US 3917979 A
The electrical control system for a motor operated adjustable hospital bed is energized by AC voltage from a grounded AC power supply. A separate ground connection between the bed frame or chassis and the power supply ground establishes the bed chassis at earth ground potential during normal operating conditions. The patient is protected against shock hazards in the event that he contacts an AC voltage (from the bed's control system or from any external electrical apparatus) at the same time that he touches the bed chassis, thereby causing ground leakage current to flow through his body. This protection is achieved by constantly monitoring the ground leakage current flowing through the ground connection and opening that connection anytime the leakage current exceeds a predetermined threshold level, such as around 9 microamperes. In addition, the AC power supplied to the bed is interrupted.
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United States Patent 1 1 Volk, Jr.
[ PROTECTION CIRCUIT FOR AN ADJUSTABLE MOTORIZED HOSPITAL BED  Inventor: Joseph A. Volk, Jr., Florissant, Mo.
 Assignee: Borg-Wamer Corporation, Chicago,
 Filed: Mar. 18, 1974  Appl. No: 452,223
 U.S. Cl 317/18 D; 317/9 A; 317/10  Int. CI. .1 HOZH 3/08; HOZH 7/20  Field of Search... 317/18 R, 18 D, 18 A, 18 B,
 References Cited UNITED STATES PATENTS 2,999,189 9/1961 Gerrard n 317/18 B 3,515,942 6/1970 Gordon 317/18 D 3,525,018 8/1970 Murphy et a1. r r 317/18 D 3,579,037 5/1971 Hackman et a1 317/18 R 3,617,808 11/1971 Yoder 317/18 D 3,697,808 10/1972 Lee 317/18 A Fault Indicator 20v. Power pp y Hot Earth Ground Bed Chassis 1 1 Nov. 4, 1975 Primary Examinerl. D. Miller Assistant E.raminerPatrick R. Salce Attorney, Agent, or Firm-James E. Tracy ABSTRACT The electrical control system for a motor operated adjustable hospital bed is energized by AC voltage from a grounded AC power supply. A separate ground connection between the bed frame or chassis and the power supply ground establishes the bed chassis at earth ground potential during nonnal operating conditions. The patient is protected against shock hazards in the event that he contacts an AC voltage (from the beds control system or from any external electrical apparatus) at the same time that he touches the bed chassis, thereby causing ground leakage current to flow through his body This protection is achieved by constantly monitoring the ground leakage current flowing through the ground connection and opening that connection anytime the leakage current exceeds a predetermined threshold level, such as around 9 mi croamperes. In addition, the AC power supplied to the bed is interrupted.
8 Claims, 1 Drawing Figure Control System For Adjustable Motorized Hospital Bed Ground PROTECTION CIRCUIT FOR AN ADJUSTABLE MOTORIZED HOSPITAL BED BACKGROUND OF THE INVENTION An adjustable motorized hospital bed, the positioning of which may be remotely controlled by means of an electrical control system operated by the patient, must be completely safe to preclude any possibility of the patient being subject to hazardous electrical shocks from the voltages present in the control system or in the conductors supplying AC power thereto. To that end, and in accordance with conventional practice, the metal bed frame or chassis is usually grounded to the power supply ground or building earth ground so that circuit component failures or insulation breakdowns cannot establish the bed frame at a dangerously high potential relative to earth ground. Unfortunately, even though a patient may be made immune to or protected against malfunctioning or faults that tend to cause the application of undesired voltages to the bed chassis, he may still be subject to lethal currents due to the surrounding electrical environment.
To elucidate, the patient may contact, due to a fault condition, a voltage in the beds control system, or he may come in contact with a voltage from some faulty electrical device or appliance, such as a television set. radio, lamp, etc. On the other hand, the patient may receive a voltage from medical electronic equipment at tached to the patient for diagnostic or therapeutic purposes. Any one of these conditions can result in lethal current passing through the patients body. For example, assume that one of the patients hands touches an ungrounded metallic cabinet housing for a television receiver whose power cord is frayed such that the hot line conductor is in contact with the metallic housing. If the patient simultaneously places some other portion of his body in contact with the grounded bed frame a circuit path will be completed through his body, causing current to flow therethrough which may be of a magnitude sufiicient to cause death. Of course, a very dan gerous situation exists and the patient is particularly vulnerable when he is connected to medical electronic apparatus via a catheter or an implanted electrode. It has been found that current flow as low as microam peres through a patients heart is capable of causing ventricular fibrillation The present invention constitutes a significant improvement over the protection circuits heretofore developed for motor operated hospital beds, since for the first time a patient may be totally protected from any fault or failure either in the bed itself or in any electrical apparatus used by or on the patient.
