System for monitoring and controlling position of hoists

Disclosed is a system for monitoring and controlling movement of a hoist along a line of treatment tanks of an electroplating apparatus. Hoist-mounted optical sensing units of the system have pickups formed of opposed lines of infrared emitting diodes and phototransistors which interact with elements of plates fixed at work stations along the tanks to provide station identification and centering information relative to the hoist. A microprocessor-based controller on the hoist processes data from the sensing units and from a rotary encoder attached to a hoist drive motor and, under direction of an off-hoist main computer, rapidly and accurately positions the hoist at selected stations in a desired operating sequence.

BACKGROUND OF THE INVENTION 
This invention relates to apparatus for controlling the position of 
conveyors and particularly to improved systems for monitoring and 
controlling the position of hoists which move articles to be treated along 
electroplating tanks. 
Various mechanisms such as limit switches, bar code readers, and other 
optical devices have been used for determining the position of conveyors 
or elevator-conveyors such as hoists along a path to and from work 
stations. In the electroplating industry, for example, such mechanisms are 
utilized in controlling the movement of a hoist mounted over or on one 
side of a line of several tanks so that articles carried by the hoist may 
be immersed in baths of various tanks according to a desired sequence. 
Each of the positioning systems currently in use have disadvantages. For 
example, mechanical devices such as limit switches which engage code 
plates in parallel are bulky and not very reliable. Bar code readers and 
similar optical coding devices depend on the reflectivity of 
code-generating materials and thus may be subject to errors from 
accumulation of dust, oil, or mist on the reflective materials or from 
spurious reflections. Bar code readers also cannot, unless complicated 
optics are used, identify the position of a hoist when the hoist is 
stationary. Transmissive optical systems which utilize one or more 
photoelectric switches traveling with the hoist and a stationary light at 
each work station are bulky and expensive; they typically require a 
separate electrical line for each light. Moreover, such systems do not 
provide positive station identification when the moving hoist is stopped 
by an on-station light, and positioning errors are possible if an 
incorrect light is illuminated. 
Accordingly, it is an object of the invention to provide an improved system 
for monitoring the position of a conveyor such as a hoist. 
It is a particular object of the invention to provide a system for 
monitoring and controlling the position of a hoist along a line of 
electroplating tanks which gives positive identification of station 
position. 
It is an object of the invention to provide a compact, reliable, low cost 
system for accurately positioning a conveyor such as a hoist along a 
plurality of work stations. 
SUMMARY OF THE INVENTION 
The invention concerns improved apparatus for monitoring and controlling 
the position of a conveyor such as a hoist which moves articles to be 
treated along a line of tanks. Included in the control system is a sensing 
unit having an array of opposed radiation emitters and detectors mounted 
on the hoist and spaced along the direction of hoist movement. Stationary 
radiation blocking means are provided at each of several work stations 
along the path of the hoist to pass within the gap between the emitters 
and detectors as the hoist approaches a work station and to selectively 
block radiation from at least some of the emitters. A controller processes 
signals from the sensing unit to provide accurate information on the 
"on-station" positioning of the hoist and the identity of the station. The 
controller and one or more sensing units form part of a position control 
system which also includes a rotary encoder and motor drive attached to a 
hoist motor and which, under supervision of an off-hoist computer 
controller, rapidly and accurately positions the hoist at selected work 
stations according to a desired sequence of operation. 
In a preferred embodiment two hoist-mounted sensing units are included in a 
monitoring and control system for an overhead hoist of an electroplating 
line. Each sensing unit has several transmission-type optical pickups 
comprising an infrared emitting diode and a phototransistor facing the 
diode and operable to detect its infrared emission. One sensing unit 
carries eight pickups arranged in a generally horizontal line and which 
interact with up to eight elements of stationary code plates attached to 
each work station as the plates pass through the gap between the diodes 
and phototransistors. Two end elements of the code plates function to 
block infrared radiation of the end diodes of the unit to indicate that 
the hoist is positioned "on-station", and the pattern of radiation blocked 
by up to six remaining code plate elements provides positive 
identification of the station. The second sensing element includes sixteen 
closely spaced optical pickups which interact with a centering plate 
attached to each work station to provide accurate positioning information. 
Also mounted on the hoist is a microprocessor/controller which is 
electronically connected to the sensing units, a rotary encoder which 
provides the microprocessor/controller with data on hoist position both at 
and between stations, and a variable frequency motor drive which, on 
instructions from the microprocessor/controller, operates a motor to move 
a hoist to selected work stations.

