Patient support device and method of operation

A patient support device of a radiation therapy treatment system includes an electromechanical motor and control system for raising and lowering the support device in the Z direction. The control system utilizes regenerative braking concepts, converting the motor into a generator as the support device is lowered such that no matter the load, the support device will be lowered at a constant speed. The control system also allows for lowering of the support device in the powered off situation (i.e., when there is no power to the support device).

FIELD OF THE INVENTION

This invention relates to a radiation therapy imaging and treatment system. More specifically, the invention relates to a patient support device for use with such a system.

BACKGROUND OF THE INVENTION

Medical equipment for radiation therapy treats tumorous tissue with high energy radiation. The dose and the placement of the dose must be accurately controlled to ensure both that the tumor receives sufficient radiation to be destroyed, and that damage to the surrounding and adjacent non-tumorous tissue is minimized. Intensity modulated radiation therapy (“IMRT”) treats a patient with multiple rays of radiation each of which may be independently controlled in intensity and/or energy. The rays are directed from different angles about the patient and combine to provide a desired dose pattern. In external source radiation therapy, a radiation source external to the patient treats internal tumors. The external source is normally collimated to direct a beam only to the tumorous site. Typically, the radiation source includes either high-energy X-rays, electrons from certain linear accelerators, or gamma rays from highly focused radioisotopes, though other types of radiation sources are possible.

One way to control the position of the radiation delivery to the patient is through the use of a patient support device, such as a couch, that is adjustable in one or more directions. The use of a patient support device is well known in the medical field, with similar patient support devices being used in CT scanning devices and Magnetic Resonances Imagers (MRIs). The patient support device allows the patient to be moved into and out of the field of the radiation to be delivered and in some cases, allow for adjustments of patient position during a radiation treatment.

SUMMARY OF THE INVENTION

When a patient support device, such as a couch, is used in this manner, there are many variables that need to be accounted for. For example construction materials and configuration of suitable electronics necessary to operate the couch must be carefully selected to ensure smooth operation of the couch, and precise measurement of couch position (when the couch has multiple movable parts). When these features are thoughtfully considered in the environment of radiation delivery, the patient support device can be a key tool in improving patient outcomes.

The present invention provides a patient support device comprising a base, a table assembly supported by the base and configured to support a patient, a motor electrically coupled to and operable to control motion of the table assembly, a controller electrically coupled to the motor, the controller operable to generate a signal to brake the motor when power to the motor is interrupted, and a brake control module. The brake control module is electrically coupled to the motor and the controller and is operable upon reactivation of the motor. The brake control module includes a passive dynamic load module electrically coupled to the motor to increase speed of the motor, a rectification module electrically coupled to the motor and operable to convert AC voltage to DC voltage when the AC voltage reaches a predetermined value, a controlled dynamic load module electrically coupled to the passive dynamic load module, and a switch electrically coupled to the controlled dynamic load module and operable to connect and disconnect the controlled dynamic load module to the motor to control a braking operation of the motor.

In another aspect, the present invention provides a radiation therapy treatment system comprising a patient support device and a control system. The patient support device includes a table assembly configured to support a patient, and a motor electrically connected to the table assembly and operable to control movement of the table assembly. The control system is electrically connected to the motor and operable to control a speed of the motor and provide linear motion of the table assembly when power to the couch is interrupted.

DETAILED DESCRIPTION

Although directional references, such as upper, lower, downward, upward, rearward, bottom, front, rear, etc., may be made herein in describing the drawings, these references are made relative to the drawings (as normally viewed) for convenience. These directions are not intended to be taken literally or limit the present invention in any form. In addition, terms such as “first,” “second,” and “third” are used herein for purposes of description and are not intended to indicate or imply relative importance or significance.

In addition, it should be understood that embodiments of the invention include hardware, software, and electronic components or modules that, for purposes of discussion, may be illustrated and described as if the majority of the components were implemented solely in hardware. However, one of ordinary skill in the art, and based on a reading of this detailed description, would recognize that, in at least one embodiment, the electronic based aspects of the invention may be implemented in software. As such, it should be noted that a plurality of hardware and software based devices, as well as a plurality of different structural components may be utilized to implement the invention. Furthermore, and as described in subsequent paragraphs, the specific mechanical configurations illustrated in the drawings are intended to exemplify embodiments of the invention and that other alternative mechanical configurations are possible.