SUMMARY OF THE INVENTION The protection circuit of the invention is to be incorporated in an adjustable motorized hospital bed wherein a grounded AC power supply is coupled to the beds electrical control system for applying AC operating voltage thereto, and wherein a ground connection couples the bed chassis to the power supply ground. Means are provided for monitoring any ground leakage current flowing through the ground connection, and switching means responds to the monitoring means for interrupting the ground connection anytime the ground leakage current exceeds a predetermined threshold level.
DESCRIPTION OF THE DRAWING The features of the invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further advantages and features thereof, may best be understood, however, by reference to the following description in conjunction with the accompanying drawing which schematically illustrates a protection circuit, constructed in accordance with the invention, and the manner in which the circuit is incorporated in the electrical system of an adjustable motorized hospital bed.
DESCRIPTION OF THE ILLUSTRATED EMBODIMENT Block 10 represents a conventional grounded AC power supply providing a single-phase alternating voltage having a magnitude of approximately volts RMS and a commutating frequency of 60 cycles per second or hertz. This AC voltage, or what is commonly called line voltage, is usually available at a wall outlet in any hospital room where an adjustable hospital bed is located. The line voltage is produced across the output terminals to which the conductors labeled Hot and Neutral are connected. The third output terminal of power supply 10 is connected to the building ground or earth ground, as is also the case with the Neutral output terminal. In conventional manner, the voltage at the Hot terminal will alternate in generally sinusoidal fashion above (or positive) and below (or negative) relative to the earth ground reference plane.
Under normal conditions the line voltage is applied to the primary winding 14 of an isolation transformer 15 through a series-connected bidirectional semiconductor or solid state switching device 18 which preferably takes the form of a triac having first and second main terminals, labeled T and T respectively, and a control or gate terminal G. A triac may be considered as two parallel PNPN structures oriented in opposite directions to provide symmetrical bidirectional electrical characteristics to permit current flow between the main terminals in either direction. It operates basically as two silicon controlled rectifiers or SCRs connected in parallel, but with the anode and cathode of one SCR connected to the cathode and anode. respectively, of the other SCR. In the absence of any applied voltages, a triac assumes its Off condition in which a very high impedance exists between main terminals T and T to effectively constitute an open switch. When a voltage of either polarity is impressed across the main terminals at the same time that triggering or gate current of appropriate magnitude flows between terminals G and T, in either direction, the triac turns ON to permit current flow between terminals T, and T in response to the voltage applied thereto and in the direction determined by the voltages polarity. When the triac conducts, a very low impedance is presented between its main terminals so that it essentially functions as a closed switch.
Isolation transformer 15, while not essential to the operation of the present invention, isolates the beds electrical system from the AC voltage supplied over the neutral and hot conductors. Of course, it provides no isolation with respect to any voltage that the patient may encounter from some external electrical apparatus also powered by power supply 10. Of course, the AC supply will usually be common to all the electrical equipment in the hospital room. Inasmuch as transformer I5 merely serves to isolate the beds electrical system from power supply 10, it preferably has a lzl turns ratio. The terminals of secondary winding 16 connect over respective ones of line conductors L, and L to the electrical control system for the adjustable mo torized hospital bed, depicted in the drawing by the single block labeled 21. Under normal operating conditions when there are no circuit component failures and when the patient does not come in contact with a voltage and provides a path for ground leakage current. the line voltage of 120 volts is applied. via transformer 15, to control system 21 to effect energization thereof in order that the positioning of the hospital bed may be under control of the patient occupying the bed.
In a typical arrangement, the control system includes a motor-drive system and a pushbutton-actuated control device capable of remotely controlling the motordrive system. By selectively depressing the pushbuttons. the patient is able to control the various adjustments. Usually. the mattress-supporting structure of the bed is articulated, being divided into four interconnected sections or panels, namely a back section, a center or seat section, an upper knee or thigh section and a lower knee or foot section. One motor-driven adjustment that may be controlled by the patient raises or lowers the two knee sections where they join together, thereby controlling the position of the patients knees. Another adjustment. under the patients control, pivots or tilts the back section with respect to the center section so that the patient's back and head may be raised or lowered. In most cases, a third motor-driven adjustment may be controlled by the pushbutton-actuated control device to vertically adjust the entire mattresssupporting frame.
