773 33.Exoskeletons for Human Performance Augmentation Homayoon Kazerooni Although autonomous robotic systems perform 33.1 Survey of Exoskeleton Systems.............. 773 remarkably in structured environments(e.g.,fac- 33.2 Upper-Extremity Exoskeleton................775 tories),integrated human-robotic systems are 33.3 Intelligent Assist Device....................... 776 superior to any autonomous robotic systems in un- structured environments that demand significant 33.4 Control Architecture for Upper-Extremity 778 adaptation.The technology associated with exo- Exoskeleton Augmentation................. skeleton systems and human power augmentation 33.5 Applications of Intelligent Assist Device..780 can be divided into lower-extremity exoskeletons 33.6 Lower-Extremity Exoskeleton 780 and upper-extremity exoskeletons.The reason for this was twofold;firstly,one could envision a great 33.7 The Control Scheme of an Exoskeleton....782 many applications for either a stand-alone lower- 33.8 Highlights of the Lower-Extremity Design 786 or upper-extremity exoskeleton in the immedi- 33.9 Field-Ready Exoskeleton Systems...........790 ate future.Secondly,and more importantly for the 33.9.1 The ExoHiker Exoskeleton............. 790 division,is that these exoskeletons are in their 33.9.2 The ExoClimber Exoskeleton........... 790 early stages,and further research still needs to be conducted to ensure that the upper-extremity 33.10 Conclusion and Further Reading 792 exoskeleton and lower-extremity exoskeleton can References.… 792 function well independently before one can ven- ture an attempt to integrate them.This chapter first gives a description of the upper-extremity more detailed description of the lower-extremity exoskeleton efforts and then will proceed with the exoskeleton. 33.1 Survey of Exoskeleton Systems In the early 1960s,the US Defense Department ex- The outer exoskeleton (the slave)followed the motions pressed interest in the development of a man-amplifier,of the inner exoskeleton (the master),which followed a powered suit of armor which would augment sol- the motions of the human operator.All these studies diers'lifting and carrying capabilities.In 1962,the Air found that duplicating all human motions and using Force had the Cornell Aeronautical Laboratory study master-slave systems were not practical.Additionally. the feasibility of using a master-slave robotic system as difficulties in human sensing and system complexity a man-amplifier.In later work,Cornell determined that kept it from walking. an exoskeleton,an external structure in the shape of the Vukobratovic et al.developed a few active orthoses human body which has far fewer degrees of freedom than for paraplegics [33.7].The systems include hydraulic or a human,could accomplish most desired tasks [33.1].pneumatic actuators for driving the hip and knee joints in Part From 1960 to 1971,General Electric developed and the sagittal plane.These orthoses were coupled with the tested a prototype man-amplifier,a master-slave system wearer via shoe bindings,cuffs,and a corset.The device called the Hardiman [33.2-6].The Hardiman was a set was externally powered and controlled via a predeter- of overlapping exoskeletons worn by a human operator. mined periodic motion.Although these early devices
773 Exoskeletons 33. Exoskeletons for Human Performance Augmentation Homayoon Kazerooni Although autonomous robotic systems perform remarkably in structured environments (e.g., factories), integrated human–robotic systems are superior to any autonomous robotic systems in unstructured environments that demand significant adaptation. The technology associated with exoskeleton systems and human power augmentation can be divided into lower-extremity exoskeletons and upper-extremity exoskeletons. The reason for this was twofold; firstly, one could envision a great many applications for either a stand-alone loweror upper-extremity exoskeleton in the immediate future. Secondly, and more importantly for the division, is that these exoskeletons are in their early stages, and further research still needs to be conducted to ensure that the upper-extremity exoskeleton and lower-extremity exoskeleton can function well independently before one can venture an attempt to integrate them. This chapter first gives a description of the upper-extremity exoskeleton efforts and then will proceed with the 33.1 Survey of Exoskeleton Systems ............... 773 33.2 Upper-Extremity Exoskeleton ................ 775 33.3 Intelligent Assist Device ........................ 776 33.4 Control Architecture for Upper-Extremity Exoskeleton Augmentation.................... 778 33.5 Applications of Intelligent Assist Device .. 780 33.6 Lower-Extremity Exoskeleton ................ 780 33.7 The Control Scheme of an Exoskeleton.... 782 33.8 Highlights of the Lower-Extremity Design 786 33.9 Field-Ready Exoskeleton Systems ........... 790 33.9.1 The ExoHiker Exoskeleton.............. 790 33.9.2 The ExoClimber Exoskeleton........... 790 33.10 Conclusion and Further Reading ............ 792 References .................................................. 792 more detailed description of the lower-extremity exoskeleton. 