Patent Publication Number: US-6907630-B2

Title: Load compensation system for power chair

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is related to concurrently filed U.S. Patent Applications entitled “Line Voltage Compensation System for Power Chair” and “Smooth Start System for Power Chair.” The entire disclosures of these U.S. patent applications are incorporated into this application by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to powered chairs and tables, and more particularly, to examination chairs and tables that may be automatically elevated, lowered or tilted. 
     BACKGROUND OF THE INVENTION 
     Patient comfort and practitioner efficiency remain paramount considerations within the healthcare industry. To this end, powered examination chairs featuring automatically moveable back, foot or other support surfaces have developed to facilitate clinical applications. Many such chairs may be positioned at a predetermined height above the floor. Support surfaces of the chair can often be manipulated to adjust the position of the person seated within, and many chairs can be lowered or raised in order to reduce the distance between a seated patient and the floor or healthcare professional. 
     An examination chair typically includes adjustable side rails positioned to restrain the movement of the patient seated in the chair. The side rails of the chair may e manually or automatically moved to a position away from the seat of the chair to facilitate the person getting in and out of the chair. 
     The speed at which a chair is designed to move is conventionally set at a nominal, or target speed. This target speed generally consists of a range of expected speeds, and is ideally optimized for efficient and predictable chair movement. As such, a voltage is supplied to a motor to produce a speed that generally falls within the target range. More particularly, the supplied voltage theoretically induces an amount of revolutions per minute in the motor that will cause the chair to generally move at the target speed. 
     However, the speed that conventional chairs actually move can vary dramatically from this target range. This inconsistency is often attributable to the weight of the patient or other some other load acting on the chair. The load incident on the chair causes the number of revolutions per minute to vary. The speed at which the chair moves reflects this variance. Namely, the load placed on the motor causes voltage to be diverted from its intended purpose of generating revolutions per minute. 
     Some conventional target speeds factor in the affect of an estimated load when determining the voltage or magnetic force level. Notably, this estimated load is a static figure. That is, the voltage is set according to a single, standard or median load. In this manner, voltage supplied to the motor of a conventional chair is set at a level that will generally achieve the target speed for a patient whom is precisely the standard weight. 
     The weight of patients, however, can vary dramatically from the standard weight estimate to which the motor is geared and powered. As the power level is set exclusively to the standard load, deviation from that standard load translates into the motor moving the chair at a rate that deviates from the target speed. That is, the chair moves at a faster or slower rate than the target speed. This variance and unpredictability poses an inconvenience and distraction to healthcare professionals and patients, alike. 
     Speed variance may also be encountered or exacerbated in circumstances where a chair is lowered or raised. Gravitational forces acting in concert with the patient and chair weight cause the motor to have to work relatively harder in order to raise the patient. Consequently, the speed of the chair is slower than the target speed when being raised. Conversely, the motor works less when lowering the chair. The speed of the chair is thus faster than the target speed when the chair is lowered. 
     As a consequence, what is needed is an improved manner of automatically adjusting the position of a power chair that mitigates the affect of load forces on chair/motor speed. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved method and apparatus for automatically positioning a powered chair that addresses the above shortcomings of the prior art. In one sense, an embodiment of the present invention positions a chair at a desired speed irrespective of load forces acting on the chair. An exemplary load force may include the weight of a patient, as well as other gravitational and mechanical forces associated with chair travel. The desired speed is achieved by apportioning voltage to the motor according to the load. For example, a constant motor speed may be achieved by compensating for patient weight and chair travel direction. 
     More particularly, a load signal indicative of the load on the chair is used to determine a voltage, or magnetic force, that should be included in a power signal. That power signal is applied to a motor to produce a desired speed. Such determination processes as are consistent with the principles of the present invention may include determining the voltage applied to and/or the current drawn by the motor. Namely, a voltage associated with the current draw of the motor is subtracted from the voltage signal supplied to the motor. Because the resultant applied voltage is proportional to the current. drawn by the motor from the voltage supplied to the motor, the applied voltage is proportional to or otherwise indicative of the speed of the motor. 
     This determined, or applied voltage may them be compared to a reference voltage. The reference voltage is typically associated with a desired speed. The desired speed may relate to either or both the motor speed and the speed at which the chair moves. The duty cycle of a power signal supplied to the motor is modified according to the voltage comparison. Where advantageous, voltage and/or other load determinations may be correlated to power levels stored within a memory. 
