US3354740A - Conveyor drive and indexing mechanism for liquid scintillation spectrometers - Google Patents

Description

1967 R. E. CANNON CONVEYOR DRIVE AND INDEXING MECHANISM FOR LIQUID SCINTILLATION SPECTROMETERS Filed Aprll 11, 1966 N mo T N NN A Vfi W E D N O W A R ATTORNEY United States Patent CONVEYOR DRIVE AND INDEXING MECH- ANISM FOR LIQUID SCINTILLATION SPEC- TROMETERS Raymond E. Cannon, San Diego, Calif., assignor to Beckman Instruments, Inc., a corporation of California Filed Apr. 11, 1966, Ser. No. 541,595 9 Claims. (Cl. 74-436) This invention relates to a conveyor drive and indexing mechanism of the type used for liquid scintillation spectrometers and, more particularly, to such a mechanism employing a modified Geneva drive for providing a smooth output motion around the indexing point with a high degree of positioning accuracy while using a relatively low torque gear motor drive and simple mechanism.

One of two major tasks of automatic sample conveying devices used with liquid scintillation spectrometers is accurately positioning the samples in an index position over an opening in the base of the conveyor table for further handling. As an example, a conveyor chain, which may be driven to and from the index position, has used a lever arm which comes in contact with the sample holder and then actuates a limit switch shutting off the current to the associated drive motor. Examples of this type of device are disclosed in US. Patents 3,188,468 Packard, and 3,206,006-Meedcr et al. In the latter, the indexing switch is actuated by the driving cam which serves to position the conveyor over the index position. This is a simple and inexpensive approach but has many disadvantages, among which are the requirement that the drive motor be effectively braked, mechanically with a solenoid brake, or dynamically.

Mechanical brakes, available on electric motors, do not stop with a high degree of accuracy and solenoid brakes simply do not work reliably. Dynamic braking may be effective in stopping the motor but does not hold the motor once it is stopped. Even if the braking problem is mastered in this type of direct drive system, the starting and stopping of the conveyor is very abrupt since the resulting motion is linear in nature.

Indexing the chain from station to station has to be done with repeatable accuracy and the use of limit switches actuated by mechanical arms or toggles acting on the conveyor chain will also not perform reliably, since the distance moved bythe conveyor chain is large with respect to the sensing arm acting on it, and therefore even very small inconsistencies in the switch or actuator reflect large errors in conveyor chain position. The precise point at which a limit switch actuates will change with use which is inherent in most spring-controlled devices.

Another problem is that the limit switches display an adverse characteristic termed bounce which means that when the switch is actuated the contacts make and break several times, yielding several signals to the associated electronics which are looking for only one signal. This confuses the electronics to the point that additional circuitry must be designed to blank out all but the first signal. Limit switch actuator arm toggles are in general also easily damaged and require frequent adjustment, which is undesirable as it may involve a field service call.

It is the main purpose of the invention to provide a new and improved conveyor drive and indexing mechanism for liquid scintillation spectrometers which will give accurate indexing of the chain from station to station, using a low-cost modified Geneva drive capable of driving the conveyor in a non-linear manner and with resulting smooth motion and accurate positioning around the index point.

This and other objects are achieved by providing a ice conveyor drive and indexing mechanism of the type used to position sample bottles in a liquid scintillation counting system which is characterized by: a drive motor having a motor output shaft; a drive wheel having at least three engaging mechanisms attached thereto, spaced evenly around its circumference, said drive wheel. connected to said motor output shaft for driving said drive wheel; a driven wheel containing at least three slots extending inwardly from its circumference and spaced evenly therearound for cooperating with said engaging mechanisms, said engaging mechanisms and slots being located to effectively provide a dwell time over several degrees of rotation of said drive wheel during which there is an insignificant amount of motion of said driven wheel; and, means connecting the output of said driven wheel to a conveyor drive sprocket whereby the output motion at said driven wheel and conveyor drive sprocket is nonlinear, effectively having dwell times, and has slow acceleration and deceleration curves.