33.1 Survey of Exoskeleton Systems In the early 1960s, the US Defense Department expressed interest in the development of a man-amplifier, a powered suit of armor which would augment soldiers’ lifting and carrying capabilities. In 1962, the Air Force had the Cornell Aeronautical Laboratory study the feasibility of using a master–slave robotic system as a man-amplifier. In later work, Cornell determined that an exoskeleton, an external structure in the shape of the human body which has far fewer degrees of freedom than a human, could accomplish most desired tasks [33.1]. From 1960 to 1971, General Electric developed and tested a prototype man-amplifier, a master–slave system called the Hardiman [33.2–6]. The Hardiman was a set of overlapping exoskeletons worn by a human operator. The outer exoskeleton (the slave) followed the motions of the inner exoskeleton (the master), which followed the motions of the human operator. All these studies found that duplicating all human motions and using master–slave systems were not practical. Additionally, difficulties in human sensing and system complexity kept it from walking. Vukobratovic et al. developed a few active orthoses for paraplegics [33.7]. The systems include hydraulic or pneumatic actuators for driving the hip and knee joints in the sagittal plane. These orthoses were coupled with the wearer via shoe bindings, cuffs, and a corset. The device was externally powered and controlled via a predetermined periodic motion. Although these early devices Part D 33
774 Part D Manipulation and Interfaces were limited to predefined motions and had limited suc-joint trajectories without the use of any sensory systems cess,balancing algorithms developed for them are still from its wearer. used in many bipedal robots [33.8]. The hybrid assisted limb(HAL)was developed at the Seireg et al.also created an exoskeleton system for University of Tsukuba ([33.13,141).This 15 kg battery- paraplegics where only the hip and knee were pow- powered suit detects muscle myoelectrical signals on ered by hydraulic actuators in sagittal plane [33.9]. the skin surface below the hip and above the knee.The The hydraulic power unit consists of a battery-powered signals are picked up by the sensors and sent to the direct-current (DC)motor,pump,and accumulator.computer,which translates the nerve signals into sig- A bank of servo-valves drives the actuators at the knee nals of its own for controlling electric motors at the and hip.The device was controlled to follow a set of hips and knees of the exoskeleton,effectively amplify- a Fig.33.1 (a)Hardiman;(b)An exoskeleton system designed for paraplegics by Seireg et al.[33.9]:(c)HAL Part D33.1 Fig.33.2 (a)An exoskeleton for patient handling [33.10,11];(b)RoboKnee [33.12]
774 Part D Manipulation and Interfaces were limited to predefined motions and had limited success, balancing algorithms developed for them are still used in many bipedal robots [33.8]. Seireg et al. also created an exoskeleton system for paraplegics where only the hip and knee were powered by hydraulic actuators in sagittal plane [33.9]. The hydraulic power unit consists of a battery-powered direct-current (DC) motor, pump, and accumulator. A bank of servo-valves drives the actuators at the knee and hip. The device was controlled to follow a set of a) b) c) Fig. 33.1 (a) Hardiman; (b) An exoskeleton system designed for paraplegics by Seireg et al. [33.9]; (c) HAL a) b) Fig. 33.2 (a) An exoskeleton for patient handling [33.10, 11]; (b) RoboKnee [33.12] joint trajectories without the use of any sensory systems from its wearer. The hybrid assisted limb (HAL) was developed at the University of Tsukuba ([33.13,14]). This 15 kg batterypowered suit detects muscle myoelectrical signals on the skin surface below the hip and above the knee. The signals are picked up by the sensors and sent to the computer, which translates the nerve signals into signals of its own for controlling electric motors at the hips and knees of the exoskeleton, effectively amplifyPart D 33.1
Exoskeletons for Human Performance Augmentation 33.2 Upper-Extremity Exoskeleton 775 ing muscle strength.In addition to electromyography tor coupling the upper and the lower portions of a knee (EMG)signals,the device further includes potentiome-brace.The control of this powered knee brace requires ters for measuring the joint angles,force sensors for the ground reaction force measured by two load cells. measuring the ground reaction forces and a gyroscope The system uses a positive-feedback force controller to and accelerometer for measuring the torso angle.Each create an appropriate force for the actuator. leg of HAL powers the flexion/extension motion at the Kong et al.developed a full lower-limb exoskeleton hip and knee in the sagittal plane through the use of system that works with a powered walker [33.15].The DC motors integrated with harmonic drives.The ankle walker houses the electric actuators,the controller,and includes passive degrees of freedom. the batteries,reducing the weight of the exoskeleton Yamamoto et al.[33.10,11]have created an exoskel- system.A transmission system transmits power to the eton system for assisting nurses during patient handling. wearer's joints from the actuators in the walker.The The lower limbs include pneumatic actuators for the exoskeleton is powered at the hips and knees in sagittal flexion/extension of the hips and knees in the sagittal plane.The input to drive the system is a set of pressure plane.Air pumps are mounted directly onto each actua-sensor that measure the force applied by the quadriceps tor to provide pneumatic power.User input is determined muscle on the knee. via force sensing resistors coupled to the wearer's skin. Agrawal et al.have conducted research projects on The measurement from force sensing resistor (FSR)statically balanced leg orthoses that allow for less effort and other information such as joint angles are used to during swing [33.16].In the passive version,the device determine the required input torques for various joints.uses springs in order to cancel the gravity force associ- Pratt et al.developed a powered knee brace for ated with the device links and the person leg.Through adding power at the knee to assist in squatting [33.12]. experiments the authors showed that the device reduced The device is powered by a linear series-elastic actua- the required torque by the wearer substantially. 