     Another of the same embodiment that is consistent with the principles of the present invention may receive a load signal indicative of a direction in which movement of the support apparatus is desired. This input signal may be correlated to a power level and/or stored reference voltage. The determined power level may be used to generate a power signal that drives the motor. In this manner, a constant speed may be achieved irrespective of the load forces associated with the direction in which the chair moves. 
     By virtue of the foregoing there is provided an improved chair positioning system that addresses shortcomings of the prior art. These and other objects and advantages of the present invention shall be made apparent in the accompanying drawings and the description thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with a general description of the invention given above, and the detailed description of the embodiment given below, serve to explain the principles of the invention. 
         FIG. 1  shows a schematic diagram of a chair system in accordance with the principles of the present invention. 
         FIG. 2  shows a block diagram of the controller of FIG.  1 . 
         FIG. 3  shows a database schematic having application within the controller of FIG.  2 . 
         FIG. 4  is a flowchart having a sequence of steps executable by the system of  FIG. 1  for automatically positioning a chair at a desired speed using a determined voltage measurement. 
         FIG. 5  is a flowchart having a sequence of steps suited for execution by the system of  FIG. 1  for automatically positioning a chair at a desired speed using a lookup table. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows chair system  10  that may be positioned at a desired speed in accordance with the principles of the present invention. The chair system  10  includes a moveable column  12  to which a support surface  14  is mounted. Upholstered sections  16  are removable and mounted to the support surface  14 . As shown in  FIG. 1 , the support surface  14  comprises a back support  18  and a head support  21  that pivotally attach to a seat support  20 . The support surface  14  additionally includes a foot support  22 , which also pivotally attaches to the seat support  20 . The chair system  10  illustrated in  FIG. 1  is equipped with powered tilt and elevation and may be positioned according to a number of settings. 
     The block diagram of  FIG. 1  shows a motor  24  configured to power an actuator  26 . A motor  24  comprises a direct current (DC) motor. One skilled in the art, however, will appreciate that any manner of electric motor, including alternating current (AC) motors, may be alternatively used in accordance with the principles of the present invention. 
     An actuator  26  consistent with the principles of the present invention includes any device configured to initiate movement of the support surface  14 . The actuator  26  may include a screw shaft and gearing for enabling the motor to rotate the screw shaft. For this purpose, a nut may be mounted on each shaft for converting the rotary motion of the shaft into linear motion of an actuator arm  28 . The actuator arm  28 , in turn, positions the support surface  14 . While only one motor  24  and actuator  26  are shown in  FIG. 1 , one skilled in the art will appreciate that several such motors and/or actuators may be used to position a chair system  10  in accordance with the principles of the present invention. 
     A source  30  supplies voltage to a transformer  32 , which powers the chair system  10  of FIG.  1 . An exemplary transformer  32  steps down voltage from the power source  30  for hardware convenience and operating considerations. A suitable source  30  may include DC or AC input voltage. 
     More particularly, the motor  24  of the chair system  10  receives power from motor control circuitry  34  of a controller  36 . The motor control circuitry  34  produces a power signal having a fixed frequency and adjustable pulse width. As such, the controller  36  of the embodiment shown in  FIG. 1  generates pulse width modulated (power) signals including a variable duty cycle. The power signal delivers a variable voltage. to the motor  24 . Using this pulse width modulated scheme, the motor speed is held constant despite changes in motor load. For purposes of this specification, motor “speed” may alternatively be referred to as “revolutions per minute;” 
     The controller  36 , in turn, may receive external control inputs from a series of switches, pedals and/or sensors comprising user input devices  38 . Such input may comprise a load signal in an embodiment of the present invention. Other load signal sources may include output from voltage and load sensing circuitry (included within the controller  36 , as shown in the embodiment of FIG.  1 ). As discussed herein, a load signal is associated, derived from, suggestive or otherwise indicative of load forces incident on the chair system  10 . Moreover, the term “load” for purposes of this specification may include: a patient weight, voltage, current, speed signal (such as generated using a tachometer), force or other measurement relating to energy, power, voltage or magnetic force required by a motor  24  in moving a support surface  14 . 
