It is known to use sensors coupled to a mattress for monitoring a bed occupant. For example, Triplett et al. (U.S. Pat. No. 4,175,263) disclose a pressure sensing mechanism for placement on a hospital bed. The pressure sensing mechanism comprises a first pressure sensing pad for sensing a patient's weight when the patient is near the centre of the bed, and a second pressure sensing pad for sensing the patient's weight when the patient is near the edges of the bed. An alarm may be triggered when the patient moves to a location near the edge of the bed.
Tucknott et al. (U.S. Pat. No. 4,633,237) disclose a patient bed alarm system comprising a matrix of sensors woven into a mat for placement on a bed. The matrix of sensors is coupled to a micro computer for calculating the position of a patient in the bed. An alarm may be triggered when the patient is about to leave the bed.
Alihanka et al. (U.S. Pat. No. 4,320,766) disclose a capacitive motion sensor placed under a mattress or the like for monitoring the movements of a person.
McMahon et al. (U.S. Pat. No. 5,435,317) disclose a device for detecting a respiratory dysfunction of a person located in a bed. The device comprises a detection unit which is provided under the mattress of the bed, and a stimulation unit, which acts, in response to a signal from the detection unit, to impart a rocking motion to the bed. The detection unit is embodied as a pad-like device provided under the mattress.
Bellin et al. (U.S. Pat. No. 5,558,996) disclose a patient monitoring device wherein sensors are located in a bed sheet with which a subject comes in contact. One sensor produces a signal corresponding to respiratory induced, pulmonary motion, and myocardial pumping sounds. A second sensor produces a signal corresponding to changes in body position. A processor amplifies and filters the induced signals resulting in resolved output highly correlated to respiration rate, heart beat rate, and changes in body position.
Reimer et al. (U.S. Pat. No. 5,917,180) disclose a pressure sensor which relies on detecting multiply scattered light within an optical cavity. Changes in the volume of the cavity are sensed by the change in sampled light intensity. Pressure sensitive mats with a high density of sensors are assembled using optical fiber technology.
Musick (U.S. Pat. No. 5,844,488) discloses a narrow, pressure-sensitive sensor pad for installation on top of and across the width of a mattress proximate the midsection of a reclining patient. The pad has both central and edge switching areas. The central pressure sensitive switch indicates the presence of a patient in the center of the bed. When a patient moves toward either edge of the bed, an edge switch is activated which generates an early warning signal indicating to attending personnel that a patient has moved from the center of the bed to an edge and may be attempting to exit the bed unattended.
Hammett (U.S. Pat. No. 5,144,284) discloses a bed covering device adapted to cover a mattress. The device has compliant flat pressure sensitive means disposed its lower surface for detecting compressive force exerted by a person, and electrical connector means emergent from the pressure sensitive means and configured to connect in modular fashion with a monitoring device and alarm.
Rudeke (PCT publication No. WO9010281A1) discloses a bed alarm intended for use with patients in hospitals and like establishments for indicating when a patient leaves his or her bed. The alarm includes a pressure sensor which functions to detect whether a patient is lying in bed or not and which is intended to be positioned in the bed and preferably between the bed-bottom and the mattress.
Rincoe et al. (U.S. Pat. No. 5,993,400) and Scott (U.S. Pat. No. 6,067,019) disclose devices that utilize arrangements of sensors to detect the impending egress of a bed occupant. Joseph et al. (U.S. Pat. No. 5,410,297) disclose a weight-sensitive capacitive sensor for tracking the position of an occupant. Dixon et al. (U.S. Pat. No. 6,208,250) disclose a system comprising two sensors and a processor used to determine the location of a patient on a bed.
Rosenthal (U.S. Pat. No. 3,961,201) discloses a switch located between a mattress and a bedframe that is used to detect when a patient moves close to the edge of a bed. Feldl (U.S. Pat. No. 4,020,482) discloses an air bladder below a mattress that signals egress of a patient when air pressure in the bladder falls below a threshold. Nicholas (U.S. Pat. No. 4,381,434) discloses a device including a spring-loaded plate with a limit switch, all mounted below a mattress.
Despite the volume of existing patent literature, there are significant shortcomings in the known devices. For many devices, reliable operation requires installation on top of the mattress. This has two major shortcomings: firstly, the devices are often stiff (i.e. resistant to flexing) and hard, thereby causing discomfort to the bed occupant. In some applications (for example, monitoring of demented occupants) this can cause physical and psychological irritation to the bed occupant. Secondly, because the devices are on top of the mattress, they rapidly suffer material fatigue and fail under normal use.