Automatically Positionable Chair

This invention relates to the field of automatic positioning chairs for medical application. Such chairs are sometimes referred to by the general term "table" and when the word "chair" is used herein, it is intended to include such a "table". The invention has particular application to podiatry chairs which may be automatically elevated or tilted and which may have automatically moveable back supports or foot supports.

Prior art chairs of the type with which this invention is concerned use feedback sensors for measuring the position of moveable surfaces and signaling the measured position to a motor controller. Automatic positioning control is achieved by comparing a position command with a measured position to develop an error signal. The position motors are driven until the error signal has been reduced to zero. At this point, the controller knows that the desired position is achieved.

Prior art chairs of the above described type have also been provided with control units having switches which may be operated to move various support surfaces to desired locations. When the desired locations are achieved, the operator activates a switch directing the control system to store the positions of all moveable surfaces. Thereafter the chair may be returned to the same positional configuration by operating the same or another switch.

There has been a need for automatically positionable chairs capable of returning to a programmed position without the use of feedback sensors. Such sensors complicate the design and increase the cost of the chair. It is apparent that such a chair may be operated "open loop" without position sensors by simply measuring the time required to move the chair surfaces to a desired position and treating such movement time as a measurement of position. However, it is desired to move such chairs to a variety of preprogrammed positions under various load conditions which may be encountered during movement with patients of differing weights seated thereon. Drive motors are prone to operate at different speeds under different load conditions. This is especially true for electric motors which are designed with the anticipated operating load in mind and which do not have substantial overcapacity. It is impractical to utilize elapsed movement time as a measure of surface position when the motor speed changes with load.

This invention provides a programmably positionable chair which may be directed to move to selectively preprogrammed positions without any use of position feedback. Such positioning control is achieved by providing electric motors for moving the positionable support surfaces and measuring the motor current during surface movement. It has been found that variable load conditions affect the motor current as well as the motor speed and that there is a relationship between motor current and motor speed which is independent of load for a given supply voltage. In accordance with this invention, the relationship between motor current and motor speed for a supply voltage of interest is experimentally established. This relationship is programmed into the controller, so that motor current measurements may be used as an indication of motor speed. The motor speed, so determined, is integrated over the movement time to establish surface position. The chair support surfaces are driven to computed positions on an open loop basis without any position feedback.

A chair in accordance with this invention may use anywhere from one to four or more electric motors for driving a like number of support surfaces. In a typical application, a podiatry chair in accordance with this invention, may have a programmable back position and a programmable foot support position. Moreover, the seat may rest upon a base which may be automatically elevated to a programmed position and automatically tilted to a programmed tilt angle. A plurality of programmed position combinations may be established. In a preferred embodiment, the controller utilizes a programmed microprocessor, and the current flows through the electric motors are measured by Hall effect devices.

It is therefore an object of this invention to provide an automatically positionable chair which may be controlled to assume selectively programmed positions without the use of position feedback and to do so irrespective of load conditions.

Other and further objects of the invention will be apparent from the following description, the appended claims and the attached drawings.

FIG. 1 is a perspective view of a podiatry chair;

FIG. 2 is a side elevation view of a podiatry chair in a flat position;

FIG. 3 is a bottom elevation view of a podiatry chair taken along lines 3--3 of FIG. 2;

FIG. 4 is a schematic electric diagram illustrating connections for a hand held program controller;

FIGS. 5A-5C are an electrical schematic diagram of a microprocessor connected for open loop control of drive motors for a programmable podiatry chair;

FIGS. 6A and 6B are an electrical schematic diagram of circuitry for transmitting motor current measurements to the microprocessor arragement illustrated in FIGS. 5A-5C; and

FIG. 7 is an electrical schematic diagram of connections for a watchdog circuit.

A podiatry chair having programmable positions in accordance with the present invention is shown in FIG. 1. The chair 20 includes a base 22 to which a chair frame 24 is mounted. A plurality of upholstered sections 26 are removably mounted to chair frame 24 by hook and loop fabric strips or the like, so that upholstered sections 26 may be easily removed for cleaning or repair. The chair frame 24 comprises a back 30 pivotally attached to a seat 32. A foot support 60 is attached to seat 32. The chair model illustrated in FIG. 1 has a powered back, powered tilt, and powered elevation. A slightly different model of podiatry chair, as illustrated in FIGS. 2 and 3 may also have a powered foot support.

Referring now to FIGS. 2 and 3, chair 20 is provided with four electric drive motors: including a base motor 61, a back motor 66, a tilt motor 67 and a foot motor 62. Motors 66, 67, 61 and 62 are reversibly operated to power a back actuator 34, a tilt actuator 36, a base actuator 38, and a foot actuator 33 respectively. Controls for the drive motors comprise a series of switches mounted in a hand control unit 40 (FIG. 1) detachably mounted on the side of foot support 60. Hand control unit 40 also includes switches for programming the automatic control, as hereinafter described.