     Where desirable, the chair system  10  may include position sensors  50  and limit switches  52  for detecting and limiting the positions and movement of the support surface  14 . One embodiment consistent with the principles of the present invention includes a weight sensor  54 . An exemplary weight sensor  54  is configured to determine at least a portion of a load comprising the weight of a patient seated in the chair system  10 . The weight sensor  54  may comprise an alternative or additional source of input used to generate a load signal. 
       FIG. 2  is a block diagram of the controller  36  of FIG.  1 . As shown in  FIG. 2 , the controller  36  may include one or more processors  60 . The controller  36  may additionally include a memory  62  accessible to the processor  60 . The memory  62  may include a database  64  and/or cache memory  66 . For instance, a database may contain lookup values for correlating a sensed load or voltage to a direction and/or power level. Another exemplary database may include a lookup feature for correlating a voltage magnitude to a signal profile. For example, a voltage magnitude may be correlated to a duty cycle parameter. Cache memory  66  may be used to temporarily store a sensed voltage or current, for instance. 
     The memory  62  may also include program code  68 . Such program code  68  is used to operate the chair system  10  and is typically stored in nonvolatile memory, along with other data the system  10  routinely relies upon. Such data may also includes operating parameters  70  such as predefined reference voltages, crash avoidance and program addresses. Program code  68  typically comprises one or more instructions that are resident at various times in memory  62 , and that, when read and executed by the processor  60 , cause the controller  36  to perform the steps necessary to execute functions or elements embodying the various aspects of the invention. 
     The controller  36  also receives and outputs data via various input devices  72 , a display  74  and an output device  76 . A network connection may comprise another input device  72  that is consistent with the principles of the present invention. Exemplary input device  72  may include hand and foot pedals  38 , as well as input from a voltage detection circuit  40  and/or a voltage sensor  54 . A suitable display  74  may be machine and/or user readable. Exemplary output(s)  76  may include a port and/or a network connection. As such, the controller  36  of an embodiment that is consistent with the principles of the present invention may communicate with and access remote processors and memory, along with other remote resources. 
     The controller  36  of  FIG. 2  includes motor voltage sensing circuitry  42  that comprises a device configured to measure voltage applied to and/or the rotational speed of the motor  24 . The controller  36  further includes motor load sensing circuitry  48 . The motor load sensing circuitry  48  comprises a device that measures current through and/or the rotational speed of the motor  24 . While the controller  36  of  FIG. 2  includes voltage sensing circuitry  42  and load sensing circuitry  48 , one skilled in the art will appreciate that other embodiments that are consistent with the invention may alternatively include voltage and load sensing circuitry equivalents external to the controller. Moreover, one of skill in the art will appreciate that the functionality of the voltage sensing circuitry  42  and load sensing circuitry  48 , as with all functionality of the controller  36  and electrical components of the chair system  10 , may alternatively be realized in an exclusively or hybrid software environment. Furthermore, a controller for purposes of this specification may include any device comprising a processor. 
     The processor  60  optically or otherwise interfaces with and provides instructions to the motor control circuitry  34 . The motor control circuitry  34  receives input from the motor load sensing circuit  48  and the motor voltage sensing circuitry  42  to determine an applied voltage signal that is directly proportional to the actual speed of the motor  24 . The motor control circuitry  34  further compares the applied voltage signal to a stored reference voltage. If they do not match within predefined parameters, the controller  36  may generate an error signal. The motor control circuitry  34  processes the error signal to determine how to modulate the pulse width (and duty cycle) of the power signal. 
       FIG. 3  is a database schematic  80  having application within the memory  62  of FIG.  2 . The exemplary schematic  80  includes a column  82  of load fields that are logically linked to either or both: a field comprising a direction in column  84  and a power level field, as shown in column  86 . As such, the database schematic  80  provides differing power levels  86  with respect to differing load values  82  to achieve a desired speed. 
     Additionally, because the direction of chair movement can affect speed, the database schematic  80  includes different power levels  86  for the same load value  88 , depending on the specified direction  84 . Typically, a power level correlated to a load being raised in a chair  10  will be larger than a power level correlated to the same load and a downward direction. As discussed herein, the difference may be attributable to gravitational forces and/or a mechanical advantage associated with gearing. Similarly, a power level logically associated with a heavier load will be higher than a power level associated with a lighter load. 