The novel features which are believed to be characteristic of the invention are set forth with particularity in the appended claims. The invention, and further objects and advantages thereof, can best be understood by reference to the following description and accompanying drawing which illustrates an exploded isometric view of the mechanism of the invention, having a portion of the driven wheel cut away for clarity of illustration.

Turning now to the drawing, there is illustrated a drive mechanism, which is a four-position intermittent motion device with effectively short dwell cycles and slow acceleration and deceleration curves. It will be understood, of course, that the invention is not limited to a device having four positions but that any number of positions of three or over might be employed, and it is not intended to limit the invention thereto. However, four positions will provide a sinusoidal output motion for a linear rotational input motion whereas other numbers of positions do not necessarily do so.

The drive is a modified Geneva drive, which differs from other Geneva drives in that it has no locking cam to hold it in a dwell position. It is positively locked only at a single point, although it has a high output to input mechanical advantage which makes it difficult, if not impossible, to drive the device from output to input.

The drive consists of a drive wheel 10, mounted on a motor output shaft 11, through holes 12 and 13 in lower and upper mounting plates 14 and 15, which contains bushings 16 and 17, respectively, for cooperating with the shaft 11. The shaft 11 goes through the drive wheel 10 at its center through a hole 18. Around the circumference of drive wheel 10 there are four holes 19, posi-- tioned apart, in which an engaging mechanism in the form of pins 20 are mounted extending downwardly from the lower surface of the wheel 10. The wheel 10 also contains four photocell holes 21, positioned 90 apart and in between the holes 20. These holes 21 serve to cooperate with a lamp 22, mounted in a socket 23 and by a bracket 24 and supporting column 25 to the upper mounting plate 15.

Lower mounting plate 14 contains a cooperating hole 26, under the lower side of which a photocell detector 27 is mounted. The lower and upper mounting plates 14 and 15 are connected together by means of spacers 28 and, in turn, are connected to a conveyor base-plate (not illustrated) by means of spacers 29. Such a conveyor base plate is illustrated in a copending application entitled, Control Tower and Method for Programming Automatic Radioactivity Measuring SystemR. Nather, Ser. No. 541,674, filed Apr. 11, 1966, and assigned to the assignee of the present invention. This copending application also discloses a conveyor chain, such as may be driven by the present invention, and associated drive sprocket, guide rails and idler wheels. A typical chain is illustrated in more detail in a copending application entitled, Sample Conveying Chain for Liquid Scintillation Spectrometer- R. Cannon, Ser. No. 541,843, filed Apr. 11, 1966, and assigned to the assignee of the present invention.

Returning to the drawing, a driven wheel 30, containing four slots 31 extending inwardly from its circumference partway along radii thereof, is mounted on a shaft 32, through a central hole 33 therein. The shaft 32 rides in bushings 34 and 35, which are mounted in holes 36 and 37 in lower and upper mounting plates 14 and 15, respectively. 7

A geared wheel 38 is also mounted on shaft 32 between wheel 30 and bushing 35 and contains a gear 39 near its top for rotation around the shaft 32. Gear 39 is herein illustrated as having twelve teeth. The wheels 10, 30 and 38 are locked to the shafts 11 and 32 by the looking pins 40, which prevent them from rotating relative to their shafts.

The shaft 11 is driven by a gear motor 41 which is made up of a undirectional drive motor 42 and associated gear box 43 connected to drive the motor output shaft 11. The motor can be bidirectional to reverse chain direction. In the illustrated embodiment, the gear motor is a 25 r.p.m. gear motor delivering a 20-inch pound torque which is sufficient to drive a conveyor of the type illustrated in the above referenced copending applications. A driving chain 44 engages the teeth of the gear 39 and goes to a second cooperating gear (not illustrated) which is connected to a conveyor driving sprocket such as illustrated in the above referenced copending application Ser. No. 541,674. The cooperating gear has 30 teeth. Since the pins 20' and slots 31 aredesigned to provide a fourposition device upon the rotation of the wheels and 30 through one revolution, the 2 /2 to 1 gear reduction between the gear 39 and its cooperating gear further reduces this to 10 positions during one revolution, as illustrated on the conveyor drive sprocket and indexing wheel of the aforementioned application, Ser. No. 541,674.

