US3207917A - Static rotary limit switch - Google Patents

Description

Sept. 21, 1965 I J. w. MARTIN, JR 3,207,917

STATIC ROTARY LIMIT SWITCH Filed March 30, 1961 vtion to achieve the required control operations.

United States Patent O 3,207,917 STATIC ROTARY LIMIT SWITCH John W. Martin, Jr., Oak Park, Ill., assigner to Danly Machine Specialties, Inc., Chicago, lll., a corporation of Illinois Filed Mar. 30, 1961, Ser. No. 99,420 8 Claims. (Cl. 307-885) My invention relates to a static rotary limit switch and more particularly to an improved rotary limit switch which fails safe.

In operation of a power press, or the like, control operations are to be performed at various points in the cycle of operation of the press. For example, when the press operator pushes the run buttons, for reasons of safety the press control circuit is arranged so that he must continue to press these buttons until such time as the pressl slide passes through bottom dead center. After the press slide goes through its bottom dead center position, a circuit insures that the press will return to the top of its stroke even though the operator releases the run buttons. A further requirement in control of a power press is that in normal operation the press slide stop at the top of its stroke. This is accomplished by de-energizing the brakereleasing solenoid at the proper point in the cycle of operation of the press to permit the springs to set the press brake.

In the prior art the control operations outlined above are accomplished by mechanical rotary limit switches carried by a shaft coupled to the main drive of the press. These cams are so arranged as to actuate switches mechanically at the proper points in the cycle of press opera- These mechanical rotary limit switches embody the usual defects of mechanically operated electric switches in that they require a mechanical making and breaking of contact and also they require a mechanical interengagement vof relatively movable parts to cause the contacts to engage. As a result the parts become worn-out and also the electrical contacts become dirty and deteriorate as a result of arcing. The further danger exists that switch contacts may fuse so that the required control operation does not take place in the intended manner.

It has been suggested that the mechanical limit switches known in the prior art be replaced by magnetic limit switches in which a pulse is generated by the passage of a member formed of magnetic material adjacent a sensing device to produce the desired control operations. While 'an arrangement of this nature overcomes the disadvantages of mechanical rotary limit switches, it embodies other disadvantages. First, since the magnitude of the pulse which is generated by a switch of this type is a function of the speed of movement of the magnetic member, it may be that the pulse generated at slow speeds is too small to produce the desired operation. Further a system in which a signal is required to initiate the control operation does not fail safe, since a failure in the power supply results in no control operation and the desired result is not produced.

I have invented a static rotary limit switch which overcomes the disadvantages of limit switches of the type known in the prior art. My rotary limit switch does not require mechanical engagement between relatively moving parts for its operation. My switch employs no electrical contacts which must be engaged and disengaged upon operation of the switch. My switch normally produces an output signal and the absence of a signal causes the control operation to take place. Owing to this arrangement, my rotary limit switch provides fail safe operation. The output signal of my static rotary limit switch is independent of the speed of the shaft carrying 3,207,917 Patented Sept. 21, 1965 ICC the switch actuating element. switch is highly sensitive.

One object of my invention is to provide a static rotary limit switch which overcomes the disadvantages of limit switches known in the prior art.

Another object of my invention is to provide a static rotary limit switch which employs no contacting relatively movable parts.

Another object of my invention is to provide a static rotary limit switch which affords fail safe operation.

A still further object of my invention is to provide a static rotary limit switch which normally produces an output signal which is independent of the speed of the shaft carrying the switch actuating element.

Still another object of my invention is to provide a static rotary limit switch which is extremely sensitive.

Other and further objects of my invention will appear from the following description.

In general, my invention contemplates the provision of a static rotary limit switch in which a rotary magnetic disk has a lobe normally positioned adjacent a nulled differential device to cause the device to produce an eifective output signal for operation of the controlled element. As the disk rotates and when the control operation is to take place, a discontinunous portion of the disk arrives at a position adjacent the differential device so that the output signal becomes ineffective and the desired control operation takes place. The dillerential device is energized from the source of power for the control circuit so that if this source fails, then the output of the differential device disappears and the desired control operation such as stopping of a press takes place and the system fails safe.

In the accompanying drawings which form part of the instant specification and which are to be read in conjunction therewith and in which like reference numerals are used to indicate like parts in the various Views:

FIGURE 1 is a partially schematic perspective view of my static rotary limit switch.

FIGURE 2 is a sectional view of the differential device of my static rotary limit switch.

FIGURE 3 is a schematic view of one form of control circuit with which my static rotary limit switch may be used.