     An exemplary load value in field  88  may comprise the weight of a patient and/or the support surface  14 . The load  88  may also include a sensed voltage value. For example, the load  88  may include a voltage level sensed in connection with the operation of the motor  24 , including the voltage supplied to the motor  24 . The load value  88  may also include forces indicative of some mechanical advantage, such as those attributable to gearing or some other support structure. For instance, it may require more work to move a support surface  14  from its lowest position than when the support surface  14  is at a relatively higher, intermediate position. 
     One skilled in the art will appreciate that the load value  88  may be particular to a specific support surface  14 . For instance, the load value  88  may be associated with one or more of: a back support  18 , a seat support  20 , an armature  19 , or any other moveable component of the chair system  10 . A typical direction field  90  comprises “up” or “down.” However, one of skill in the art may appreciate that other directions having a horizontal vector component may be included where appropriate and as dictated by the nature of the support surface  14  being moved. 
     The load  88  and input direction  90  may be logically linked, or correlated, to a power level field  92 . The power level contained in the field  92  may comprise a voltage and/or signal protocol associated with a desired speed. For instance, such a signal protocol may include a duty cycle. The signal protocol may be used to generate a power signal at the controller  36 . The power level may further comprise a stored reference voltage. 
     As such, a load value  88  indicative of a patient&#39;s weight may be processed in conjunction with a desired direction  90  to determine a power level  92  that is required to maintain a desired speed. Thus, the controller  36  determines a power level  92  that will compensate for variance in loads and/or directions in a manner that addresses the problems of the prior art. 
     The fields of the database  80  may be populated using clinically established and/or independently computed data. Moreover, while the database schematic of  FIG. 3  may have particular application within certain embodiments of the present invention, one skilled in the art will recognize that the controller  36  may alternatively determine a power level by processing directional and load value inputs without using a database  80 . For example, input load and/or directional data may be multiplied by or otherwise processed using scaled factors to arrive at a comparable or identical power level. 
     As shown in  FIG. 3 , an embodiment of the present invention enables different desired speeds for a support surface  14  to be set according to respective, different directions. For instance, a doctor may prefer that the desired speed at which a given a support surface  14  lowers be slower than a second desired speed at which the support surface elevates. Moreover, different desired speeds may be set for different support surfaces. For instance, a foot support  22  may be programmed to move at a higher speed than a back support  18 . 
     When used in conjunction with position sensors  50  or another location determining device or process, different desired speeds may apply to different portions of a chair&#39;s travel. For example, the final ten inches of a chair&#39;s descent may be executed at a slower desired speed than the prior two feet of descent. Thus, features of the present invention allow maximum flexibility in designing and setting desired speed(s). To this end, a user in the factory or field may customize speeds via an input device  38 . 
     In an embodiment where no directional data is available or needed, a load level  94  may be directly correlated to a respective power level  96 . Similarly, a load level may alternatively be correlated directly to a direction field, where applicable. In any case, one skilled in the art will appreciate a number of alternative logical associations that may be realized in a computer context in accordance with the underlying principles of the present invention. 
       FIG. 4  is a flowchart  100  having a sequence of steps configured to move a support surface  14  at a constant, desired speed. Turning more particularly to the flowchart  100 , a user may initiate processes that are consistent with the present invention at block  102 . Such processes may include booting relevant program code  68 , as well as receiving user/automated inputs  72 , such as commands to move a support surface  14 . Other processes performed at block  102  may include initializing applicable memory  62 . For example, initialization processes may prompt the recall from memory  62  of a reference voltage, V ref , as shown in block  104 . The reference voltage is typically preset during manufacturing. However, the reference voltage may be programmatically modified, where desired. In either case, an exemplary reference voltage is typically set in proportion to a desired speed. 
     More particularly, voltage applied across the motor  24  is roughly proportional to the revolutions per minute (rpm&#39;s) of the motor  24 . The rpm&#39;s, in turn, are translatable into a distance traveled by a support surface  14  in a determinable period of time. Thus, the reference voltage can be set at a magnitude that generally or precisely corresponds to a desired speed. 
     An embodiment consistent with the principles of the present invention may use a stepped-down or derivative voltage level as the reference voltage. For instance, a voltage of 48 volts delivered to the motor  24  may correspond to a reference voltage of 5 volts. This stepped-down voltage may have signal processing advantages. 