The fact that at the index position the motion is minimal may be illustrated by considering that when index is indicated by the lamp 22 shining through the holes 21 and 26 on the photocell 27, two of the pins 20 are next to the-driven wheel 30, one just entering one slot 31 and the other just leaving the next slot 31. At this time, there is practically no motion of the driven wheel 30, and not until the pin 20 entering the slot 31 proceeds substantially down the slot is there apreciable movement of the driven wheel 30. With radial slots the motion of the driven wheel 30 is sinusoidal in nature.

The drive wheel 10 may be made of cast aluminum and the driven wheel 30 may be an injection molded wheel. The device, as illustrated and described, is somewhat simpler than the conventional Geneva drive, having 1 much shorter effective dwell time with no provision for positive locking during the entire dwell time. However, it has the degree of precision necessary to accurately perform the sample conveying and indexing function required by the application described.

The necessity for knowing when a sample is in position :loes not disappear with the use of the modified Geneva irive and some means of determining the fact is essen- ;ial. The drive does, however, relieve the sensing device of B decision making and command responsibilities asking )nly that it substantiate a fact. This is possible because of the effective dwell time inherent in the drive. When he driven wheel 30, or output of the modified Geneva irive, is on index, the input or drive wheel 10 can be rotated approximately 30 before any noticeable movenent occurs at the output of driven wheel 30 in the four position embodiment described above. The result is that position sensing is far less critical than with a linear irive.

The lamp 22 and photocell 27 provides a highly reliable means to provide an electronic signal which will detect the positioning with a high degree of accuracy due to the small amount of motion around the index point.' The holes 21, in the drive wheel 10, allow the passage of light only when the driven wheel 30 is on index. When the photocell 27 is exposed to light from the lamp 22, its resistivity changes and the change is detected by associated electronics (not illustrated) providing the necessary signal.

The 2 /2 to 1 reduction between the gear 39 and its associated gear, in addition to changing the four-position drive to ten positions at the final sprocket, further reduces by a factor of 2 /2 any eror in the modified Geneva drive which may be allowed by taking advantage of comparatively loose manufacturing tolerances which will then not affect the resulting indexing accuracy.

The foregoing mechanism has the advantage of very gentle acceleration and deceleration and the components are subjected to far less stress and impact than with a linear driving system. This is particularly important when moving samples per minute which is a common requirement of such systems. Neither accurately braked motors, nor motors with holding brakes, need be used. The device can only be effectively driven from the input side and force applied to the output side will not affect the index position due to the high mechanical advantage. At the point where one of the pins 20 is just entering a slot 31 and another pin 20 is just leaving a slot 31, and only then, is this mechanism positively locked. Normal Geneva drives are locked for about of input rotation. The present invention is also only effectively locked for 10 or 15 by the mechanical advantage, which is more than sufiicient in the present application.

Adjustment of the index points is never needed and, in fact, is not possible with the mechanism. The indexing is established at the manufacturing level by the nature of the drive and the fixed ratio of the final drive with respect to the driven wheel 30. The photocell sensing device is also exceptionally reliable since there are no moving parts to wear out. One possible exception might be the lamp 22; however, it may be operated at a reduced voltage thereby extending its life enormously. For repetitive indexing applications, Geneva type drives are unchallenged in their superiority over other methods. For the present application, the modified Geneva described herein, is even more advantageous than a conventional Geneva, which would have to turn faster and would involve more lost motion, resulting in more jerky driving of the conveyor chain than with the present device.

Since the principles of the invention have now been made clear, modifications which are particularly adapted for specific situations without departing from those principles will be apparent to those skilled in the art. The appended claims are intended to cover such modifications as well as the subject matter described and to only be limited by the true spirit of the invention.

What is claimed is:

1. A conveyor drive and indexing mechanism for liquid scintillation spectrometers including:

a drive motor having a motor output shaft;

a drive wheel having at least three engaging mechanisms attached thereto, spaced evenly around its circumference, said drive wheel connected to said motor output shaft for driving said drive wheel;

a driven wheel containing at least three slots extending inwardly from its circumference and spaced evenly therearound for cooperating with said engaging mechanisms, said engaging mechanisms and said slots being located to effectively provide a dwell time over several degrees of rotation of said drive wheel during which there is an insignificant amount of motion of said driven wheel; and,

means connecting the output of said driven wheel to a conveyor drive sprocket whereby the output motion of said driven wheel and said conveyor drive sprocket is non-linear, effectively having dwell times, and has slow acceleration and deceleration curves.