Actuators 34 and 36 include a screw shaft and gearing means or the like for enabling the respective motors to rotate their shafts. A nut is mounted on each shaft for converting the rotary motion of the shaft into linear motion of actuator arms 68 and 69. Actuator arms 68 and 69 in turn position back 30 and tilt seat 32. Foot actuator 33 operates in a similar manner for angular positioning of foot support 60.

Actuator 38 is mounted in base 22 and is secured to upper plate 86. Base actuator 38 converts the rotary motion of base motor 61 into linear motion for operation of a scissors mechanism 88, as illustrated in FIG. 2.

The control system for the chair is illustrated in FIGS. 4, 5A-5C, 6A, 6B and 7. The heart of the control system is a microprocessor 100 which is illustrated in FIG. 5B as having pin connections corresponding to those of a microprocessor sold by Zilog, Inc. under the trademark Super8. Clock pulses for microprocessor 100 are provided by a 12 MH.sub.z crystal.

Pin numbers 14-17, 32, 33, 35 and 36 of microprocessor 100 are connected to a cable 203, which in turn is connected to solenoid operated switches 151-158. Microprocessor 100 selectively closes switches 151-158 (FIG. 5C) for powering and directionally controlling base motor 61, back motor 66, tilt motor 67, and foot motor 62. Closure of switch 151 causes rotation of base motor 61 in a direction which causes upward movement of base 22. Base 22 is driven downwardly by opening switch 151 and closing switch 152. Switches 153 and 154 similarly cause upward and downward motion of back 30, while switches 155 and 156 cause upward and downward tilting of seat 32, and switches 157, 158 cause downward rotation of foot support 60.

Electrical current for motors 61, 66, 67 and 62 flows through choke coils 161, 166, 167 and 162 respectively. Current flows through these choke coils are sensed by Hall effect devices 121, 126, 127 and 122 respectively (FIG. 5A). Output signals from the Hal effect devices are applied to operational amplifiers 221-224 which detect overcurrent conditions of the type which may occur when a foreign object interferes with motion of the chair. Output signals from operational amplifiers from 221-224 are applied to pin numbers 24-27 respectively of microprocessor 100 to stop the overcurrent condition by opening the appropriate ones of switches 151-158.

Hand control 40 is connected to microprocessor 100 by means of lines 201, 202. The connection of those lines into hand control unit 40 is illustrated in FIG. 4. Unit 40 has fifteen switches 131-137 and 141-148, which are sequentially tested by 8-bit parallel to serial converters 109, 110. Microprocessor 100 transmits a sequence of clock pulses on line 201 to integrated circuits 109, 110 which return a code on line 202 identifying the state of each of the switches in hand control unit 40. The functions of these switches are identified in Table I.

As indicated by Table I, an operator may move foot support surface 60 in the upward direction by closing switch 147. Thereafter, upward movement may be interrupted by activating the stop switch 135. The operator may similarly operate switches 142-148 in combination with stop switch 135 to achieve any desired relative positions of the moveable chair surfaces. Once a desired positioned combination has been achieved, a calculated position set is programmed into the system by actuating the "Program" switch 134 and any one of the four positions select switches 131, 133, 136, 137. The latter four switches enable a podiatrist to program up to four position combinations into the control system. These four position combinations may be selected to meet his specific professional needs. The chair may be directed to return automatically to any of these positioned combinations by activating the associated one of switches 131, 133, 136 or 137. An "Auto Return" switch 132 is provided for commanding a return to the home position.

It is a feature of this invention that the chair may be controlled automatically to move to selectively programmed positions wthout using any position sensing transducers. This is accomplished by digitizing the analog output signals from Hall effect devices 121, 126, 127 and 122 and using these digitized outputs to solve a position control algorithm. The algorithm as hereinafter described, associates motor current with speed and integrates the speed over time to establish a computed position.

Digitizing of the analog output signals from the Hall effect devices is performed by analog-to-digital converter 106 (FIG. 6A). However, prior to digitizing, the Hall effect signals are processed by peak amplifiers 225-228. These peak amplifiers are connected to one-microfarad capacitors 229-232 respectively for providing output signals which represent the average peak currents through choke coils 161, 166, 167 and 162. These average peak current signals are applied to lines 233-236 for application to the input terminals of A/D converter 106. A/D converter 106 produces digitized equivalents of the average peak choke coil currents on lines 240-247 for application to pin numbers 1-8 of microprocessor 100.

Microprocessor 100 also receives programming codes from integrated circuit 102 which is a 16K by 8-bit EPROM. EPROM 12 operates under control of an address decode latch 101. The computing circuitry also includes an address chip select decoder 103 for selection of EPROM 102 and another address chip select decoder 104 for selection of a watchdog circuit comprising a clock divider 105 (FIG. 7). The watchdog circuit generates a microprocessor reset signal, if it doesn't receive a periodic toggle signal from integrated circuit 104. This disables the motors in the event of a logic error or a failure of memory. However, if a problem is caused by a "glitch" induced by an external cause, the system will resume operating automatically after the reset.