     The reference voltage is used as a point of comparison for the actual, or applied voltage delivered to the motor  24 . One skilled in the art will appreciate that an embodiment that is consistent with the principles of the present invention may include a device that directly senses voltage delivered to or from the motor  24 . Alternatively, the processes associated with sensing the motor voltage and current draw may be supplanted or augmented with sensor or user input indicative of a patient&#39;s weight or other load data. As such, the above processes of blocks  106  and  108  represent just one manner of determining actual voltage or load in accordance with the principles of the present invention. 
     As a first step towards determining the actual voltage delivered to the motor  24 , the voltage sensing circuitry  42  may measure at block  106  a motor voltage, V m , delivered to the motor. As discussed herein, the measured motor voltage may be stepped down to accommodate circuitry specifications. In any case, a portion of the motor voltage delivered to the motor  24  is lost or consumed by the motor  24  during operation. At least a portion of such loss in motor voltage is attributable to load. That is, the motor  24  must draw additional current. This increased current draw reduces rpm production in order to accommodate load forces communicated to the motor  24  via the actuator  26 . As such, the amount of current or voltage needed to manage the load forces can be used to determine the percentage of voltage provided to the motor  24  that actually goes towards producing rpm&#39;s and, ultimately, speed. 
     To determine these losses in one embodiment that is consistent with the principles of the present invention, a current sensor  44  measures the current, I, drawn by the motor  24 . The drawn current flows in response and in proportion to the voltage levels applied to the motor  24  and caused by the loading of the actuator  26 . Because the resistive characteristics of the motor  24  and load sensing circuitry  48  are known, a voltage attributable to the load, V I , can be determined using Ohm&#39;s Law. Namely, the voltage loss is determined according to: V I I×R (motor and load sensing circuitry) . 
     The actual, or applied voltage used for motor speed may then be determined by subtracting the load voltage from the motor voltage. This step is included in the comparison of the applied voltage to the referenced voltage at block  114  of FIG.  4 . 
     Though not shown in  FIG. 4 , the determined motor voltage, load current and load voltage may be stored and communicated to the controller  36 , where appropriate. Similarly, an embodiment that is consistent with the principles of the present invention may likewise store the applied voltage for future reference or other use. 
     As shown at block  114 , the comparison of the applied voltage (V m −V I ) to the voltage reference (V ref ) may determine if the duty cycle of a power signal delivered to the motor  24  should be modified. For example, where the applied voltage is less than the reference voltage, the motor control circuitry  34  of the controller  36  may increase the duty cycle at block  118  according to the difference between the applied voltage and the reference voltage, as determined at block  116  of FIG.  4 . Of note, this determined difference may take into account any scaling or other processing used to step down a motor voltage, as discussed in connection with block  106 . Moreover, one of skill in the art will appreciate that, where so configured, the difference may alternatively be used to step up motor voltage in another embodiment that is in accordance with the principles of the present invention. 
     If the applied voltage at block  120  is alternatively determined to be greater than the reference voltage, then the duty cycle of the power signal may be decreased at block  122 . Such may be the case where a child or person of smaller stature is seated within the chair system  10 . The duty cycle may be decreased at block  122  in proportion to the difference between the actual voltage and the reference voltage. 
     Where so configured at block  124 , a load signal comprising an error signal may be initiated by motor control circuitry  34  in response to a discrepancy between the applied and reference voltages. The error signal generated at block  124  will automatically initiate modification of the duty cycle in proportion to the load at block  118  or block  122 . Where the applied voltage is alternatively equal to or otherwise within acceptable tolerances of the reference voltage, the duty cycle of the power signal is maintained, as indicated at block  126  of FIG.  4 . 
     In any case, the motor control circuitry  34  responds to a command to increase or decrease the duty cycle of the motor  24  by generating a pulse width modulated signal as shown at block  128 . The resultant power signal is then communicated to the motor  24  at block  130 . In this manner, the actuator  26  is continuously driven at block  132  at the desired speed. 
     The sequence of steps of the flowchart  100  of  FIG. 4  may be accomplished automatically and in realtime. Thus, the power supplied to the motor  24  is continuously and automatically adjusted to maintain the desired speed. Moreover, this dynamic adjustment may be accomplished in a manner that is transparent to the patient and/or healthcare professional. That is, the load (including the motor voltage across the motor  24 , where applicable) is constantly monitored in a feedback loop that continuously apportions power to the motor  24  to maintain the desired speed. 