DESCRIPTION OF PREFERRED EMBODIMENTS 
In the disclosure set forth below, those aspects of the invention relating 
to the monitoring and identification of the position of a hoist along a 
line of tanks will first be described in detail. This will be followed by 
a discussion of a preferred position control system incorporating the 
position identification subsystem. 
FIG. 1 illustrates in schematic form a preferred radiation sensing unit and 
a station code plate 20 which form basic elements of the monitoring and 
control system of the invention. The sensing unit includes an array of 
several pickups (eight being shown by way of example) each including a 
radiation emitter 30 mounted opposite a radiation detector 36 and defining 
an elongated gap 40. As will be shown and described in greater detail 
hereinafter, the sensing unit is mounted on a conveyor or hoist which may 
be moved to and stopped at any of several stations such as work stations 
of tanks of an electroplating line. A stationary code plate such as the 
plate 20 is attached to each work station and oriented to pass within the 
gap 40 of the sensing unit as the hoist approaches the station. The plate 
20 is formed with elements 44 which block or interrupt radiation from 
selected emitters such as the emitters 30a, 30c, 30f, and 30h when the 
hoist arrives "on-station". Preferably opposite ends of each plate such as 
the plate 20 include elements (44a, 44h) to block radiation from the 
corresponding end emitters 30a and 30h. The interruption of radiation 
detection by both detectors 36a and 36h provides a true indication that 
the movable hoist or conveyor is positioned at a work station. At the same 
time, positive identification of the specific station is indicated by the 
pattern of radiation sensed by the remaining detectors 36b -36g (for the 
station shown in FIG. 1 detectors 36b, 36d, 36e, and 36g sense radiation). 
Since from zero to six elements may be included in the plate, up to 
sixty-four different stations may be coded using the "eight position" 
plate of FIG. 1. 
Infrared emitting diodes are currently preferred for use as radiation 
emitters 30 of the sensing unit, and infrared-sensitive phototransistors 
are preferred as detectors 36. These devices are inexpensive, readily 
available, and have long operating lives. By way of example, suitable 
diodes and phototransistors for the sensing unit are available from 
Motorola, Inc., as part numbers MLED930 and MRD300, respectively. Other 
types of radiation-transmitting pickups may, however, be employed in the 
invention; for example, magnetic pickups can be utilized. 
A suitable spacing for the line of emitters 30 (and the opposed line of 
detectors 36) is at one inch centers, and the emitters and detectors may 
be mounted in a channel or box (see member 92 in FIG. 3) providing a gap 
of about seven-eighths inch between the opposed emitters and detectors. 
The elements 44 of a station code plate 20 in such an arrangement have a 
width of about one inch in the direction of movement of the sensing unit 
85 so that the end elements 44a and 44h extend about half an inch beyond 
their corresponding emitters (30a, 30h) and detectors (36a, 36h) when the 
hoist (and sensing unit) are "on-station". 
For certain applications requiring greater hoist-positioning accuracy than 
is assured by the above-described sensing unit with one inch plate 
elements 44, a second sensing unit may be provided having closely-spaced 
radiation emitters and an opposed similar group of detectors. This second 
sensing unit, termed a "station centering unit" is preferably used in 
conjunction with the earlier-described sensing unit (termed a "station 
identification unit") and provides fine position readings for accurately 
centering a hoist at a station. 
One suitable emitter array 45 for a station centering unit (FIG. 2) has two 
rows 46 and 47 of eight emitters staggered to provide an effective spacing 
of one quarter inch between emitters along their direction of movement. A 
centering plate such as the plate 48 is mounted at each work station and 
indicates centering of the emitter array 45 (and thus of the hoist to 
which it it is attached) to an accuracy of one quarter inch or better when 
the plate 48 blocks radiation from the emitters 46e and 47d as shown in 
FIG. 2. Such accuracy helps assure proper positioning of articles when 
they are lowered into tanks for treatment and may help reduce space 
required for the electroplating line. 
As is indicated in the modified block diagram of FIG. 3, a sensing unit 
such as the station identification unit 56 and, if needed, a station 
centering unit 58, are carried by a conveyor or hoist such as the overhead 
hoist 60. The hoist 60 is mounted over and is movable along a line of 
several treatment tanks, three of which are shown at 62a, 62b, and 62c. 
According to conventional practice the hoist 60 also is operable, by 
conventional mechanisms such as a motor and chain drive not illustrated 
herein, to lower articles into a bath or chemical solution carried by each 
tank 62 so that specific treatments such as stripping, cleaning, 
activating, plating, and rinsing may be performed in a prescribed 
sequence. Also shown in FIG. 3 are fixed station code plates 20a, 20b, 20c 
and station centering plates 48a, 48b, 48c and a microprocessor/controller 
70 which receives signals from the units 56 and 58 along electrical lines 
74, 76, respectively. (Other components illustated in FIG. 3 and their 
operating relationship with the microprocessor/controller 70 are described 
hereinafter in the discussion on hoist control.) 