FIG. 1illustrates a radiation therapy treatment system10that can provide radiation therapy to a patient14. The radiation therapy treatment can include photon-based radiation therapy, brachytherapy, electron beam therapy, proton, neutron, or particle therapy, or other types of treatment therapy. The radiation therapy treatment system10includes a gantry18. The gantry18can support a radiation module22, which can include a radiation source24and a linear accelerator26(a.k.a. “a linac”) operable to generate a beam30of radiation. Though the gantry18shown in the drawings is a ring gantry, i.e., it extends through a full 360° arc to create a complete ring or circle, other types of mounting arrangements may also be employed. For example, a C-type, partial ring gantry, or robotic arm could be used. Any other framework capable of positioning the radiation module22at various rotational and/or axial positions relative to the patient14may also be employed. In addition, the radiation source24may travel in path that does not follow the shape of the gantry18. For example, the radiation source24may travel in a non-circular path even though the illustrated gantry18is generally circular-shaped. The gantry18of the illustrated embodiment defines a gantry aperture32into which the patient14moves during treatment.

The radiation module22can also include a modulation device34operable to modify or modulate the radiation beam30. The modulation device34provides the modulation of the radiation beam30and directs the radiation beam30toward the patient14. Specifically, the radiation beam30is directed toward a portion38of the patient. Broadly speaking, the portion38may include the entire body, but is generally smaller than the entire body and can be defined by a two-dimensional area and/or a three-dimensional volume. A portion or area desired to receive the radiation, which may be referred to as a target or target region, is an example of a region of interest. Another type of region of interest is a region at risk. If a portion includes a region at risk, the radiation beam is preferably diverted from the region at risk. Such modulation is sometimes referred to as intensity modulated radiation therapy (“IMRT”).

The modulation device34can include a collimation device42as illustrated inFIG. 2. The collimation device42includes a set of jaws46that define and adjust the size of an aperture50through which the radiation beam30may pass. The jaws46include an upper jaw54and a lower jaw58. The upper jaw54and the lower jaw58are moveable to adjust the size of the aperture50. The position of the jaws46regulates the shape of the beam30that is delivered to the patient14.

In one embodiment, and illustrated inFIG. 2, the modulation device34can comprise a multi-leaf collimator62(a.k.a. “MLC”), which includes a plurality of interlaced leaves66operable to move from position to position, to provide intensity modulation. It is also noted that the leaves66can be moved to a position anywhere between a minimally and maximally-open position. The plurality of interlaced leaves66modulate the strength, size, and shape of the radiation beam30before the radiation beam30reaches the portion38on the patient14. Each of the leaves66is independently controlled by an actuator70, such as a motor or an air valve so that the leaf66can open and close quickly to permit or block the passage of radiation. The actuators70can be controlled by a computer74and/or controller.

The radiation therapy treatment system10can also include a detector78, e.g., a kilovoltage or a megavoltage detector, operable to receive the radiation beam30, as illustrated inFIG. 1. The linear accelerator26and the detector78can also operate as a computed tomography (CT) system to generate CT images of the patient14. The linear accelerator26emits the radiation beam30toward the portion38in the patient14. The portion38absorbs some of the radiation. The detector78detects or measures the amount of radiation absorbed by the portion38. The detector78collects the absorption data from different angles as the linear accelerator26rotates around and emits radiation toward the patient14. The collected absorption data is transmitted to the computer74to process the absorption data and to generate images of the patient's body tissues and organs. The images can also illustrate bone, soft tissues, and blood vessels.

The system10can also include a patient support device, shown as a couch82, operable to support at least a portion of the patient14during treatment. While the illustrated couch82is designed to support the entire body of the patient14, in other embodiments of the invention the patient support need not support the entire body, but rather can be designed to support only a portion of the patient14during treatment. The couch82moves into and out of the field of radiation along an axis84(i.e., Y axis). The couch82is also capable of moving along the X and Z axes as illustrated inFIG. 1.

With reference toFIGS. 3-6, the couch82includes a table assembly92coupled to a base93via a platform95. The table assembly92includes an upper support94movably coupled to a lower support98. With particular reference toFIG. 5, the upper support94is a substantially flat, rectangular support member on which the patient is supported during treatment. The upper support94is movable with respect to the lower support98to move the patient into and out of the radiation beam30during treatment. In the illustrated embodiment, the upper and lower supports94,98are composed of a carbon fiber composite, though other suitable compositions of the supports are possible. The upper support94includes an upper surface102and a lower surface106that contacts an upper surface110of the lower support98. As shown in the illustrated embodiment, the lower surface106includes a bearing layer114that is intended to reduce friction between the lower surface106and the upper surface110of the lower support98when the upper support94is moved with respect to the lower support98. Specific details of the bearing layer114and its application are discussed in co-pending U.S. patent application Ser. No. 12/204,617, the entire contents of which are incorporated herein by reference.