As one example of a construction that control system 21 may take, reference is made to copending patent application Serv No. 380,310, filed July 18, 1973 in the name of Kenneth W. Padgitt. In that application, optically coupled circuits are employed to electrically isolate the patients hand control unit from the much higher current motor-drive circuitry.
The metal bed chassis or frame, which of course is conductive, is grounded to the power supply ground through a ground connection that includes a seriesconnected resistor 23 and a series-connected solid state switch 24, preferably in the form of a triac. As is the case with triac l8, triac 24 is normally maintained in its ON condition so that the ground connection is continuous and ground leakage current may flow to earth ground. Preferably, resistor 23 has a resistance of 500 ohms but by shunting it with the oppositely poled diodes 24 and 25, the apparent resistance is reduced to approximately 0.l ohm at voltages greater than one volt peak-to-peak. In other words, when the voltage across resistor 23 reaches a magnitude sufficient to turn diodes 24 and 25 ON, the ground current flows primarily through the diodes. Capacitor 27 merely serves as an RF (or radio frequency) bypass so that operation of the protection system will be limited to the low frequency (60 hertz) of the AC power supply. To explain. if, for example, RF radiation from a television set is picked up by the ground connection, the RF signal will be shunted through capacitor 27 so that no voltage develops across resistor 23.
A DC power supply 29 is coupled to the neutral and hot outputs of main AC power supply in order to produce a DC voltage, preferably around l0 volts, for operating the protection circuit. Of course. all of the terminals in the drawing marked are tied to the positive output of DC supply 29. A bistable device, such as a bistable multivibrator or flip flop 31, controls triacs l8 and 24. Flip flop 31 has two stable operating conditions. designated for convenience as the set and reset conditions respectively, Once the flip flop is triggered to its reset condition by means of a triggering pulse applied over its reset input, the flip flop will remain in that condition subsequent to the termination of that pulse and until another triggering pulse is applied over its set input, whereupon it will be actuated to its set condition, where it will remain until reset.
A reset circuit automatically resets the flip flop when power supply 29 becomes energized. At that time, capacitor 32 charges through resistor 33 toward the 10 volts positive potential of DC source 29. The positivegoing voltage at the junction of capacitor 32 and resistor 33 is threshold detected in amplifier 35 to produce a suitable reset pulse which is applied via resistor 36 to the reset input of flip flop 31. Diode 37 provides a discharge path for capacitor 32 when power is removed from the circuit. For reasons to be appreciated, a manual reset is also provided for flip flop 31. Manually operated reset switch 38 is a normally open pushbutton momentary contact switch. By depressing switch 38, the positive voltage of DC supply 29 is applied to the differentiating circuit formed by resistor 39 and capacitor 41 which in turn produces a triggering pulse for resetting the flip flop.
The two outputs 43 and 44 of flip flop 31 produce phase opposed, rectangular wave signals each of which varies between positive and negative amplitude levels, relative to the earth ground reference plane, as the flip flop is actuated between its set and reset conditions. The two output signals are amplified by a pair of buffer amplifiers 45 and 46 such that their output voltages will be positive and negative, respectively, when flip flop 31 is in its reset condition, whereas the outputs of those buffers will be reversed when the flip flop is in its set condition.
With flip flop 31 in its reset condition, the positive output voltage of buffer 45 is applied via current limiting resistor 48 to the base of NPN transistor 49 to turn it ON and effect current flow through lamp S1 of opto isolator 5 2. The lamp illuminates and causes photo resistor 53 of the opto isolator to exhibit a resistance sufficiently low to permit gate current to flow through resistor 55 and between the gate and main terminal T of triac 18, as a result of which the triac turns ON and completes the coupling between AC supply 10 and primary 14 so that power is supplied to the electrical control system 21. Simultaneously, the negative output voltage of buffer 46 is applied via current limiting resistor 56 to the base of NPN transistor 57 to maintain that transistor in its OFF condition. This permits the gate of traic 24 to receive gate current through biasing resistors 58 and 59 to maintain the triac conductive and the ground wire continuous.
Hence, under normal conditions flip flop 31 will be established in its reset condition and this in turn results in both of the triacs being in their ON states so that the AC power supply will be coupled to control system 21 and at the same time the ground connection between the chassis and earth ground remains uninterrupted.
It will now be shown that in the event of any one of a variety of different faults or malfunctions in either the beds electrical system or in any external electrical equipment or device that the patient engages, flip flop 31 will be triggered to its set condition to disconnect the AC power supply from control system 21 and to interrupt the ground connection. Initially, the protection circuit responds to any component failure or breakdown that effects the application of an undesirable voltage to the bed frame. To that end, a pair of seriesconnected, oppositely poled diodes 61 and 62 are connected in shunt with secondary winding 16 of the isolation transformer, the junction of the diodes being cou pled to the ground connection via a resistor 63 and a capacitor 65.