Referring now to FIGURE l of the drawings, my static rotary limit switch indicated generally by the reference character 10 includes a diiierential device indicated generally by the reference character 12 having a primary winding 14 connected to the termials 16 and 18 of a suitable source of alternating current potential. The transformer 12 includes two secondary windings 20 and 22 connected in series opposed relationship by a conductor 24 and provided` with output terminals 26 and 28; The transformer 12 has a core 30 which as will be explained hereinbelow is fixed in a neutral position at which an ineffective output signal appears at the terminals 26 and 28 in response to energization of primary winding 14.

The switch 10 includes a disc 32 formed from a suitable magnetic material. I mount the disk 32 on a shaft 34 by means of a key or the like 36. Shaft 34 is driven from the main driving train (not shown) of the machine such for example as a power press (not shown) to be controlled. Disk 32 includes a lobe 38 and a discontinuous portion or notch 40. It will be appreciated that the return flux path of the device 12 is external to the device. When the lobe 38 is positioned adjacent the device 12 in the return ilux path, it unbalances the transformer 12 so that an output signal appears at the terminals 26 and 28. When the shaft 34 rotates to a position at which the discontinuous portion or notch 40 is adjacent the device 12, then the unbalance no longer exists, and the output signal at terminals 26 and 28 disappears.

My static rotary limit .cal construction of the differential transformer 12.

Referring now to FIGURE 2, I have shown the physi- It includes a coil 42 carried by a suitable bobbin 44 made of epoxy resin or the like. The coil 42 which makes up the windings 14, 20 and 22 may be potted in a suitable potting compound such as epoxy casting resin. The core 30 is formed from a high grade magnetic alloy. In assembling transformer 12 so that it normally provides substantially no output, the core 30 is inserted within the bobbin 44 and with the primary winding energized the output leads are connected to a suitable device such as an oscilloscope for indicating the output voltage. The core 30 is then adjusted until a null is observed. When this occurs, potting compound is poured into a space between the bobbin and the core and permitted to set firmly to hold the core 30 in its null position. Any suitable potting compound known to the art may be employed for this purpose.

Referring now to FIGURE 3, I have shown one form of control circuit in which may static rotary limit switch may be advantageously employed. Control of a machine such for example as a power press (not shown) is achieved by energizing a solenoid 46 which in a manner known to the art releases the press brake against the action of springs (not shown) which normally set the brake. Simultaneously with the release of the brake the press clutch may` be engaged to cause the press to run. One control operation which is required is that the press slide stop at the top of its stroke. This is the particular control operation which I have illustrated in FIGURE 3 of the drawings.

An input voltage at source terminals 52 and 54 is coupled by a transformer 50 having a primary winding 48 and a secondary winding 58 to a full wave rectifier, indicated generally by the reference character 56, to provide a direct current control circuit potential between a positive conductor 64 and a negative conductor 66. A protective diode 60 and a p-n-p transistor, indicated generally by the reference character 62 control the application of the potential across conductors 64 and 66 to the solenoid winding 46, which is the element to be controlled. A diode 68 prevents transients generated in winding 46, when its circuit is interrupted, from exceeding the reverse Voltage rating of transistor 62.

Conductors 70 and 72 apply the alternating output voltage of transformer 50 to the primary winding 14 of differential transformer 12. I impress the signal at the output terminals 28 and 26 of differential transformer 12 between the emitter 76 and base 78 of a transistor amplifier indicated generally by the reference character 74. A

provide a regulated output Voltage between a negative conductor 90 and the positive conductor 64. A capacitor 92 smooths this voltage. A circuit including a variable sensitivity control resistor 94 applies the potential between conductors 64 and 90 across the emitter 76 and collector 82 of transistor 74. A transformer 98 having a primary winding 96 in the collector circuit of transistor 74 couples the transistor output to a bridge rectifier 100 across the secondary winding 102 of transformer 98.

A conductor 104 anchors the positive output terminal of rectifier 100 to conductor 64. A capacitor 106 stores the peak output value of rectifier 100 and applies it between the emitter 112 and the base 108 of a transistor 110 in a sense to render this transistor conductive. As conduction through transistor 110 increases the potential across a resistor 114 in the circuit of the collector 116 of transistor 110 increases and the collector potential rises.

I apply the potential at collector 116 to the base 118 of a p-n-p transistor 120 to produce apotential between the emitter 122 of transistor 120 and the base 118 which tends to render transistor 120 nonconductive in response to a rise in potential of collector 116. As the current through transistor 120 decreases the voltage developed across a resistor 124 in the circuit of the collector 126 of the transistor 120 decreases and the collector potential drops. I impress the collector potential of transistor 120 on the base 128 of transistor 62. As is known in the art a drop in potential at the base of a p-n-p transistor tends to increase the current drawn by the transistor.

I so select the circuit parameters that when the lobe 38 of disk 32 is adjacent the core 30 of transformer 12 the signal output at terminals 28 and 26 is effective ultimately to produce enough current flow through transistor 62 to energize solenoid 46. Conversely, when the discontinuity or notch 40 is adjacent the core the output at terminals 28 and 26 is ineffective ultimately to energize solenoid 46.