       FIG. 5  shows a sequence of process steps in accordance with the principles of the present invention. That is, the flowchart  140  of  FIG. 5  includes method steps suited for automatically achieving a desired speed irrespective of load and directional requirements of a chair positioning operation. In one respect, the processes of  FIG. 5  achieve the desired speed using a lookup table. That is, directional data and/or other load information are correlated to stored power levels. 
     Turning more particularly to the flowchart  140  of  FIG. 5 , a user may enter input at block  142 . Exemplary user input may comprise directional data input via hand or foot input devices  38 . For instance, input received at block  148  may indicate a user&#39;s desire to raise the chair  10 . Other or the same such user input initiate program code  68  and memory processes of the chair system  10  at block  144  of FIG.  5 . As shown at block  146  of  FIG. 5 , the user input is communicated to the controller  36 . The controller  36  may store the input at block  147  within its memory  62  where advantageous. 
     In response to such input at block  150 , the program code  68  of one embodiment that is consistent with the invention may correlate the directional data a load value determined at block  168 . As discussed below in greater detail, the load value may include a patient weight, voltage or other measurement relating to work required by a motor  24  in moving a support surface  14 . As more particularly shown in the embodiment of  FIG. 5 , the directional data comprising a raise or lower command is correlated to the determined load value to retrieve a power level field  92  of a database  64 . Such a database  64  may include a plurality of stored power levels and load values. Each stored power level of the database  64  logically associates with the respective load value. The controller  36  then retrieves from the database  64  the power level correlated to the desired speed in response to receiving the load value. 
     Turning particularly to block  152  of  FIG. 5 , the power level field  92  and load value  88  may further be logically associated with a field  90  corresponding to the received raise command. Similarly, a input command processed by the controller  36  at block  154  to lower the chair  10  may cause the program code  68  to correlate a lower direction field and the load to a second power level at block  156 . 
     Where no direction is indicated, or directional input is not considered when achieving a desired speed in accordance with the principles of the present invention, a software implementation consistent with the principles of the present invention may correlate the load directly to a power level. Such a scenario is shown at block  158  of FIG.  5 . 
     In any case, the system retrieves the appropriate power level associated with the desired speed from memory  62  at block  160 . The retrieved power level is used to generate the power signal at block  162 , which is communicated to the motor  24 . As discussed herein, the power level may comprise a recalled reference voltage. As such, the power signal of one embodiment that is consistent with the principles of the present invention may be generated according to the voltage comparison processes discussed above in connection with FIG.  4 . In any case, the motor  24  drives the actuator  26  at block  166  as the chair system  10  dynamically monitors the load at block  168 . 
     As discussed herein, all or a portion of the load forces acting upon the chair system  10  are determined at block  168 . The load may be sensed or otherwise determined at block  168  by detecting the motor voltage and current loss, as discussed previously in connection with FIG.  4 . Alternatively, the load may be determined at block  168  of  FIG. 5  using a weight sensor or a voltage sensor positioned inline with the motor output. One skilled in the art will appreciate that any number of methods of determining load may alternatively be included within processes that are consistent with the principles of the present invention. 
     While the present invention has been illustrated by the description of embodiments thereof, and while the embodiments have been described in considerable detail, it is not intended to restrict or in any way limit the scope of the appended claims to such detail. Moreover, when the term “chair” is used above, it is intended to include the terms “table” and “bed.” Additional advantages and modifications will be readily apparent to those skilled in the art. 
     For example, a load signal in another embodiment that is consistent with the principles of the present invention may comprise input from an error signal and/or position sensors  50 . That is, the position sensors  50  may be used determine the speed at which the support surface  14  moves. As discussed herein, the detected speed is proportional to rpm&#39;s generated by the motor  24 . These rpm&#39;s, in turn, are proportional to the voltage used to generate speed. In any case, the detected speed or determined voltage value may be fed back to the controller  36  via the load signal. The controller  36  may then compare the speed conveyed in the load signal to a reference value. The reference value may be associated with a desired speed. If the controller  36  determines that there is a disparity between the load signal and the reference, the controller  36  may increase or decrease the voltage delivered to the motor according to the determined disparity. 
     The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and method, and illustrated examples shown and described. For instance, any of the exemplary steps of the above flowcharts may be augmented, replaced, omitted and/or rearranged while still being in accordance with the underlying principles of the present invention. Accordingly, departures may be made from such details without departing from the scope or spirit of Applicant&#39;s general inventive concept.