A preferred mounting arrangement of two sensing units on a movable overhead 
hoist 60 is illustrated in FIG. 4, and FIG. 5 shows the alignment of the 
elements of two station plates with these sensing units. As indicated in 
FIG. 4, a partial sectional view along a line of tanks such as the tank 62 
(lower right), the overhead hoist 60 includes a frame 80, a pair of wheels 
82 which ride generally horizontally along a fixed I-beam 86 of the 
superstructure of the plating line, and a set of guide wheels 88 which 
ride along the lower member 90 of the I-beam 86. The eight pickup sensing 
unit 56 for station is mounted in a box or channel member 92 which is 
attached to the hoist frame 80 by a bracket 94. Its infrared diodes 98 
emit radiation in a generally horizontal direction for interaction with 
the vertically-oriented station code plates such as the plate 20 and 
detection by the opposed phototransistors 100. The stationary code plate 
20 is secured to a perforated mounting strip 104 extending along the 
plating line and which in turn is attached to a bracket 106 secured to 
fixed superstructure 108 of the plating line. Also attached to the 
mounting strip 104 are station centering plates such as the plate 48 
having an L-shaped cross section (For other views of the centering plate 
48 see FIGS. 6 and 7). As shown in FIGS. 4 and 5, the plate 48 has an 
element 110 oriented horizontally to intercept radiation directed 
vertically by the infrared emitting diodes 112 of the sixteen-pickup 
station centering unit 58. These diodes 112 and an opposed array of 
phototransistors 114 are mounted in a channel member 120 orthogonal to the 
member 92 and attached to the hoist frame 80 at a position (FIG. 5) 
displaced from the member 92 a distance along the direction of movement of 
the hoist 60. 
As disclosed earlier, the station code plates serve two functions as they 
move with the hoist along a line of work stations. First, their end 
elements, upon blocking radiation from corresponding end emitters of a 
sensing unit, indicate that the sensing unit and the hoist to which it is 
attached have arrived at a work station. Second, the station code plates, 
by the pattern of emissions blocked by their inner six elements, provide 
positive identification of the particular station at which the hoist is 
positioned. 
A preferred technique for forming station code plates such as that 
illustrated in FIG. 1 is to provide a set of half plates 124 with each 
half plate having one to four elements as shown in FIGS. 8-15 and to mount 
two half plates 124 in a side-by-side arrangement. An advantage of using 
the half plates 124 of FIGS. 8-15 is that just seven different half plate 
configurations are required to form up to sixty-four separate plate 
patterns (and thus to identify sixty-four different stations). 
An electrical schematic of the eight-pickup station identification sensing 
unit 56 (FIG. 16) shows eight infrared emitting diodes 98 connected to 
receive electrical power from an input line 130 and eight opposed 
phototransistors 100 operable to sense radiation emitted by the diodes 98. 
In the arrangement shown, the phototransistors 100 of the sensing unit 56 
are connected in pairs so that just four output lines 132a, 132b, 132c, 
and 132d directed to the microprocessor 70 (FIG. 3) are required rather 
than eight. Each alternative infrared emitting diode is in turn connected 
to either of two return lines 134 and 136 so that, under control of 
electronic switches in the power source (not shown) for the diodes, 
electrical power may be switched at high speed to alternate between the 
diodes 98a, 98c, 98e, 98g and diodes 98b, 98d, 98f, 98h. This arrangement, 
in addition to reducing the number of lines required to the 
microprocessor/controller 70, increases the operational life of the 
infrared emitting diodes 98. 
FIG. 17 shows a similar arrangement for the sixteen-pickup station 
centering sensing unit 58, with the phototransistors 114 connected in 
groups of four and the infrared emitting diodes 112 wired such that power 
is alternately supplied to every fourth diode in the array. Thus at any 
time during operation of the station centering unit 58, just one 
phototransistor in each group may receive radiation from its corresponding 
infrared emitting diode. However, since power-switching among the diodes 
occurs at a substantially greater speed than that at which the hoist 60 
moves, it appears to the hoist as if radiation is emitted continuously by 
each diode. 
FIG. 3 illustrates a preferred system for controlling the position of a 
hoist along a line of tanks and which incorporates the position 
identification and centering subsystems described to this point. The 
position control system includes a control computer 140 which is 
preferably positioned at a convenient off-hoist location and is connected 
to the hoist-mounted microprocessor/controller 70 by a ribbon cable 144. 
