Electricity: circuit makers and breakers – Interlocking – Between switches and housing
Reexamination Certificate
1999-09-27
2001-03-06
Friedhofer, Michael (Department: 2832)
Electricity: circuit makers and breakers
Interlocking
Between switches and housing
C200S043070, C200S01600R, C200S341000
Reexamination Certificate
active
06198058
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to switch assemblies and, more particularly, relates to a method and apparatus for monitoring a contact in a switch assembly.
2. Discussion of the Related Art
Electrical switches, such as pushbuttons or rotary switches used for the control of industrial equipment, are typically mounted onto a front panel of a cabinet so that the manipulated portion of the switch (termed the “operator”) projects out from and is accessible at the front of the cabinet.
For a pushbutton switch, a hole may be punched in the cabinet of sufficient diameter to accommodate the pushbutton and a surrounding threaded shaft. The shaft and pushbutton are inserted through the hole, and a threaded retaining nut is placed over the shaft and tightened to securely affix the switch to the panel. The panel is thus sandwiched between the switch body and the retaining nut.
The end of the switch operator protruding inside of the panel may be snapped or otherwise mounted onto one side of a latch assembly, and a contact block or a plurality of contact blocks are mounted onto the other side of the latch assembly. The contact blocks are electrically connected to the circuit or circuits that the switch is to control.
Contact blocks typically comprise housings that contain normally open and/or normally closed contacts. A normally open contact may be used, for example, when a user wishes to activate a specified function by actuating the operator, thereby closing the normally open contact. When the operator switch is deactivated, a plunger returns to its normal position, thereby opening the normally open contact and terminating the controlled function.
A normally closed contact may be used when a user wishes to stop an ongoing function. One common example of a normally closed contact is an Emergency Stop (EStop) function which is activated when the user wishes to immediately terminate the controlled function due, e.g. to a malfunction in the process or the development of a situation that may cause damage to the product line or the operating equipment. In this situation, when the switch operator is actuated, the normally closed contact opens and remains open until the operator is returned to its normal state, thereby closing the normally closed contact and resuming the controlled function.
In such systems, the user assumes a risk that the normally closed contact may become mechanically disengaged from the switch operator. Such a situation may occur, for example, if the latch assembly is damaged or not properly mounted onto the switch operator and therefore becomes detached during operation. Alternatively, the contact block may be damaged or improperly mounted. Even though, in these situations, the contact block is mechanically disconnected from the switch operator, the normally closed contact remains closed, thereby permitting the continuous operation of the controlled function. As a result, when the normally closed contact is functioning as an E-Stop, for example, the controlled function will remain in operation even though the contact block is no longer mechanically engaged with the switch operator.
Currently, one known way to ensure an operable state of a normally closed switch is to test it by intermittently activating the switch operator. If, after activation, the controlled function is nonresponsive, then the user will become aware of a problem in the switch assembly and may take corrective measures. However, this method of detection is quite inefficient and results in considerable unnecessary down-time, thereby increasing cost. Furthermore, this method is unreliable as situations may arise that require the activation of an E-Stop that has become non-operational since the last test.
The need has therefore arisen to implement a method and apparatus for detecting when the normally closed contact becomes mechanically disengaged from the switch operator in an efficient and reliable manner.
OBJECTS AND SUMMARY OF THE INVENTION
It is therefore a first object of the present invention to provide a switch assembly having a switch that: 1) monitors a normally closed contact to determine when the contact becomes mechanically disengaged from a switch operator, and 2) permits normal operation of the controlled function.
It is a second object of the invention to permit the switch and normally closed contact to be mounted either within the same housing or in separate housings.
It is a third object of the invention to provide a single switch that is able to monitor a plurality of contacts.
In accordance with a first aspect of the invention, the switch comprises a normally open contact that is electrically connected in series to the normally closed contact to be monitored, and to the function that the switch assembly is to control. When the switch is connected to the switch operator, preferably via a latch assembly in a known manner, the normally open contact is closed, thereby completing the circuit for the controlled function. To perform a specified operation of the function, the switch operator is actuated to open the normally closed contact. If, during operation, the normally closed contact becomes mechanically disengaged from the switch operator, the normally open contact will open, thereby opening the circuit and terminating operation of the controlled function. The user, noticing the stoppage, will then be alerted that a problem exists in the switch assembly and may take corrective action.
In accordance with a second aspect of the invention, the normally open contact and normally closed contact may either reside in the same housing or in separate housings. If both contacts are in the same housing, the switch preferably comprises a column that is disposed within the housing and that comprises a plunger that is permitted to engage a switch operator stem. A contact spring within the column is interposed between the two contacts in the housing and biases each contact towards its closed position. A return spring, disposed within the housing, biases the normally open contact towards its open position and, because it provides a greater force than the contact spring, maintains the normally open contact in its open position. Additionally, the return spring biases the column upwards so that the plunger extends outside the housing to engage the stem.
When the latch assembly is mounted onto the switch operator, the stem depresses the column via the plunger, and the column compresses the return spring. The contact spring then closes the normally open contact, which is now in mechanical communication with the switch operator. When the operator is activated, the column is further depressed and the normally closed contact, also now in mechanical communication with the operator, opens to perform a specified operation to the controlled function. If the housing becomes disconnected from the switch operator, thereby mechanically disengaging the normally closed and normally open contacts from the switch operator, the plunger will return to its normal position, thereby biasing the return spring to open the normally open contact, opening the circuit, and terminating the controlled function. Additionally, if the plunger breaks, the return spring again will bias the column upwards, thereby opening the normally open contact and terminating the controlled function.
Alternatively, the normally open and normally closed contacts could be disposed within separate housings. In the housing containing the normally closed contact, a plunger that is connected to a column and partially disposed within the housing is connected to the switch operator such that the normally closed contact is opened when the switch operator is actuated. A second plunger is partially disposed within the housing containing the normally open contact such that, when the housing is connected to the switch operator, the plunger closes the normally open contact. If the latch assembly becomes mechanically disconnected from the switch operator, the second plunger will also become disconnected, and the normally o
Graninger Frank J.
Green Richard M.
Forman Adam J.
Friedhofer Michael
Horn John J.
Rockwell Technologies LLC
Walbrun William R.
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