Circuit for enabling safe actuation of a machine

Abstract

A circuit requiring the simultaneous use of both hands of an operator for enabling the safe and repetitive operation of a press or other similar type of machine. The circuit is comprised of first and second single pole double throw momentary contact switches, a capacitor, means for charging the capacitor, and a relay having a coil and a normally open contact. Each switch has first, second and third terminals, wherein the first and second switch terminals are normally electrically connected together while the second and third switch terminals are normally electrically disconnected. When each of the switches is actuated by application of hand pressure thereto, the second and third terminals are momentarily electrically connected together while the first and second terminals are momentarily disconnected. The capacitor is electrically coupled between the second terminals of the first and second switches, and the charging means is electrically connected between the first terminals of the first and second switches. The coil is electrically connected between the third terminals of the first and second switches for providing a discharge path for the capacitor when the second and third terminals of the first switch and the second and third terminals of the second switch are momentarily electrically connected together. The normally open relay contact is placed in circuit relationship with the press, whereby upon discharge of the capacitor through the coil, the normally open relay contact closes and enables operation of the press, and upon return of both of the first and second switches to their respective normal positions, the charging means recharges the capacitor to await enabling of the next operation of the press.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a control circuit for enabling safe and repetitive operation of a machine, and, more particularly, to a circuit requiring the use of both hands of an operator to enable the safe operation of a press or the like.

2. Description of the Prior Art

In the past, when operating large machines, such as presses, an operator would normally depress a button with one hand and enable a machine operation to proceed. However, since the operator's other hand was not needed to start the operation of the press, at times this hand would be dangerously positioned by the operator in the path of the machine operation, thus resulting in serious injury to or loss of the hand. In order to prevent such injury or loss, it became required that machine operation could not commence until the operator simultaneously depressed two start or operating buttons so as to prevent positioning of either of his hands in the operating area of the machine. However, this second operating button would typically be electrically connected directly to and electrically in series with the first operating button. Under these circumstances, the operator would simply use a piece of tape to continually hold down and effectively short the second operating button out of the circuit without lengthening or otherwise adversely affecting the machine operating cycle. Thus, the operator could simply enable operation of the press by using one hand, while the other hand would again be free to be dangerously positioned within the operating area of the press.

OBJECT OF THE INVENTION

It is therefore an object of this invention to provide a circuit which requires the simultaneous use of both hands of the operator to enable safe actuation of a press, thereby insuring that neither hand of the operator can be positioned within the operating area of the press.

Other objects of the invention will be pointed out hereinafter.

SUMMARY OF THE INVENTION

According to a broad aspect of the invention, there is provided a circuit requiring the simultaneous use of both hands of an operator for enabling the safe and repetitive operation of a press or other similar type of machine. The circuit is comprised of first and second single pole double throw momentary contact switches, a capacitor, means for charging the capacitor, and a relay having a coil and a normally open contact. Each switch has first, second and third terminals, wherein the first and second switch terminals are normally electrically connected together while the second and third switch terminals are normally electrically disconnected. When the switch is actuated by application of hand pressure thereto, the second and third terminals are momentarily electrically connected together while the first and second terminals are momentarily disconnected. The capacitor is electrically coupled between the second terminals of the first and second switches, and the charging means is electrically connected between the first terminals of the first and second switches. The coil is electrically connected between the third terminals of the first and second switches for providing a discharge path for the capacitor when the second and third terminals of the first switch and the second and third terminals of the second switch are momentarily electrically connected together. The normally open relay contact is placed in circuit relationship with the press, whereby upon discharge of the capacitor through the coil, the normally open contact closes and enables operation of the press, and upon return of both of the first and second switches to their respective normal positions, the charging means recharges the capacitor to await enabling of the next operation of the press.

The charging means is comprised of a step down transformer having a primary and a secondary winding, a resistor and a polarizing rectifier. The resistor and rectifier are in series circuit relationship with the secondary winding, and the primary winding is in series circuit relationship with a source of AC power.

BRIEF DESCRIPTION OF THE DRAWING

The drawing is an electrical schematic diagram of the circuit in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention will now be explained with reference to the drawing.

Referring to the drawing, the circuit is comprised of first and second single pole double throw momentary contact switches 10 and 12, a capacitor 14, a relay 16 having a coil 18 and a normally open contact 20, and means for charging the capacitor. The charging means is comprised of a voltage step down transformer 22 having primary and secondary windings 24 and 26, a polarizing rectifier 28 and a current limiting resistor 30. The transformer serves to reduce the voltage magnitude of the AC power source, so as to prevent damage to the capacitor and relay coil, and provide a safe low voltage level across switches 10 and 12.

Switches 10 and 12 are hand operated and can be the same type as Model CR115HO1404 manufactured and sold by the General Purpose Control Products Department of the General Electric Company. Switch 10 can have three screw-type terminals 32, 34 and 36, and a single electrically conductive pole 38 which is always electrically connected to terminal 36. Similarly, switch 12 can have three screw-type terminals 40, 42 and 44, and a single electrically conductive pole 46 which is always electrically connected to terminal 44. Pole 38 is normally in electrical contact with terminal 32 and thereby forms a normally closed contact between terminals 32 and 36 and a normally open contact between terminals 34 and 36. Upon transmittal of hand pressure to pole 38, pole 38 comes into momentary electrical contact with terminal 34 and is momentarily disconnected from terminal 32, thereby opening the contact formed between terminals 32 and 36 and closing the contact formed between the terminals 34 and 36. Similarly, pole 46 of switch 12 is normally in electrical contact with terminal 40, and forms a normally closed contact between terminals 40 and 44 and a normally open contact between terminals 42 and 44. Again, upon transmittal of hand pressure to pole 46, pole 46 comes into momentary electrical contact with terminal 44, and is momentarily electrically disconnected from terminal 40 so as to close the contact formed between terminals 42 and 44 and open the contact formed between terminals 40 and 44.