During normal operation when no fault exists, no current flows from the isolated secondary circuit and through resistor 63 and capacitor 65 due to the presence of series-opposed diodes 61 and 62. Specifically, during the half cycles when line conductor L, is positive relative to conductor L diode 62 blocks any current flow, whereas during the alternate half cycles when line conductor L is positive with respect to conductor L diode 61 prevents the flow of current. When a fault occurs in the control system such that one of its components fails and in so doing establishes a lowimpedance coupling (either capacitive. resistive, induc' tive or direct short) between the bed frame and either one of line conductors L, and L pulsating DC fault current flows between the junction of diodes 61 and 62 and the ground connection. Consider, for example, that conductor L becomes coupled to the bed chassis. Diode 61 thus becomes short circuited with the result that current flows from line L; and through diode 62, resistor 63 and capacitor 65 during the half cycles when line L is positive relative to line L,. Of course, the magnitude of this fault current is dependent on the impedance in the undesirable coupling between line L, and the bed frame.
Resistor 67 forms a filter with capacitor 65 to develop at circuit junction 68 a positive DC voltage, of an amplitude proportional to the fault current, for application to the set input of flip flop 31. Assuming that the fault current exceeds a given threshold level, such as 9 microamperes, this positive DC voltage will be sufficient to trigger flip flop 31 from its reset to its set condition. Diode 71 couples junction 68 to the positive output of DC power supply 29 to insure that the voltage applied to the set input never exceeds volts. When the flip flop sets, the potentials at outputs 43 and 44 re verse their polarities such that buffer 45 now applies a negative voltage to the base of transistor 49 and buffer 46 impresses a positive voltage on the base of transistor 57.
Transistor 49 thus turns OFF to extinguish lamp 51, whereupon photo resistor 53 introduces a resistance in the gate circuit for triac l8 sufficiently high to render the triac non-conductive. Primary winding 14 thus becomes disconnected from power supply 10. At the same time. the positive voltage applied to transistor 57 turns that transitor ON. shorting out the gate current for traic 24 to cause it to become cutoff, as a consequence of which the ground connection interrupts. When triac [8 is switched OFF. the full volts is applied to the fault indicator circuit comprising the lamp 72 (which may be a neon bulb) and current limiting resistor 73. The lamp illuminates at this time to provide a visual signal that a malfunction or failure exists. When the troublesome component is located and repaired or replaced, reset switch 38 may be depressed to reset the flip flop and reestablish the application of line voltage to primary winding 14 and at the same time to recon- 6 nect the ground connection between the bed frame and the power system ground.
Assume now that the patient comes in contact with the hot output of power supply 10 due to malfunction or fault in the beds electrical circuits or in some external equipment. As noted, power supply 10 will usually be common to all of the electrical apparatus in the hospital room where the bed and patient are located. If some other part of the patients body then touches the bed chassis, a circuit path will be completed through the patient from the hot line to earth ground. However, in accordance with a salient feature of the invention the patient will be protected from the ground leakage current and he will suffer no injury whatsoever, even though the circuit path may include the patients heart. Total patient protection is achieved by constantly monitoring any ground leakage current flowing through the patients body and the ground connection (which will be in series) and opening the ground connection as soon as any such current exceeds a predetermined threshold level, preferably 9 microamperes.
To explain, by providing resistor 23, shunted by diodes 24 and 25, in series with the ground connection, ground leakage current may be monitored by-sensing the voltage across the resistor. When such current flows, the AC voltage across resistor 23 is applied via a coupling capacitor to amplifier 76 which is bridged by a resistor 77 to obtain negative feed back to establish the amplifiers gain characteristic. The amplified AC output signal is applied through coupling capacitor 78 to potentiometer 79 where the signal is provided at adjustable tap 81 at an amplitude depending on the positioning of the tap. It is then supplied via coupling capacitor 82 to another amplifier 83 for further amplifcation, resistor 84 introducing negative feedback for establishing the amplifiers gain characteristic. The amplified ground leakage signal, developed by amplifier 83, is applied through capacitor 85 to diodes 86 and 87 and resistor 88. The diodes provide a peak-to-peak detector to clamp the negative peaks of the amplified signal to ground and to rectify the resulting signal, as a consequence of which a positive voltage is developed to which capacitor 65 charges through resistor 88. A positive voltage is thus produced at circuit junction 68 having an amplitude proportional to the amount of ground leakage current flowing through the ground connection. By appropriately adjusting potentiometer 79, the sensitivity of the protection circuit may be varied. Preferably, it is adjusted so that a ground leakage current as low as 9 microamperes produces a positive voltage at junction 68 sufi'icient to actuate flip flop 31 to its set condition which, of course. results in turning OFF triacs l8 and 24 to decouple the AC power supply from control system 21 and to open the ground lead. Setting the threshold level at around 9 microamperes insures that the circuit path through the patient will be broken before any injury is sustained. The protection circuit may easily be constructed so that it trips and opens the ground connection within 0.0]6 second.