33.2 Upper-Extremity Exoskeleton In the mid-1980s,researchers at Berkeley initiated sev- movements accordingly,but the force he/she feels is eral research projects on upper-extremity exoskeleton much smaller than what he/she would feel without the systems,billed as human extenders [33.17-23].The device.In another example,suppose the worker uses main function of an upper-extremity exoskeleton is the device to maneuver a large,rigid,and bulky ob- human power augmentation for the manipulation of ject,such as an exhaust pipe.The device will convey heavy and bulky objects.Since upper-extremity ex- the force to the worker as if it was a light,single- oskeletons are mostly used for factory floors,warehouse, point mass.This limits the cross-coupled and centrifugal and distribution centers,they are hung from overhead forces that increase the difficulty of maneuvering a rigid cranes.As can be seen in later sections,lower-extremity body and can sometimes produce injurious forces on exoskeletons focus on supporting and carrying heavy the wrist.In a third example,suppose a worker uses payloads on the operator's back(like a backpack)during the device to handle a powered torque wrench.The de- long-distance locomotion.Upper-extremity exoskele-vice will decrease and filter the forces transferred from tons,which are also known as assist devices or human the wrench to the worker's arm so the worker feels the power extenders,can simulate forces on a worker's low-frequency components of the wrench's vibratory arms and torso.These forces differ from,and are usu-forces instead of the high-frequency components that ally much smaller than the forces needed to maneuver produce fatigue [33.24].These assist devices not only a load.When a worker uses an upper-extremity exoskel- filter out unwanted forces on a worker.but can also be eton to move a load,the device bears the bulk of the programmed to follow a particular trajectory regardless weight by itself,while transferring to the user as a nat-of the exact direction in which the worker attempts to ural feedback a scaled-down value of the load's actual manipulate the device.For example,suppose an auto- Part weight.For example,for every 20kg of weight from an assembly worker is using an assist device to move a seat object,a worker might support only 2 kg while the de-to its final destination inside a car.The assist device can 出 vice supports the remaining 18 kg.In this fashion,the bring the seat to its final destination,moving it along worker can still sense the load's weight and judge his/her a preprogrammed path with a speed that is proportional
Exoskeletons for Human Performance Augmentation 33.2 Upper-Extremity Exoskeleton 775 ing muscle strength. In addition to electromyography (EMG) signals, the device further includes potentiometers for measuring the joint angles, force sensors for measuring the ground reaction forces and a gyroscope and accelerometer for measuring the torso angle. Each leg of HAL powers the flexion/extension motion at the hip and knee in the sagittal plane through the use of DC motors integrated with harmonic drives. The ankle includes passive degrees of freedom. Yamamoto et al. [33.10,11] have created an exoskeleton system for assisting nurses during patient handling. The lower limbs include pneumatic actuators for the flexion/extension of the hips and knees in the sagittal plane. Air pumps are mounted directly onto each actuator to provide pneumatic power. User input is determined via force sensing resistors coupled to the wearer’s skin. The measurement from force sensing resistor (FSR) and other information such as joint angles are used to determine the required input torques for various joints. Pratt et al. developed a powered knee brace for adding power at the knee to assist in squatting [33.12]. The device is powered by a linear series-elastic actuator coupling the upper and the lower portions of a knee brace. The control of this powered knee brace requires the ground reaction force measured by two load cells. The system uses a positive-feedback force controller to create an appropriate force for the actuator. Kong et al. developed a full lower-limb exoskeleton system that works with a powered walker [33.15]. The walker houses the electric actuators, the controller, and the batteries, reducing the weight of the exoskeleton system. A transmission system transmits power to the wearer’s joints from the actuators in the walker. The exoskeleton is powered at the hips and knees in sagittal plane. The input to drive the system is a set of pressure sensor that measure the force applied by the quadriceps muscle on the knee. Agrawal et al. have conducted research projects on statically balanced leg orthoses that allow for less effort during swing [33.16]. In the passive version, the device uses springs in order to cancel the gravity force associated with the device links and the person leg. Through experiments the authors showed that the device reduced the required torque by the wearer substantially. 