The table assembly92is movable in the X, Y, and Z directions, as illustrated inFIG. 1. Positioning of the table assembly92, and thus the position of the patient, with respect to the gantry18and the radiation beam30must be precise to ensure that the radiation is delivered to the proper areas of the patient. The movement of the table assembly92is controlled by the couch operator using a control keypad140, illustrated inFIGS. 7-10.

Once the user actuates the buttons144of the keypad140, the table assembly92will move at the direction of the user. In conventional couch designs, a hydraulic lifting system is utilized to move the table assembly92in the Z direction. The hydraulic lifting system is a convenient way to achieve some control over the lowering of the table assembly92, and has the benefit of allowing the table assembly92to be lowered when there is no power delivered to the system10. When the power to the couch82is disrupted while a patient is in the treatment position, the table assembly92needs to be lowered to allow the patient to exit the couch, and such lowering must be done in a controlled manner. However, hydraulic systems are more expensive to implement, are less reliable, and are less accurate in their range of motion.

With reference toFIGS. 3 and 11, the couch82according to the present invention, includes a lowering mechanism160with resistive braking capabilities to allow for the controlled lowering of the table assembly92in powered off situations. More specifically, the lowering mechanism160utilizes an electromechanical roller screw configuration. This configuration has the benefits of being less expensive to implement, being more reliable (e.g., the reliability of a roller screw implementation), and allowing for more accurate control of couch motion and position than the conventional hydraulic lifting mechanisms. The lowering mechanism160as described herein is responsible for motion in the vertical direction (i.e., the Z direction).

The lowering mechanism160includes power braking resistors to dissipate energy from a motor170to control the downward motion of the table assembly92. The braking resistors act as a damping or deceleration device, taking the energy output of the motor170and allowing for controlled lowering of the table assembly92, even in the powered off situation. This allows for regulated control of a free running motor that provides linear motion of the mechanical system under nonlinear external loads, even if the power to the system is interrupted. The braking resistors are designed so that no matter the load on the table assembly92, the downward speed of the table assembly92remains the same. By keeping the speed constant, even with a dynamic load, control of the motion is achieved.

Using the lowering mechanism160, the motor170becomes a generator during the lowering process of the table assembly92(i.e., if the power is uncontrolled, the power and speed increase as the couch drops). To prevent this, there needs to be a change in the load resistance applied proportionately to the power generated. When the generator has too high of a load, it begins braking. The effective value of the resistance is changed by connecting and disconnecting a power resistor. If the power resistor were constantly applied to the lowering mechanism160, the speed of the table assembly92would increase as the table assembly92is lowered (simulating a free fall) that could cause the table assembly92to crash at the bottom of the path of motion. By alternating the connection of the power resistor to the lowering mechanism160, the table assembly92is protected from crashing. By applying a non-linear load to the lowering mechanism160, the speed drop of the table assembly92is linear such that the resistance linearizes what was previously non-linear motion. The frequency with which the power resistor is connected to the lowering mechanism160changes the effective resistance within the braking circuitry.

The lowering mechanism160also includes support arms164that couple the table assembly92to a riser168of the base93. As shown in the illustrated embodiment, the lowering mechanism160includes two pairs of support arms164, with each arm164within a pair of arms being parallel to the other. As the table assembly92is raised and lowered, a longitudinal axis of each arm164within a pair remains parallel to the other arm, and a plane P1formed by the longitudinal axis of one pair of arms does not intersect a plane P2formed by the longitudinal axis of the other pair of arms.

Movement of the table assembly92in the Z axis, as described in some detail above, utilizes an electromechanical roller screw. The Z axis motion is controlled by a dual feedback mechanism. Incremental feedback is provided by the roller screw, and a direct drive encoder looks at angle and provides absolute feedback. All axes of the couch82have step-move capabilities due to their control mechanisms. In the Z direction, doing a step-move will correct for cobra motion in the Y axis direction.