The presence of back to back diodes 24 and 25 allows resistor 23 to be bypassed for heavy currents in the event the protection circuit malfunctions. Assume, for example, that triac 24 fails and becomes permanently shorted between its main terminals. The low impedence path provided by the diodes establishes a good ground connection in order to protect the external equipment frtliii any damage to their components.
Of course, while triacs are preferred for interrupting the hot line and the ground lead, a wide variety of switching devices could be used instead. For example, relays could be employed. n the other hand, a light emitting diode photo coupler may be employed. As still another example, each switch may comprise two NPN transistors whose emitter-collector paths are in series with each other and with the ground connection, the emitter of one transitor being directly connected to the emitter of the other. Turning both of the transistors ON completes the ground connection. With this arrangement, the offset bias voltage of one transistor is balanced by the bias voltage of the other so that there is no voltage drop between the two collectors.
The invention provides, therefore, a novel protection circuit for detecting anytime that ground leakage current, above a predetermined threshold level, flows through a patient's body, whereupon the current-carrying path is immediately opened to protect the patient from shock hazards and serious injury. In addition, the protection circuit detects when ever line voltage is applied to the bed frame, in which event the AC power supply automatically disconnects from the bed.
While a particular embodiment of the invention has been shown and described, modifications may be made, and it is intended in the appended claims to cover all such modifications as may fall within the true spirit and scope of the invention.
I. A protection circuit for an adjustable motorized hospital bed wherein a grounded AC power supply is coupled to the bed 5 electrical control system for applying AC operating voltage thereto, and wherein a ground connection couples the bed chassis to the power supply ground, comprising:
means, including a series-connected resistor in the ground connection, for monitoring any ground leakage current flowing through said ground connection;
switching means responsive to said monitoring means for interrupting the ground connection anytime the ground leakage current exceeds a predetermined threshold level;
and a pair of oppositely poled diodes, each connected in shunt with said resistor, for limiting the voltage developed across said resistor.
2. A protection circuit according to claim 1 in which said switching means also decouples the beds electrical control system from the AC power supply when the ground leakage current exceeds said predetermined threshold level.
3. A protection circuit according to claim 2 wherein the AC operating voltage is applied to the bed s electrical control system over a pair of line conductors, and including means for actuating said switching means to decouple the control system from the AC power supply anytime either one of the line conductors becomes grounded to the bed chassis.
4. A protection circuit according to claim 2 wherein the AC power supply is coupled to the bed s electrical control system via an isolation transformer, the primary winding of which is disconnected from the AC power supply by said switching means when the ground leakage current exceeds said predetermined threshold level.
5. A protection circuit according to claim 4 and including a pair of series-connected, oppositely poled diodes in shunt with the secondary winding of said transformer, the junction of said diodes being coupled to said ground connection, wherein said monitoring means monitors any fault current flowing between said junction and said ground connection, and in which said switching means responds to said monitoring means to decouple the bed's electrical control system from the AC power supply anytime the fault current exceeds a given threshold level.
6. A protection circuit according to claim 4 wherein the primary winding of said isolation transformer is coupled to the AC power supply via hot and neutral leads, and wherein a pair of solid state switches in series with the hot lead and the ground connection, respectively, are rendered non-conductive when the ground leakage current exceeds said predetermined threshold level.
7. A protection circuit according to claim 1 in which said switching means is actuated by said monitoring means to interrupt the ground connection when the ground leakage current exceeds approximately 9 microamperes.
8. A protection circuit according to claim 1 in which said switching means includes a solid state switch in series with the ground connection, a bistable device for controlling said solid state switch and having set and reset operating conditions, and means for establishing said bistable device normally in its reset operating condition to render said solid state switch conductive, thereby maintaining the ground connection continuous; and in which said monitoring means actuates said bistable device to its set condition, when the ground leakage current exceeds said predetermined threshold level, to render said solid state switch non-conductive and to interrupt the ground connection.