure pressure via 204 and 242 to the differential 
pressure sensor 252. 
Autozero subsystem 254 provides control for the position of the valve 240 
in the following manner. Periodically, such as every 30-60 seconds, the 
valve is switched from its "normal" connection (to the sensing hose at 
port 242) to its second port (connection to ambient air at 244). When 
valve port 244 is selected, ambient air pressure is present at both 
differential pressure sensors ports 248 and 250. The output of the 
pressure sensor at 214 should therefore be indicative of a zero pressure 
differential. 
At this time, when the differential pressure sensor 252 outputs a reading 
indicative of a zero pressure differential, a temporary storage device 260 
within the auto zero subsystem stores the zero-indicative value. (Ideally, 
though not in practice, this value should be zero. Autozero subsystem 254 
compensates for occasions when it is not zero.) 
During normal (reading) operation, the valve 240 is switched back to port 
242, so that containment enclosure pressure passes through sensing hose 
204 to the first port 248 of the differential pressure sensor. Whatever 
actual negative pressure is present is then output from differential 
sensor 252 and amplified by amplifier 216. Any improper offset of the 
differential pressure sensor or amplifier is compensated for by 
subtracting the value stored in temporary storage device 260 from the 
current measurement along path 262. A zero offset subtractor 264 receives 
the current measurement on path 262 and the stored zero-indicative value 
along path 258, and subtracts one from the other to arrive from a 
corrected, zero-adjusted measurement pressure. In this manner, the effects 
of any "slow wandering" (wandering slow enough that no significant change 
occurs between updates of the offset correction) of the zero value of the 
entire measurement apparatus is compensated. 
The strobing of information into the display indicator 15, and the changing 
of control information into converter/controller 220, is properly 
synchronized to the switching of valve 240. Respective indicator or 
control data is input to these devices only when the differential pressure 
sensor 252 has stabilized its output after connection to sensing hose 204. 
In this manner, spurious effects of the zeroing portion of the auto zero 
function do not adversely effect the indicator or motor control functions. 
Alternatively, an additional zero-order hold memory (a parallel-in shift 
register, for example) 266 may be inserted at the output of zero offset 
subtractor 264. Proper negative pressure differential information, 
generated when the differential pressure sensor is connected to the 
containment enclosure interior, is stored in register 266. Thus, the 
timing and strobing may be applied to register 266 rather than to a 
plurality of elements such as indicator 15 and motor power 
converter/controller 220. 
As an optional enhancement, a linearizing system 268 may be employed. 
Linearizing system serves to reduce sensor errors due to non-linearities 
in the system, especially in the differential pressure sensor 252. The 
linearization is capable of implementation by those skilled in the art and 
need not be further detailed herein. Those skilled in the art will readily 
appreciate that a conversion function may be implemented using, for 
example, a look-up table composed only of programmable read only memories 
(if implemented digitally) or an analog circuit implemented with a desired 
transfer function (if implemented using analog components). 
The elements particular to FIG. 2B which were not present in FIG. 2A may be 
implemented as follows. 
______________________________________ 
Element Implementation 
______________________________________ 
Sensor 252 MPX 10 or MPX 2010 silicon pressure sensors, 
from Motorola, Inc., of Phoenix, Arizona 
Valve 240 Micro-3-Way, (solenoid valve), from the Lee 
Company, Westbrook, Connecticut 
______________________________________ 
Of course, variations from these particular implementations may be made by 
those skilled in the art without varying from the spirit and scope of the 
present invention. 
As stated above, amplifier 216, auto zero subsystem 254 with storage 
element 260, zero offset subtractor 264, temporary storage device (e.g., 
shift register or sample-and-hold device) 266, and linearizer 268 may be 
implemented using common elements known to those skilled in the 
electronics art, although a specific exemplary implementation is 
illustrated in FIG. 3, described in detail below. 