33.2 Upper-Extremity Exoskeleton In the mid-1980s, researchers at Berkeley initiated several research projects on upper-extremity exoskeleton systems, billed as human extenders [33.17–23]. The main function of an upper-extremity exoskeleton is human power augmentation for the manipulation of heavy and bulky objects. Since upper-extremity exoskeletons are mostly used for factory floors, warehouse, and distribution centers, they are hung from overhead cranes. As can be seen in later sections, lower-extremity exoskeletons focus on supporting and carrying heavy payloads on the operator’s back (like a backpack) during long-distance locomotion. Upper-extremity exoskeletons, which are also known as assist devices or human power extenders, can simulate forces on a worker’s arms and torso. These forces differ from, and are usually much smaller than the forces needed to maneuver a load. When a worker uses an upper-extremity exoskeleton to move a load, the device bears the bulk of the weight by itself, while transferring to the user as a natural feedback a scaled-down value of the load’s actual weight. For example, for every 20 kg of weight from an object, a worker might support only 2 kg while the device supports the remaining 18 kg. In this fashion, the worker can still sense the load’s weight and judge his/her movements accordingly, but the force he/she feels is much smaller than what he/she would feel without the device. In another example, suppose the worker uses the device to maneuver a large, rigid, and bulky object, such as an exhaust pipe. The device will convey the force to the worker as if it was a light, singlepoint mass. This limits the cross-coupled and centrifugal forces that increase the difficulty of maneuvering a rigid body and can sometimes produce injurious forces on the wrist. In a third example, suppose a worker uses the device to handle a powered torque wrench. The device will decrease and filter the forces transferred from the wrench to the worker’s arm so the worker feels the low-frequency components of the wrench’s vibratory forces instead of the high-frequency components that produce fatigue [33.24]. These assist devices not only filter out unwanted forces on a worker, but can also be programmed to follow a particular trajectory regardless of the exact direction in which the worker attempts to manipulate the device. For example, suppose an autoassembly worker is using an assist device to move a seat to its final destination inside a car. The assist device can bring the seat to its final destination, moving it along a preprogrammed path with a speed that is proportional Part D 33.2
776 Part D Manipulation and Interfaces Fig.33.4 One-handed upper-extremity exoskeleton where Fig.33.3a,b Two-handed upper-extremity exoskeleton a griper allows for grasping of heavy objects [33.21] where artificially built friction forces between the load and the arms allow for grasping objects [33.25] The upper-extremity exoskeleton will significantly re- duce the incidence of back injury in the workplace, to the magnitude of the worker's force on the device.Al- which will in turn greatly decrease the annual cost of though the worker might be paying very little attention to treating back injuries. the final destination of the seat,the device can still bring Upper-extremity exoskeletons were designed based the seat to its proper place without the worker's guid-primarily on compliance control [33.26-29]schemes ance.The upper-extremity exoskeleton reflects on the that relied on the measurement of interaction force worker's arm forces that are limited and much smaller between the human and the machine.Various experi- than the forces needed to maneuver loads.With it,auto- mental systems,including a hydraulic loader designed assembly and warehouse workers can maneuver parts for loading aircrafts and an electric power extender built and boxes with greatly improved dexterity and preci- for two-handed operation,were designed to verify the sion.not to mention a marked decrease in muscle strain. theories(Fig.33.3 and Fig.33.4). 33.3 Intelligent Assist Device The intelligent assist devices (IAD)are the simplest system includes an ergonomic handle,which contains non-anthropomorphic form of the upper-extremity sys- a high-performance sensor for measuring the magnitude tems that augments human capabilities [33.30,31]. of the vertical force exerted on the handle by the operator. Figure 33.5 illustrates an intelligent assist device (IAD). A signal representing the operator force is transmitted At the top of the device,a computer-controlled elec- to a computer controller,which controls the actuator of tric actuator is attached directly to a ceiling,wall,or the IAD.Using the measurement of the operator force an overhead crane and moves a strong wire rope pre- and other calculations,the controller assigns the neces- cisely,and with a controllable speed.Attached to the sary speed to either raise or lower the wire rope to create wire rope is a sensory end-effector where the opera- enough mechanical strength to assist the operator in the tor hand.the IAD.and the load come into contact.The lifting task as required.If the operator pushes upwardly end-effector includes a load interface subsystem and an on the handle,the assist device lifts the load;and if the operator interface subsystem.The load interface sub-operator pushes downward on the handle.the assist de- Part D33.3 system is designed to interface with a variety of loads vice lowers the load.The load moves appropriately so and holding devices.Hooks,suction cups,and grippers that only a small preprogrammed proportion of the load are examples of other connections to the end-effector as force(weight plus acceleration)is supported by the oper- shown in Fig.33.6.In general,to grab complex objects, ator,and the remaining force is provided by the actuator special tooling systems should be made and connected to of the IAD.All of this happens so quickly that the op- the load interface subsystem.The operator interface sub- erator's lifting efforts and the device's lifting efforts are