The lowering mechanism160includes a motor control system169as illustrated inFIG. 13. The motor control system169includes the motor170and motor controller198, which controls the motor170in both the regular mode and in the case of the free-running motor-turned-generator mode under the mechanical non-linear variable external load PEXT174when the main power (3-Phase AC bus178and system VDC bus182) to the couch82is interrupted.FIG. 21is a graph illustrating various curves of the mechanical non-linear variable external load174(PEXT).

The motor control system169includes a system enable interlock186where pin1is connected to SYS enable bus190, pin2is connected to system VDC bus182, pin3is connected through the enable bus194to pin5of the motor controller198, and pin4is connected through bus202to a coil206dof a motor power switch206and to a coil210cof a motor brake release (MBR) interlock switch210.

The motor controller198includes pin1, pin2, and pin3connected to the 3-Ph AC bus178, pin4connected to system VDC bus182, pin6connected through bus214to normally-open contact210aof the MBR interlock switch210, and pin7, pin8, and pin9connected through 3-Phase Motor Controller Bus218to appropriate normally-open contacts206a,206b,206cof the motor power switch206.

The motor170is connected through a shaft222to the mechanical non-linear variable external load PEXT174. A three-phase power bus226connects the motor170to the appropriate common contacts206a,206b,206cof the motor power switch206. The motor170includes a motor brake release (MBR)230connected through bus234to common contact210aof the MBR interlock switch210.

The motor control system169also includes an emergency dynamic braking control unit238connected through 3-Phase Power bus242to appropriate normally-closed contacts206a,206b, and206cof the motor power switch206. A delta connected 3-Phase resistive dynamic load246is connected to appropriate phases of the 3-Phase power bus242. A star connected 3-Phase capacitive dynamic load250is connected to appropriate phases of the 3-Phase power bus242. A 3-Phase rectifier254, with common points of diodes D1-D4, D2-D5, and D3-D6, is connected to appropriate phases of the 3-Phase Power bus242. The common points of the diodes D4, D5, D6are connected to ground and the common points of the diodes D1, D2, D3are connected to the power-control-sensor bus258.

The power-control-sensor bus258is connected to a voltage sensor dynamic load262(RVSL), and the other side of the RVSLis connected to ground. The power-control-sensor bus258is also connected to a current sensor dynamic load266(CCSL), and the other side of the CCSLis connected to ground. The power-control-sensor bus258is also connected to a controlled dynamic load270(RCDL). The other side of the RCDLis connected to a normally-open contact of a power switch274, while the common contact of the power switch274is connected to ground. The power-control-sensor bus258also is connected to a positive input of an operation amplifier278.

The emergency dynamic braking control unit238includes an emergency power supply282, which contains two rechargeable batteries BT1, BT2connected in series. The negative lead of the battery BT1is connected to ground and the positive lead of the battery BT2is connected through bus286to common contact210bof the MBR interlock switch210. The normally closed contact210bis connected through bus290to the normally open contact of an emergency motor brake release switch294. The common contact of this switch294is connected through bus298to coil302cof a system VDC interlock switch302and to normally closed contact210aof the MBR interlock switch210. The common point of the batteries BT1and BT2are connected through bus306to normally open contact302bof the system VDC interlock switch302. The common contact302ais connected to system VDC through bus182and normally closed contact302ais connected to coil210cof the MBR interlock switch210through bus362. Common contact302bis connected to bus310, which supplies analog computer load314and is connected to pin1of a voltage reference318, to pin1of the operation amplifier278, and to pin1of a switched oscillator322. The voltage reference318is connected by pin2to ground and is connected by pin3through bus326to the negative input of the operation amplifier278. The operation amplifier278is connected by pin2to ground and is connected by pin3through bus330to pin3of the switched oscillator322. The switched oscillator322is connected by pin2to ground and is connected by pin4through bus334to pin1of the power switch274.

The motor170is initiated when it receives a signal from the 3-Phase AC power on the bus178, system VDC on the bus182, and SYS enable signal on the bus190. The system enable interlock186initiates the motor controller198, the motor power switch206, the MBR interlock switch210, and the system VDC interlock switch302through normally closed contact302a. The motor controller198communicates with the motor170through the bus218, normally open contacts206a,206b,206cof the motor power switch206, and bus226. The motor170begins acting through shaft222on the mechanical non-linear variable external load PEXT174.

The motor controller198also communicates with the motor brake release230through bus214, normally open contact210a, and bus234to disengage the motor brake release230. When main power 3-Phase AC on bus178and system VDC on bus182are interrupted, the switches206,210are disengaged and the motor brake release230is engaged.