In operation of my static limit rotary switch in the circuit shown in FIGURE 3 of the drawings, let us assume that the press is running and that shaft 34 is rotating with the lobe 38 of the disk 32 adjacent the transformer 12. Under these conditions transformer 12 puts out an alternating current voltage which is amplified by transistor 74 and is applied to the primary winding 96 of transformer 98 to cause the secondary winding 102 to produce an output voltage which is rectified by rectifier 100. A negative potential is applied to the base 108 of transistor 110 to cause the transistor to continue to develop a voltage across the resistor 114. This voltage causes the base 118 of transistor 120 to rise to a point at which the transistor is cut off so that the base 128 of transistor 62 is substantially at the potential of conductor 90, the transistor 62 conducts and the winding 46 is energized.

When the press slide (not shown) `arrives .at the top of its stroke the cutaway portion 40 of the disk 32 is adjacent the transformer 12 so that the transformer is no longer unbalanced and no output signal appears at the terminals 28 and 26. Under these conditions no signal is applied to primary winding 96 with the result that the output of rectifier is substantially zero and the base 108 of the transistor 110 is at such a potential that the transistor is cut olf and no potential is developed across resistor 114. Under these conditions base 118 is substantially 4at the potential of conductor 90 so that this transistor continues to develop a potential across resistor 124. With the potential at collector 126 applied to 'base 128 of transistor 62 this latter transistor iscut off and the solenoid winding 46 is de-energized to permit the press brake to set to stop the press slide.

Let us now assume that the lobe 38 of disk 32 is adjacent the transformer 12 lbut that the power supply for some reason fails. When this happens and the transformer 12 is in its unbalanced condition, there is no signal input to the primary winding 14, and the output of the transformer is ineffective. With this signal applied to winding 96, -as will be apparent from the description given hereinabove, winding 46 is de-encrgized. Thus my static limit rotary switch fails safe, since the absence of power causes the circuit to fail to put out a signal which condition also results when the limit point is reached.

It is to be noted that the presence of the lobe 38 adjacent the transformer 12 increases the flux linking winding 20 for example while at the same time decreasing the ux linking the Winding 22. Thus since the differential transformer takes the difference between two quantities which are varied in opposite directions in the course of its operation, it is extremely sensitive. It is to be understood that while I have shown a particular example of my static rotary limit switch in a circuit for insuring that a press slide stops at the top of its stroke, my switch is of general application. It may be used in any system or circuit in which a control operation is to be initiated at a certain point in a cy-cle of operation in response to rotary movement of a shaft synchronized with the operating cycle. Control may b e achieved over any desired portion of a cycle or it may be initiated at a number of points in a cycle of operation. It is to be understood also that in practice a number of cams such as cam 32 may be mounted on a single shaft which may have respective sensing unit-s associated therewith for controlling a number of different operations on a machine.

It will be seen that I have accomplished the objects of my invention. It have provided a static rotary limit switch which is an improvement over mechanical limit switches of the type known in the prior art. My switch employs no relatively movable contacting members, and it does not require making and breaking of electrical contacts for its operation. The normal output signal of my switch is independent of the speed of operation of the shaft which initiates the control. My static rotary limit switch fails safe.

It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of my claims. It is further obvious that various changes may be made in details within the scope of my claims without departing from .the spirit of my invention. It is, therefore, to be understood that my invention is not to be limited to the specic details shown and described.

Having thus described my invention, what I claim is:

1. A static limit switch for controlling the energization of a control element including in combination a balanced electromagnetic device normally producing an ineffective output signal in response to an input signal applied thereto, means coupling said output signal to said element, means for applying an input signal to said device, a member formed of magnetic material, means mounting said member for movement from a normal position adjacent said device at which the member unbalances the device to cause it to produce an effective output signal which energizes said element to a control position remote from the device at which said device returns to its balanced condition whereby said output signal is ineffective and means for moving said member.

2. A static limit -switch for controlling the energization of a control element including in combination a balanced electromagnetic device normally producing an ineffective output signal in response to an input signal applied thereto, means coupling said output signal to said element, means for applying an input signal to said device, a member formed of magnetic material and provided with a `discontinuous portion, means mounting said member for movement with respect to said device with said material normally adjacent said device to unbalance the device to cause the device to produce an effective output -signal which energizes said element and means for moving said member to a position at which said discontinuous portion is adjacent said device whereby the device returns to its balanced condition at which the output signal is inffective.