FIG. 2C illustrates in block diagram form a third embodiment of the 
negative pressure filtration device according to the present invention. In 
the embodiment of FIG. 2C, a microprocessor 270 assumes many of the 
control and analysis functions performed by discrete components in the 
embodiment of FIG. 2B. 
Referring to FIG. 2C, a bi-directional data input D of microprocessor 270 
is connected to a data bus 272. Software governing the control and 
analysis functions of the microprocessor 270 is resident in read-only 
memory (ROM) 271, which is also connected to the data bus 272 in a manner 
known to those skilled in the art. 
The data bus 272 provides a pathway by which data may be input to and 
output from the microprocessor 270. For example, the amplified 
differential pressure measurement from amplifier 216 may be converted (if 
necessary) from analog to digital form by A/D converter 284, and 
registered in a buffer 274 before being input to the microprocessor 270. 
The microprocessor performs whatever functions need be preformed in the 
particular embodiment (such as offset compensation) before outputting the 
appropriate values for the differential pressure to pressure history 
storage device 230 (which may be a random-access memory in direct 
communication with the data bus 272), and to indicator 15 (possibly 
through a buffer 278). A control signal governing the motor power 
converter/controller 220 may be buffered at 280 before being input to the 
converter/controller. 
The microprocessor 270 may also perform the timing and switching functions 
of valve 240. A binary value corresponding to the desired state of valve 
240 is output to a buffer 276, and may be converted to voltage and current 
levels by amplifier 282 to operate a solenoid which governs the position 
of valve 240. 
Certain general features of microprocessor-based technology have been 
omitted from FIG. 2C and from this description inasmuch as they are well 
known to those skilled in the electronics art. For example, no address bus 
is explicitly shown in FIG. 2C, as it is well understood that addresses 
may be used to selectively strobe clock pulses into buffers, or activate 
and deactivate tri-state buffers, so as to govern the flow of data into 
and out of the microprocessor 270 through use of data bus 272. Similarly, 
the details of implementation of software for the various functions 
desired to be performed by the microprocessor 270 may be written by those 
skilled in the art, given the functional descriptions found in this 
specification, before being programmed into ROM 271. 
The functions governed by microprocessor 270 include not only sensing the 
pressure sensor output, driving the digital indicator elements, and 
contributing to setting the motor voltage. The low-battery cut off 
function (described elsewhere in this specification, with reference to 
FIG. 2D) may also be implemented using a microprocessor. By polling a 
quantitative measurement of the battery voltage, the microprocessor may 
halt operation based on a software comparison of the read-in battery 
voltage measurement with a predetermined value below which it is desired 
to terminate operation. 
Similarly, the registers within a microprocessor are ideally suited to 
storage of the zero-differential-pressure offset, which offset can be 
subtracted from subsequent actual measurements of differential pressure 
between ambient air and containment enclosure pressure. 
The auto-zero cycling process is also readily implemented using the timing 
capabilities inherent in known microprocessor-based systems. An interrupt 
programmed for periodical intervals (such as 30-60 seconds) may cause 
specific interrupt software modules to be executed by the microprocessor 
270 which cause valve 240 to switch positions temporarily to ambient air, 
along path 244. This position is maintained until a zero pressure 
differential signal is output from differential pressure 252 through 
amplifier 216. After the zero offset reading has been input into a storage 
location in the microprocessor, the position of valve 240 is returned to 
its normal "read" position 242 for subsequent actual differential pressure 
measurements in the containment enclosure. 
Furthermore, the linearization of the sensor may be readily performed in 
software. A software-implemented look up table is a preferred method of 
mapping input readings onto a desired set of output readings, which may 
then be output to buffer 280 so as to control the motor voltage. 