Major functions of the control computer 140 include processing and storing 
information received from the microprocessor/controller 70 and generating 
and relaying hoist positioning instructions to the 
microprocessor/controller 70. A suitable computer 140 for an 
electroplating line is a PM550 programmable controller available from 
Texas Instruments Inc., and the microprocessor/controller 70 may be a 
Motorola model 6805. 
To drive the hoist 60 horizontally to a desired work station along the line 
of tanks, a motor 150 is mounted on the electroplating hoist 60, and a 
gear 156 attached to the motor shaft 160 engages a fixed drive chain 164 
extending along the line of tanks. Also connected to the hoist motor 150 
is a variable frequency motor drive 168 under control of the 
microprocessor/controller 70. 
Sensing unit 56, through its interaction with station coding plates, 
provides signals to the microprocessor/controller 70 which indicate 
on-station positioning of the hoist 60. However, in order to move the 
hoist 60 to, and stop it at, a desired work station, information is 
required on hoist position between stations--so that, for example, 
instructions may be relayed from the microprocessor/controller 70 to the 
motor drive 168 and applicable brake mechanisms to reduce the speed of the 
motor 150 in advance of arrival of the hoist 60 at the selected station. 
To provide accurate data on position of the hoist 60 between stations and 
also to verify on-station positioning, the hoist control system of the 
invention includes an encoder 170 such as an optical rotary encoder 
mounted on the shaft 160 of the motor 150 and electrically connected to 
the microprocessor/controller 70 along a line 174. A suitable encoder 170 
is a two-light system rotary encoder, Disc Instruments No. EC82-1024-5, 
available from Honeywell Inc. This rotary encoder produces 1000 pulses per 
rotation of the motor shaft 160 and is directional--i.e., its output 
indicates shaft direction as well as revolutions. Since the relationship 
is known between shaft revolutions and the distance the hoist is moved 
along the drive chain 164 by the shaft-mounted gear 156, the position of 
the hoist along the line of tanks can readily by determined from output of 
the encoder 170. In particular, signals from the encoder 170 and from the 
station identification sensing unit 56 (and, if desired, from a station 
centering sensing unit such as the unit 58) provide independent 
information regarding "on-station" positioning of the hoist 60. Agreement 
(to within specified tolerances) between these independent sources at 
various work stations confirms accuracy of the rotary encoder data, giving 
added confidence in its use to indicate hoist position between stations. 
During operation, the hoist 60 may be initially positioned at a load/unload 
station 180 where it picks up parts--e.g., a barrel loaded with metal 
articles to be plated. The control computer 140 is programmed to furnish 
signals to the microprocessor/controller 70 to instruct the hoist 60 to 
move to selected stations along the line of tanks according to a 
prescribed sequence. The microprocessor/controller 70, based on the next 
work station position desired and on information on current position 
received from sensing units 56 and 58 and the encoder 170, directs the 
motor drive 168 and in turn the motor 150 to rapidly move the hoist 60 to 
the desired new position. Since the sensing units 56 and 58 and the 
encoder 170 continue to provide updated position data as they move with 
the hoist 60 to the selected work station, the microprocessor/controller 
70 is readily able to slow the speed of the hoist 60 through control of 
the motor drive 168 and to accurately center the hoist at the desired 
position for processing of the parts it carries. 
FIGS. 18 and 19 illustrate two sensing units 190 and 192 similar to those 
described with reference to FIGS. 4 and 5 but mounted in a vertically 
stacked arrangement on a side-mounted or sidearm hoist 194. As indicated 
in FIG. 19 the station identification sensing unit 190 is offset along the 
direction of movement of the hoist 194 from the station centering sensing 
unit 192. Also, the end element 196 of a station code plate such as the 
plate 198 is elongated so as to serve as a centering element to block 
radiation from emitters of the sensing unit 192 as well as an "on-station" 
indicator. 
Thus there has been shown and described an improved system for monitoring 
and controlling the position of a conveyor such as a movable hoist of an 
electroplating line. The system utilizes low cost, reliable optical 
sensing units mounted on the hoist and having transmissive pickups which 
are compact and easy to install and test. The sensing units interact with 
fixed station plates and a hoist-mounted microprocessor/controller to 
provide both station identifications and centering information for control 
of hoist position. These subsystems, in combination with a rotary encoder 
and motor drive linked to a hoist motor and to the microprocessor 
controller, and under the direction of a supervisory off-hoist computer, 
permit fast, accurate positioning of the hoist at various stations along a 
line of tanks.