2. The mechanism of claim 1 in which said drive wheel contains at least three holes evenly spaced around its circumference and in between and substantially equidistant from said engaging mechanisms, said holes cooperating with a photocell detector and light positioned to provide an index signal.

3. The mechanism of claim 1 in which said drive motor is a gear motor.

4. The mechanism of claim 1 in which said means connecting the output of said driven wheel to a conveyor drive sprocket comprises, a gear mounted on a shaft through said driven wheel, a second gear mounted on a shaft through said sprocket, said gears having a tooth ratio sufficient to convert from the number of positions indicated by the number of said engaging mechanisms and associated slots to the number of index positions on said sprocket, and a chain interconnecting said gears.

5. The mechanism of claim 1 having four engaging 20 mechanisms and four slots.

6. The mechanism of claim 5 in which said slots are along radii of said driven wheel.

7. The mechanism of claim 6 in which said engaging mechanisms are pins mounted perpendicular to said drive wheel to engage said slots.

8. The mechanism of claim 7 in which said pins and slots are positioned to provide a sinusoidal output motion of said driven wheel for a linear rotational input motion of said drive wheel, said driven wheel having an effective dwell time centered around an index position.

9. The mechanism of claim 8 in which said drive wheel contains four holes evenly spaced around its circumference in between and substantially equidistant from said pins, said holes cooperating with a photocell detector and light positioned to provide an index signal when said driven wheel is in said index position.

References Cited UNITED STATES PATENTS 442,404 12/ 1890 Steiger 74436 2,116,127 5/1938 Steiert 7484 X 2,888,833 6/1959 Toderick 7441S X 2,890,590 6/1-959 OHarah 74436 X 3,024,402 3/1962 Adel 74-436 X 3,106,109 10/1963 Dexter 74436 X 3,206,006 9/ 1965 Meeder et a1 198-131 3,216,777 11/1965 Allen 74436 X 3,279,281 10/ 1966 Anderson et al 74-'820 3,286,536 11/196-6 Hallmann 7484 DONLEY J. STOCKING, Primary Examiner.

L. H. GERIN, Assistant Examiner.

Claims (1)

1. A CONVEYOR DRIVE AND INDEXING MECHANISM FOR LIQUID SCINTILLATION SPECTROMETERS INCLUDING: A DRIVE MOTOR HAVING A MOTOR OUTPUT SHAFT; A DRIVE WHEEL HAVING AT LEAST THREE ENGAGING MECHANISMS ATTACHED THERETO, SPACED EVENLY AROUND ITS CIRCUMFERENCE, SAID DRIVE WHEEL CONNECTED TO SAID MOTOR OUTPUT SHAFT FOR DRIVING SAID DRIVE WHEEL; A DRIVEN WHEEL CONTAINING AT LEAST THREE SLOTS EXTENDING INWARDLY FROM ITS CIRCUMFERENCE AND SPACED EVENLY THEREAROUND FOR COOPERATING WITH SAID ENGAGING MECHANISMS, SAID ENGAGING MECHANISMS AND SAID SLOTS BEING LOCATED TO EFFECTIVELY PROVIDE A DWELL TIME OVER SEVERAL DEGREES OF ROTATION OF SAID DRIVE WHEEL DURING WHICH THERE IS AN INSIGNIFICANT AMOUNT OF MOTION OF SAID DRIVEN WHEEL; AND, MEANS CONNECTING THE OUTPUT OF SAID DRIVEN WHEEL TO A CONVEYOR DRIVE SPROCKET WHEREBY THE OUTPUT MOTION OF SAID DRIVEN WHEEL AND SAID CONVEYOR DRIVE SPROCKET IS NON-LINEAR, EFFECTIVELY HAVING DWELL TIMES, AND HAS SLOW ACCELERATION AND DECELERATION CURVES.

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