Also, the use of a microprocessor facilitates the storage of the sequence 
of differential pressure readings for generation of a differential 
pressure history. Storage device 230, which may be the random access 
memory (RAM) which is commonly used in association with any 
microprocessor. As known by those skilled in the art, a communications 
cable may be directly connected to the microprocessor-based system. The 
negative pressure history for a given cleaning session may be output 
through any of a number of communications controllers (such as UARTs or 
USARTs) to a printer or non-volatile storage device at the opposite end of 
the communications cable as pictured in FIG. 2C. However, a path is shown 
in FIG. 2C exiting storage device 230 to be directly connected to external 
devices. This illustration presupposes some form of direct memory access 
(DMA), a process which is known to those skilled in the electronics arts. 
FIG. 2D is a circuit diagram illustrating a particular embodiment which 
does not employ the full feedback loop shown in FIGS. 2A, 2B, and 2C. It 
may be considered a simplified version of those earlier-described 
embodiments, although it possesses the advantage of conservation of batter 
charge due to use of a switching type of converter. 
Briefly, FIG. 2D comprises a control unit outlined in dotted lines. A 
potentiometer, labelled EXTERNAL SPEED ADJUST, allows the user to specify 
a voltage which ultimately helps to determine the magnitude of negative 
pressure desired for the containment enclosure. A power source is shown as 
a second input to the control unit. The control unit receives the negative 
pressure setting from the user and (employing a switching regulator 
control IC) converts the power source voltage (here, a DC voltage of, 
e.g., 6 volts) into an output voltage (adjustable to a range on the order 
of 1-4 volts) for controlling the speed of the indicated BLOWER MOTOR. 
Roughly the right-most two-thirds of the circuitry shown in the control 
unit is dedicated to conversion of the power source voltage to the motor 
control voltage; the circuitry in the left-most third of the control unit 
is directed to the low voltage cutoff function which avoids possible 
irreversible damage to rechargeable batteries that may result from 
overdischarge. 
Specific functions of the various components of the exemplary embodiment 
shown in FIG. 2D are next described. 
Several functions are performed by the integrated circuit IC1 (MC34063). An 
internal (on-chip) stable voltage reference is provided, for purposes of a 
comparison which in turn generates an "error" signal from which the motor 
control voltage is derived. A high-gain error amplifier in the chip 
subtracts the voltage at the device's "-IN" point from the 
internally-generated reference voltage, and amplifies the difference in 
order to drive the on-chip switching circuit. A free-running oscillator 
and associated switching control logic is provided (at pin 3). A current 
limit comparator that senses the voltage developed across an external 
current-sense resister R2 and shuts off the drive to the internal 
switching transistor when the sensed current exceeds a limit. 
In operation, IC1 adjusts the duty cycle (ratio of on-time to total cycle 
time) of switch transistor Q1 in order to regulate the voltage sensed at 
its "-IN" pin. The Application Note AN920A, "Theory and Application of the 
MC34063 and UA78540 Switching Regulator Control Circuits" from Motorola 
(Schaumberg, Ill.) is incorporated herein by reference as if reproduced in 
full below. Implementation of the embodiment shown in FIG. 2D is not 
dependent on use of the MC34063, as the various functions performed by 
this IC may be substituted by use of other IC's, in combination with 
discrete components. 
Use of switching regulator techniques, as opposed to "dissipative" 
techniques, provide embodiments of the present invention with more energy 
efficiency. Energy efficiency is especially important when rechargeable 
batteries are the power source and when portability and convenience are 
important. Battery power consumption may be reduced, and battery charge 
life thereby extended by a factor of two or more. 
Switching transistor Q1 acts to duty-cycle modulate current flow through 
the current-regulating inductor L1, in response to the drive provided by 
IC1. While IC1 has internal switching transistors, an external device Q1 
is employed to improve efficiency and power output capability, a 
substantial goal of the present invention. Q1 is advantageously chosen to 
be a MOSFET with low on-channel resistance, employed so as to minimize the 
voltage drop when conducting. 
Switching inductor L1 serves to filter the modulated current flow from Q1. 
In a conventional manner, L1 stores energy while Q1 is conducting, and 
releases energy when Q1 is off. 