The motor170is connected to the emergency dynamic braking control unit238through bus226, normally closed contacts206a,206b,206c, and through 3-Phase power bus242. To activate the emergency mode when the mechanical non-linear variable external load PEXT174begins acting through shaft222on the motor170, the user needs to push and hold the emergency motor brake release switch294. The MBR release switch294is activated through bus298to engaged coil302cof the system VDC interlock switch302, and to switch302by contact302ato disengaged coil210cof the MBR interlock switch210. The motor is reactivated when the MBR230is disengaged. The MBR230is disengaged when the MBR release switch294is released through bus298, normally closed contact210a, and bus234. The motor170begins acting under external load PEXT174, and the 3-Phase AC voltage from the motor170begins to interact with Passive Dynamic Loads338. The motor170gets first two stages of dynamic braking action on the AC Dynamic Loads342of the Passive Dynamic Loads338. The AC Dynamic Loads342includes two types of AC loads: (1) the 3-Phase Resistive Dynamic Load246which transfers AC energy from the motor into heat, and (2) the 3-Phase Capacitive Dynamic Load250which shifts AC phases from the motor170. Both heat dissipation and phase shift increases current from the motor170and this current increases eddy currents in the motor170which affect braking action in the motor170.

The motor170continuously increases speed under PEXT174until the AC voltage from the motor reaches a certain value, and then the AC voltage begins rectification by 3-Phase Rectifier254. The rectified AC voltage begins a third stage in the DC passive dynamic braking on the Voltage Sensor Dynamic Load262(VVSL) of the Passive Dynamic Loads338. The motor170proceeds to increase speed under PEXT174and the DC voltage increases too, until a certain value is reached, and then the Analog Computer Load314begins to control the dynamic braking action. The Analog Computer Load314is supplied by two sources: (1) the Emergency Power Supply282through BT1, through bus306, normally open contact302b, and bus310, and (2) motor170through bus226, normally closed contacts206a,206b,206c, bus242, 3-Phase Rectifier254, and Power-Control-Sensor Bus258.

There are two conditions of operation of the motor control system169:
Condition 1: xVRF>VVSL(1)

Only the Passive Dynamic Loads338are working. And time charge tCHof the capacitor CCSL266will have an infinite value.
Condition 2: xVRF<VVSL(2)

The Passive Dynamic Loads338and the Controlled Dynamic Loads346begin working together, and the time charge tCHof the capacitor CCSL266operates up to a value of the voltage xVRF318. This value of the time charge tCHwill be inversely proportional to a value of the load PEXT174.FIG. 22is a graph illustrating the curves of the equivalent voltage source VVSL, voltages VCSL, and time charge tCHon the current sensor load CCSLunder different values of external loads PEXT.

The time charge tCHof the capacitor CCSL266up to value of the voltage xVRFis illustrated inFIG. 19and determined by

where RSEis the resistance of the sensor equivalent, which is calculated by formula (4) illustrated inFIG. 16and formula (5) illustrated inFIG. 18. RSErepresents the internal resistance of the motor170with relationship to the PEXT174; and

VVSLis the Thevenin voltage source equivalent of the Motor170which is calculated by formula (6) illustrated inFIG. 15and formula (7) illustrated inFIG. 17.

n is determined by formula (8) (provided below) illustrated inFIG. 20.

where n is a mechanical system coefficient, n=0 . . . 1 (n is a positive number)

PINTis a system mechanical internal resistance350

PEXTis a mechanical non-linear variable external load174

xVRFis the value of the Voltage Reference318

x is an adjusting coefficient which allows adjustment of the Emergency Dynamic Braking Control Unit238for different motors and different values of the braking speed control.

FIG. 23is a graph illustrating the current on the passive dynamic loads and current on the controlled dynamic loads under different values of external loads PEXT. ICSL, illustrated inFIG. 19, of the Current Sensor Dynamic Load266is directly proportional to the value of the PEXT174
ICSL∥PEXT(9)

The operation amplifier278controls changes of the voltage on the Capacitor Sensor Dynamic Load (CCSL)262and compares it with the value of the voltage on the Voltage Reference (xVRF)318. When the voltage VCSLon capacitor CCSL262becomes equal to or higher than the value on the xVRF318, the operation Amplifier278starts Switched Oscillator322. The Switched Oscillator322is switched with a predetermined time tDDCHof the Power Switch274. The Power Switch274connects the Controlled Dynamic Load270to ground, which begins the fourth stage of the dynamic braking action of the motor170on the Controlled Dynamic Load270. The Controlled Dynamic Load270discharges Capacitor CCSLof the Current Sensor Dynamic Load266. The predetermined time discharge tDDCHshould be greater than the time constant of the discharge capacitor CCSL