It is another feature of the position control system that programmed position control information may be stored for up to 48 hours. Such storage is accomplished through an 8K.times.8 RAM 107 connected by a lead line 250 to a large capacitor 251. Capacitor 251 may have a capacitance of about 1 farad which may store sufficient charge to enable the above-noted data retention by RAM 107. Thus an operator may program the chair and then shut down overnight without losing the program data. Positional data is supplied by microprocessor 100 to RAM 107 via address/data bus lines 240-247. Address information (low order bits) is supplied by address/data bus lines 240-247 via address latch 108 and by address bus lines 260-264 (high order bits).

As noted above, this invention permits the chair surfaces to be driven to preprogrammed positions by solving algorithms which relate measured motor currents to motor speeds. It is important to know the motor speed, because variations in the weight of the patient who is seated on the chair affect the motor speed and hence the time required for the control surface to reach a preprogrammed position. Knowing the speed of the motor, it is possible to integrate this speed over the movement time to obtain a calculated position. However, it is not necessary to know the actual position, as calculation of an appropriately repeatable position parameter is sufficient. If such a position parameter is calculated during a programming run the corresponding actual position may be achieved during a working run by operating the motors and repeating the calculations until the same value of the position parameter has been calculated.

The algorithm relating motor speed to motor current will vary with motor design details but can be readily established in an experimental procedure by applying the design working voltage to the motor and operating the motor against a continuously varying load. As the load varies the supply current and shaft angular velocity are simultaneously measured and recorded. A plot of shaft angular velocity against current establishes the algorithm. The algorithm may be expressed in equation form by conventional curve fitting. As an alternative the algorithm may be simply reduced to a tabular form and stored in memory. However, the tabular form is avoided where possible, because it uses up large chunks of memory.

It has been found that for many motors the algorithm has a fairly linear form over the current range of interest, so that motor speed can e derived to a fair degree of accuracy by the expression:

where:

S is the motor speed

C is the motor current

K.sub.1 +K.sub.2 are constants to be experimentally determined

It has also been found that the above expression is not accurate for the conditions which obtain during starting and stopping. However, it is generally satisfactory to assume a constant average speed (independent of load) during these time periods. That speed, again, is determined experimentally.

In general the motor position is given by the equation:

where T is the total elapsed time.

If the starting time period is G.sub.1 and the stop period is G.sub.2, and the motor has an average speed S.sub.A during these periods, then to a fairly good approximation:

For a microprocessor computing at regularly timed intervals t

where the summations are taken over the time interval from G.sub.1 to T-G.sub.2.

In operation during a working run the microprocessor begins computing at the instant of motor start by calculating (or looking up in memory) the term S.sub.A (G.sub.1 +G.sub.2) and placing this value in a position register. It thereafter counts time intervals until t=G.sub.1 which is the known end of the motor start-up phase. Then the microprocessor calculates (K.sub.2 -K.sub.1 C) t for each computing cycle and adds this into the position register. This is continued until the position register holds a predetermined target value. The time then is (by definition) T-G.sub.2, and the drive motor is switched off automatically. For a programming run the operator switches the drive motor off manually when a desired position has been reached, and the value then in the position register is stored to become a new target value. In practice, since only a position parameter expressing a relative distance is required, G.sub.1 and G.sub.2 are expressed in terms of the number of elapsed computing cycles, and t is set equal to 1.0.

A routine implementing the above technique in SUPER 8 assembly language for foot motor 62 is listed in Table II.

In order to understand Table II it may be observed that the above-noted term K.sub.2 -K.sub.1 C may be rewritten as K.sub.1 (C-K.sub.2 /K.sub.1). The program for the foot includes routines for running the foot motor up and running the foot motor down. Within these routines there are subroutines covering the two alternative conditions wherein the measured current is greater than or less than the no load current.

Referring now to program line nos. 721-730 for foot motor up and measured current greater than no load current and making the substitutions:

It may be seen that the computer performs a series of steps which calculate the term K.sub.2 -K.sub.1 C. The calculated value is added into a register known as ADD.sub.-- FT. Other program listings, not shown, include similar routines for simultaneously controlling the position of motors G1, G6 and G7.

It will be appreciated that frictional losses, motor slippage and cumulative computing errors will cause errors in the calculated motor position which will grow with the passage of time. Such errors are small enough to be unobjectionable in an automatically positionable chair. In any event they may be removed by periodically returning the chair to the home position. When this is done the position registers are all automatically zeroed.

While the form of apparatus herein described constitutes a preferred embodiment of this invention, it is to be understood that the invention is not limited to this precise form of apparatus, and that changes may be made therein without departing from the scope of the invention which is defined in the appended claims.