Switching flyback diode D3 operates in conjunction with L1 to allow current 
flow out of L1 when Q1 is off. L1 will discharge through D3 until its 
stored flux is dissipated. L1, C1 and the operating frequency of IC1 
(typically in the hundreds of kilohertz) are chosen to operate 
satisfactorily across the range of load current drawn by the blower motor. 
Filter capacitors C1 and C6 act to filter the voltage appearing at the 
output of L1. C6 is of a type and construction to provide effective 
filtering of higher-frequency components. 
Current sense resistor R2 serves to sense the peak current flow in the 
regulator for the current limit circuit of IC1. R2 straddles SEN and VCC 
inputs of the MC34063 chip. 
Reverse protection diode D1 protects the regulator circuitry from damage 
that might occur from reversed connection to a power source. 
Oscillator timing capacitor C3 sets the free-running frequency of the 
oscillator on the MC34063. 
Potentiometers R7 and R1 act as a voltage divider to add an adjustable DC 
voltage to the sensed regulator output voltage, providing a factory 
adjustment for minimum blower motor voltage. Due to the design of IC1 the 
minimum regulated voltage is that of the internal voltage reference in IC1 
(nominally 1.25 volts). The effect of the voltage added by divider action 
in R1 and R7 is to reduce the output voltage of the regulator. 
Reference diode D2 performs two functions. In the low battery cutoff 
function, D2 conducts when the voltage applied across its terminals 
exceeds about 4.3 volts. For supply voltages at or minimally above 4.3 
volts, a small conduction current flows via R8 and R9. For higher supply 
voltages, the higher voltage drop across R9 permits conduction via the 
base-emitter junction of Q2, enabling current flow at the collector of Q2 
and turning on Q3. The values shown in FIG. 2D allow Q3 to remain off for 
supply voltages below about 5 volts, appropriate for use when the power 
source comprises 6 volt gelled-electrolyte lead-acid batteries. 
Second, as a voltage reference, D2 provides a stable voltage at R7 to 
enable the minimum motor voltage adjustment described above for R7 and R1. 
D2 is advantageously implemented as an integrated circuit that functions 
as an adjustable zener diode. R10 and R11 establish its reverse conduction 
voltage. 
Transistor Q3 serves to disconnect the switching regulator circuitry and 
blower motor from the supply voltage when the supply voltage is below the 
cutoff threshold established by the action of D2, Q2 and associated 
resistors. Q3 is a MOSFET with low on-channel resistance. Use of such a 
device minimizes the voltage drop when conducting. R5 ensures that Q3 
turns off when Q2 is not conducting, and prevents turn on in the presence 
of any collector leakage current in Q2. 
Resistor R3 serves to establish the maximum regulated voltage to the blower 
motor. Bypass capacitors C2, C5, C4, C7 provide a low-impedance path for 
flow of high frequency components of current, and serve thereby to 
minimize unwanted radio-frequency emissions of the circuit. Filter 
networks F, which may be pi-configured networks, reduce radio-frequency 
emissions of the circuit. Jumpers X1 and X2 are circuit-card jumpers shown 
to facilitate planning of fabrication, and serve only as conductors, The 
BATT CHARGE JACK connector is provided for convenience in recharging a 
battery used as a power source. 
For completeness, the values or component specifications of the various 
components illustrated in FIG. 2D are as shown in the following chart. 
______________________________________ 
Element Value/Specification 
______________________________________ 
IC1 MC 34063P1 
R1 50K trimpot, face-up, laydown, leads 
0.1" triangular pattern, linear taper, 
Panasonic EVM-31GA00B15 or equivalent; 
Digikey 36C54 
R2 0.27 ohm metal oxide film resistor, 
10%, 1 W min, 0.25" diameter (max) .times. 