During the predetermined time tDDCHthat Operation Amplifier278controls the voltage on the Capacitor Sensor Dynamic Load262, the operation amplifier278continuously compares the CCSLvoltage with value of the voltage on the Voltage Reference318(xVRF) until the voltage xVRFbecomes equal to or higher than the value CCSL262. When the predetermined time tDDCHelapses, the Switched Oscillator322disconnects Power Switch274, which disconnects RCDL270from ground, which begins the fifth stage of the dynamic braking action of the motor170on the Current Sensor Dynamic Load266(CCSL).

The instantaneous current IINSTthrough the capacitor of the Current Sensor Dynamic Load266(CCSL) is determined by:

When the voltage on the Current Sensor Dynamic Load266(CCSL) again becomes equal to or greater than the value on the Voltage Reference318(xVRF), the Operation Amplifier278restarts the Switched Oscillator322. The Switched Oscillator322is switched with predetermined time tDDCHof the Power Switch274. The Power Switch274is connected to Controlled Dynamic Load270to ground, which again begins the fourth stage of the dynamic braking action of the motor170on the Controlled Dynamic Load270. The Controlled Dynamic Load270discharges the capacitor of the Current Sensor Dynamic Load266(CCSL). That cycling will continue on the shaft222of the motor170and the mechanical non-linear variable external load PEXT174and during that cycle the Emergency Dynamic Braking Control Unit238will manage linear motion of the whole mechanical system.FIG. 24is a graph illustrating the motor speed control under different values of external loads PEXT.

Some of the unique features of the present invention include

(1) A motor-turned-generator170is a source for generating braking forces and a source of information about its conditions.

(2) The load power link, the sensors link, and the control link are on the same bus258.

(3) The voltage sensor dynamic load262(VCSL) and the current sensor dynamic load266(CCSL) are multifunctional:(a) Voltage Sensor Dynamic Load262(VCSL) is the DC passive load of the system Passive Dynamic Loads338, and the voltage sensor recognizes the internal resistance of the motor170like a voltage drop on itself.(b) Current Sensor Dynamic Load266(CCSL) is a three function device: the integrator in the analog computer load314, a current sensor, which recognizes the current value of the motor170as a time charge of the capacitor up to a certain value on the Voltage Reference318(xVRF), and the controlled dynamic load in the analog computer load314.

(4) The dynamic braking of the motor occurs in multiple stages:(a) The passive dynamic load338comprises the AC dynamic load342, which includes the 3-Ph capacitive dynamic load250(passive braking—stage 1) and the 3-Ph resistive dynamic load246(passive braking—stage 2) and the DC dynamic load, which includes the voltage sensor dynamic load262(passive braking—stage 3).(b) The controlled dynamic load346includes the controlled dynamic load270(controlled braking—stage 4) and the current sensor dynamic load266(controlled braking—stage 5).

(6) The capacitor of the current sensor dynamic load266is not a reactive load with respect to the motor170because the capacitor is charged from the motor170and is discharged through a resistor(s) of the controlled dynamic load270, which means that the capacitor does not return charged energy back to the motor170. The capacitor continues to be non-linear load.

(7) The controlled dynamic loads346include a two-cycle controlled dynamic load which provides a two-cycle controlled dynamic braking action on the motor170. The first cycle is controlled by the controlled dynamic load270, which provides the dynamic braking action on the motor170and discharges the capacitor of the current sensor dynamic load266(CCSL) during predetermined time tDDCH. The second cycle is controlled by the current sensor dynamic load266, which provides dynamic braking action on the motor170during the charge time tCHof the capacitor of the current sensor dynamic load266.

(8) When motor170operates as a motor-turned-generator it has the following properties:(a) the motor170becomes a voltage source;(b) the internal resistance of the voltage source is variable and inversely proportional to the speed of the motor and the mechanical load on the motor. In this mode, the motor170works like a sensor where the voltage reflects the speed of the motor and the current reflects the mechanical load on the shaft222of the motor.

(9) The analog computer load314includes combined properties of the analog computer and the controlled dynamic loads.

FIGS. 25-27are a flowchart and comments illustrating an embodiment of a method of the present invention.

Additional features of this invention can be found in the following claims.