Primary winding 24 of transformer 22 is electrically connected across a source of AC power via a protective line fuse 48 and a main AC power switch 50. A pilot light 52 is electrically connected across primary winding 24 to provide an indication of whether or not power is applied to the circuit. One terminal of secondary winding 26 is electrically connected to terminal 40 of switch 12, while the other terminal of the secondary winding is electrically connected to the cathode of polarizing rectifier 28. The anode of rectifier 28 is electrically coupled to contact terminal 32 of switch 10 via resistor 30. Capacitor 14 is electrically connected between contact terminals 36 and 44 of respective switches 10 and 12, while coil 18 of relay 16 is electrically connected between terminals 34 and 42 of respective switches 10 and 12. Normally open contact 20 is electrically connected across a source of AC power via a solenoid coil 54 which controls the enabling of the operation of a press or other similar machine.

The operation of this circuit will now be explained. It will be assumed that initially capacitor 14 is discharged prior to the closure of main power switch 50. Upon closure of main power switch 50, pilot light 52 lights up and a reduced voltage with respect to the AC power source is applied across secondary winding 26 of transformer 22. Since current only flows through rectifier 28 in one direction, capacitor 14 charges to the voltage appearing across the secondary winding. Due to the particular polarity of rectifier 28 within this circuit, that plate of capacitor 14, which is electrically connected to terminal 44 of switch 12, charges to a positive potential with respect to its other plate, as current flows through contact 36 to contact 32 via pole 38 of switch 10, then through resistor 30, rectifier 28, secondary winding 26, and from terminal 40 to terminal 44 via pole 46 of switch 12. Now, when it is desired to enable operation of the machine, the operator, using both hands, transmits pressure to respective poles 38 and 46 of respective switches 10 and 12 to simultaneously close the respective contacts formed between terminals 34 and 36 of switch 10 and between terminals 42 and 44 of switch 12. Upon closure of these contacts, capacitor 14 discharges through coil 18 of relay 16. This, in turn, causes normally open contact 20 to close, and AC power is applied directly across solenoid 54 of the press. Thus, operation of the press is enabled, while both hands of the operator are forced to remain outside the line of operation of the press. Now, when the operator releases both of his hands from switches 10 and 12, the respective contacts formed between terminals 32 and 36 of switch 10 and between terminals 40 and 44 of switch 12 close, and capacitor 14 begins to recharge. Once capacitor 14 is recharged, the next operation of the press is enabled following the transmittal of hand pressure to the poles of switches 10 and 12 by the operator.

If the operator chooses to tape down switch 12 so that its normally open contact formed between terminals 42 and 44 is always closed, then capacitor 14 could not become charged, and solenoid 54 of the press could never be actuated. However, if capacitor 12 were first allowed to charge, and then the pole of switch 12 were taped down to close the contact formed between terminals 42 and 44 thereof, one hand could be used to depress the pole of switch 10 and cause actuation of solenoid 54. However, the tape would have to be removed from switch 12 so that the contact formed between terminals 40 and 44 of switch 12 can close to allow recharging of the capacitor. Thus, in order to perform a series of repetitive press operations in this manner, the pole of switch 12 would have to be held down by tape for part of the operating cycle, and then the tape would have to be removed for the remaining part of the operating cycle. Such an irregular procedure would so greatly increase the operating time of the machine cycle as to be totally unacceptable, especially when the machine is to be operated continuously through repetitive cycles.

It is thus clear that the above described circuit would effectively require the use of both hands to enable operation of a press or other similar machine, thereby preventing either hand of the operator from being positioned within the line of operation of the press. This, in turn, should result in a significant decrease in serious permanent injuries to the hands of operators of various types of presses and other similar machines.

Although the invention has been described with reference to a specific embodiment thereof, numerous modifications are possible without departing from the invention, and it is desirable to cover all modifications falling within the spirit and scope of the invention.

Claims

What I claim as new and desire to secure my Letters Patent of the United States is:

1. A circuit for enabling safe operation of a machine, comprising:

a. first and second single pole double throw momentary contact switches, each switch having first, second and third terminals, said first and second switch terminals being normally electrically connected together when said second and third switch terminals are normally electrically disconnected, and said second and third switch terminals are momentarily electrically connected together during hand pressure actuation of each said switch when said first and second switch terminals are momentarily disconnected;

b. a capacitor always electrically coupled between said second terminals of said first and second switches;

c. means electrically connected between said first terminals of said first and second switches for charging said capacitor when said first and second terminals of said respective first and second switches are electrically connected together; and

d. a relay having a coil and a normally open relay contact, said coil being electrically connected between said third terminals of said first and second switches for providing a discharge path for said capacitor only when said second and third terminals of said respective first and second switches are momentarily electrically connected together, said normally open relay contact being in circuit relationship with the machine, whereby upon discharge of said capacitor through said coil, said normally open relay contact closes and enables operation of the machine, and upon return of both of said first and second switches to their respective normal unactuated positions, said charging means recharges said capacitor to await enabling of the next operation of the machine.

2. A circuit according to claim 1, wherein said charging means is comprised of:

a. a step down transformer having a primary and a secondary winding;

b. a resistor; and

c. a rectifier, said resistor and rectifier being in series circuit relationship with said secondary winding, and said primary winding being in series circuit relationship with a source of AC power.