0.75" diam (max); axial leads 0.035" 
diam (max); RCD RSF1A series or equal; 
Allied 840-4xxx 
R3 2.2K (5%; 0.25 W for R3-R11) 
R4 100K 
R5 100K 
R6 4.7K 
R7 220K 
R8 2.2K 
R9 10K 
R10 910K 
R11 390K 
C1 470 uF, 10WVDC aluminum electrolytic, 
radial leads 0.2" spacing, 12 mm max 
diam, 18 mm max length, Panasonic ECE- 
A1AFS471 or equivalent; Digikey #P1204 
C2 0.1 uF, 50WVDC ceramic, radial leads, 
0.2" spacing; Panasonic ECQ-V1H104JZ; 
Digikey P4525. 
C3 1500 pF, 20 V, 0.2" leads 
C4 0.1 uF, 50WVDC ceramic, radial leads, 
0.2" spacing; Panasonic ECQ-V1H104JZ; 
Digikey P4525. 
C5 0.1 uF, 50WVDC ceramic, radial leads, 
0.2" spacing; Panasonic ECQ-V1H104JZ; 
Digikey P4525. 
C6 0.1 uF, 50WVDC ceramic, radial leads, 
0.2" spacing; Panasonic ECQ-V1H104JZ; 
Digikey P4525. 
C7 0.1 uF, 50WVDC ceramic, radial leads, 
0.2" spacing; Panasonic ECQ-V1H104JZ; 
Digikey P4525. 
C8 0.1 uF, 50WVDC ceramic, radial leads, 
0.2" spacing; Panasonic ECQ-V1H104JZ; 
Digikey P4525. 
L1 1 mH toroidal inductor; Renco RL1386- 
1 
Q1 1RF9531 power MOSFET 
Q2 2N3906 PNP transistor 
Q3 1RF521 power MOSFET 
D1 1N5718 or 1N5719 Schottky 
D2 LM385Z 
D3 1N5718 or 1N5719 Schottky 
F1, F2, F3 EMI filters, Panasonic EXCEMT222BC, 
Digikey P9808 
EXT SPEED AD- 
10K potentiometer, 0.1 W min, 0.25 .times. 
JUST 0.5 inch shaft, linear taper, SPST on- 
off switch with 0.5 A min rating (Radio 
Shack 271-1740 switch assembly - 271- 
1715 potentiometer) 
Circuit Board 
FR-4 or G-10, 1/16" 
Spacer 0.232 diam (max) .times. 1 3/16 long inside 
diameter to clear #6 screw; may be cut 
form nylon or metal tube stock, or made 
up by stacking stock spacers 
Hookup wire #20-22 AWG stranded tinned and fused, 
insulated 
Wire for BATT 
#20-22 AWG 
CHG JACK 
______________________________________ 
Of course, variations and modifications of the described embodiment lie 
within the contemplation of the invention and within the skill of those 
skilled in the art. 
Referring now to FIGS. 3 and 3A, a particular exemplary implementation of 
the embodiment shown and described with respect to FIG. 2B is illustrated. 
Many of the particular circuit details are substantially similar to those 
in FIG. 2D, and the above discussion related to FIG. 2D applies to many of 
the circuit details shown in FIG. 3. (Certain individual components may 
not have corresponding designators, however, and the figures should be 
referred to appreciate the components' interconnection). The action of 
those elements of FIG. 3 not specifically described with respect to FIG. 
2D are next presented. 
In generating the motor control voltage, the control unit with the 
switching regulator control IC performs a basic function of comparing a 
signal indicative of the actual measured negative pressure with a voltage 
from the differential pressure setpoint control 18 (FIG. 2B). The 
difference, which may be considered an "error" in control loop 
terminology, is amplified so as to properly affect the motor control 
voltage. The comparison and amplification occurs within the motor power 
converter/controller block 220 in FIG. 2B. 
Referring again to FIG. 3, in operation, IC1 adjusts the duty cycle (ratio 
of on-time to total cycle time) of switch transistor Q3 in order to 
regulate the voltage sensed at its "-IN" pin. This voltage is a sum of the 
pressure sensor output via R7, the setting of the pressure setpoint 
control POT1, and a bias applied via R6. 