3. A static rotary limit switch for controlling the energization of a control element including in combination a balanced electromagnetic device normally producing an ineffective output signal in response to an input signal applied thereto, means coupling said output signal to said element, means for applying an input signal to said device, a disk of magnetic material having a discontinuous segment, means mounting said disk for rotary movement with respect to said device with said material normally adjacent -said device to unbalance the device to cause it to produce an effective output signal which energizes said element and means for rotating said disk to a position at which said discontinuous segment is adjacent the device whereby the device returns to its balanced condition at which the output signal is ineffective.

4. A static limit switch for controlling the energization of a control element including in combination a differential transformer having a core and a primary winding and differential secondary windings, means for applying an input signal to said primary winding, said transformer being balanced whereby it normally produces an ineffective output signal a member formed of magnetic material, means for moving said member to a normal position adjacent said core at which the member causes an effective output signal to appear across said secondary windings in response to said input signal, means for applying said output signal to said control element to energize said control element and means for moving said member to a control position remote from said core at which said output signal becomes ineffective. g

5. A static limit switch for controlling the energization of a control element including in combination a differential transformer having a core and a primary winding and differential secondary windings, means for applying an input signal to said primary winding, said transformer being balanced whereby it normally produces an ineffective output signal a member formed of magnetic material and provided with a discontinuous portion, means mounting said member for movement with respect to said core with said material normally adjacent said core to cause an effective output signal to appear across said secondary windings in response to said input signal, means for applying said output signal to said control element to energize said control element and means for moving said member to a position at which said discontinuous portion is adjacent said core whereby said output signal is ineffective.

6. A static limit switch for controlling the energization of a control element including in combination a differential transformer having a core and a primary winding and differential secondary windings, means for applying an input signal to said primary winding, said transformer being balanced whereby it normally produces an ineffective output signal a disk formed of magnetic material and provided with a discontinuous segment, means mounting said disk for rotary movement with respect to said c-ore with said material normally adjacent said core to cause an effective output signal to appear across said secondary windings in response to said input signal, means for applying said output signal `to said control element to energize said control element and means for rotating said disk to a position at which said `discontinuous segment is adjacent said windings to cause said output signal to become ineffective.

7. In a control circuit for a machine having a movable member, said circuit having an operating element conn ected to a source of energy by a normally nonconductive switching element rendered conductive in response to an effective control signal and rendered nonconductive when a control operation takes place, a static limit switch lncluding in combination a balanced electromagnetic device normally producing 4an ineffective output signal in response to an input signal applied thereto, means for applying an input signal to said device, a control member of magnetlc material, means mounting said control member for movement from a normal position adjacent said device at which the member unbalances the device to cause it to produce an effective output signal to a control position remote from the device at which it returns to its balanced position at which said output signal is ineffective, means for applying said output signal to said norm-ally nonconductive switching element as said control signal to energize said control element and means responsive to said machine member for moving said control member.

8. A control circuit for a machine having an element energized when the machine operates and having a member movable in response to operation of the machine including in combination a source of energy, means including a normally nonconductive switching element for applying energy from said 4source to said element, said switching element being rendered conductive in response to an effective control signal, a balanced electromagnetic device normally producing an ineffective output signal in response to an input signal applied thereto, means for applying an input signal from said source to said balanced device, a control member formed of magnetic material, means mounting said control member for movement from a normal position adjacent said balanced device at which the member unbalances the device to cause it to produce an effective output signal to a control position remote from the device at which it returns to its balanced position at which said output signal is ineffective, means for applying said output signal to said normally nonconductive switching element as said control signal to energize said control element and means responsive to said machine member for moving said control member.

References Cited by the Examiner UNITED STATES PATENTS JOHN W. HUCKERT, Primary Examiner.

HERMAN KARL SAALBACH, ARTHUR GAUSS,

Examiners.

Claims (1)

1. A STATIC LIMIT SWITCH FOR CONTROLLING THE ENERGIZATION OF A CONTROL ELEMENT INCLUDING IN COMBINATION A BALANCED ELECTROMAGNETIC DEVICE NORMALLY PROUDCING AN INEFFECTIVE OUTPUT SIGNAL IN RESPONSE TO AN INPUT SIGNAL APPLIED THERETO, MEANS COUPLING SAID OUTPUT SIGNAL TO SAI DEVICE, A MEANS FOR APPLYING AN INPUT SIGNAL TO SAID DEVICE, A P MEMBER FORMED OF MAGNETIC FROM A NORMAL POSITION ADJASAID MEMBER FO/R MOVEMENT FROM A NORMAL POSITION ADJACENT SAID DEVICE AT WHICH THE MEMBER UNBALANCES THE DEVICE TO CAUSE IT TO PRODUCE AN EFFECTIVE OUTPUT SIGNAL WHICH ENERGIZES SAID ELEMENT TO A CONTROL POSITION REMOTE FROM THE DEVICE AT WHICH SAID DEVICE RETURNS TO ITS BALANCED

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