R13 ensures that Q3 is not biased on by leakage currents in IC1. 
Filter capacitor C1 acts to filter the voltage appearing at the output of 
L1 in order to reduce variations that may otherwise cause audible noise in 
the blower motor. 
Resistor R6 adds a DC signal component to the appearing at the "-IN" pin of 
IC1 to provide a adjustment for minimum blower motor voltage. 
Voltage regulator VR1 provides a constant voltage supply for the sensor, 
motor subassembly, auto-zero subsystem, and the various operational 
amplifiers (denoted "OAx"). 
The sensor (transducer) amplifier block (comprising OA1, OA2, OA3, R10, 
R11, R12) functions as a differential amplifier which accepts the 
low-level sensor output and provides an amplified signal. A practical 
amplifier may require offset nulling and/or gain adjustments to compensate 
errors in the amplifier or sensor. These are not shown for simplicity. 
The digital meter subassembly is preferably calibrated in units of 
pressure, and indicates the sensed differential pressure. A digital meter 
is shown in FIG. 3, but any type of sensitive voltage- or current-actuated 
indicator can be used. 
Hold circuits K1 and K2 (Q4/C2, Q5/C3) store a signal voltage in capacitors 
C2 or C3 when the associated FET is in its off state. The downstream 
operational amplifiers are chosen to have suitably low input bias currents 
to minimize drift/droop during the holding mode. 
The zero offset subtractor circuit (comprising OA4, OA5, and R9a . . . d) 
subtracts the signal from Hold Circuit K2 from the output of hold circuit 
K1. The particular configuration shown provides a high impedance load for 
both Hold circuits. 
The potentiometer POT1 is employed by a user to establish a desired 
pressure control point (setpoint). The minimum-pressure position is when 
the slider is at the upper (+VREG) end. 
The Auto-zero Sequencer provides synchronized control signal to the 
auto-zero valve "V", and the two hold circuits, K1 and K2. This 
subassembly can be of simple electrical timing circuits that produce the 
control signal sequence shown in the time diagram. During the "zeroing 
cycle", the hold circuit K2 samples the amplified sensor output signal 
when its ports are connected together by valve, and holds this sampled 
zero offset signal during the subsequent "reading cycle". During the 
"reading cycle", the sensor is connected to read the differential pressure 
created by the blower fan, the output of hold circuit K2 is stable, and is 
subtracted from the signal passing through circuit K1, which is in its 
"read" mode. The zero offset error of the sensor is subtracted from this 
reading by the zero offset subtractor circuit. 
Reference diode VR2 provides a reference voltage for low battery cutoff 
operation. VR2 conducts when the voltage applied across its terminals 
exceeds about 4.3 volts. For supply voltages at or minimally above 4.3 
volts, a small conduction current flows via R1 and R2. For higher supply 
voltages, the higher voltage drop across R1 permits conduction via the 
base-emitter junction of Q11, enabling current flow at the collector of Q1 
and turning on Q2. The values shown will allow Q2 to remain off for supply 
voltages below about 5 volts, appropriate for use with "6 volt" 
gelled-electrolyte lead-acid batteries. 
R4 provides a small amount of hysteresis in the action of the low-battery 
cutoff circuit. 
FIG. 3A illustrates the timing of hold circuits K1 and K2 (FIG. 3) in 
relation to the two possible positions of valve 240. As shown in FIG. 3A, 
during the zeroing epoch of the valve, hold circuit K2 is allowed to 
sample and thereafter hold the zero offset output until the next zeroing 
epoch. Between zeroing epochs occur read epochs which substantially 
continuously monitor the actual negative pressure within the containment 
enclosure. 
Modifications and variation of the above-described embodiments of the 
present invention are possible, as appreciated by those skilled in the art 
in light of the above teachings. It is therefore to be understood that, 
within the appended claims and their equivalents, the invention may be 
practiced otherwise than as specifically described.