776 Part D Manipulation and Interfaces a) b) Fig. 33.3a,b Two-handed upper-extremity exoskeleton where artificially built friction forces between the load and the arms allow for grasping objects [33.25] to the magnitude of the worker’s force on the device. Although the worker might be paying very little attention to the final destination of the seat, the device can still bring the seat to its proper place without the worker’s guidance. The upper-extremity exoskeleton reflects on the worker’s arm forces that are limited and much smaller than the forces needed to maneuver loads. With it, autoassembly and warehouse workers can maneuver parts and boxes with greatly improved dexterity and precision, not to mention a marked decrease in muscle strain. Fig. 33.4 One-handed upper-extremity exoskeleton where a griper allows for grasping of heavy objects [33.21] The upper-extremity exoskeleton will significantly reduce the incidence of back injury in the workplace, which will in turn greatly decrease the annual cost of treating back injuries. Upper-extremity exoskeletons were designed based primarily on compliance control [33.26–29] schemes that relied on the measurement of interaction force between the human and the machine. Various experimental systems, including a hydraulic loader designed for loading aircrafts and an electric power extender built for two-handed operation, were designed to verify the theories (Fig. 33.3 and Fig. 33.4). 33.3 Intelligent Assist Device The intelligent assist devices (IAD) are the simplest non-anthropomorphic form of the upper-extremity systems that augments human capabilities [33.30, 31]. Figure 33.5 illustrates an intelligent assist device (IAD). At the top of the device, a computer-controlled electric actuator is attached directly to a ceiling, wall, or an overhead crane and moves a strong wire rope precisely, and with a controllable speed. Attached to the wire rope is a sensory end-effector where the operator hand, the IAD, and the load come into contact. The end-effector includes a load interface subsystem and an operator interface subsystem. The load interface subsystem is designed to interface with a variety of loads and holding devices. Hooks, suction cups, and grippers are examples of other connections to the end-effector as shown in Fig. 33.6. In general, to grab complex objects, special tooling systems should be made and connected to the load interface subsystem. The operator interface subsystem includes an ergonomic handle, which contains a high-performance sensor for measuring the magnitude of the vertical force exerted on the handle by the operator. A signal representing the operator force is transmitted to a computer controller, which controls the actuator of the IAD. Using the measurement of the operator force and other calculations, the controller assigns the necessary speed to either raise or lower the wire rope to create enough mechanical strength to assist the operator in the lifting task as required. If the operator pushes upwardly on the handle, the assist device lifts the load; and if the operator pushes downward on the handle, the assist device lowers the load. The load moves appropriately so that only a small preprogrammed proportion of the load force (weight plus acceleration) is supported by the operator, and the remaining force is provided by the actuator of the IAD. All of this happens so quickly that the operator’s lifting efforts and the device’s lifting efforts are Part D 33.3
Exoskeletons for Human Performance Augmentation 33.3 Intelligent Assist Device 777 intelligent assist device,a worker can manipulate any object in the same natural way that he/she would manip- ulate a lightweight object without any assistance.There are no push buttons,keyboards,switches,or valves to 000 Controller control the motion of the intelligent assist device:the user's natural movements,in conjunction with the device computer,controls the motion of the device and its load. Figure 33.6 shows the end-effector that measures the operator forces at all times even in the presence of load- ing and unloading shock forces.This robust end-effector also includes a dead-man switch,which is installed on End-effector the handle and sends a signal to the controller via a sig- nal cable.If the dead-man switch on the end-effector is not depressed,(i.e.,if the operator is not holding onto the handle of the end-effector),the device will be sus- pended without any motion even if loads are added to or removed from the end-effector. The IAD is engineered with variety of embedded safety features.One of the most important safety charac- Fig.33.5 Intelligent assist device:the simplest form of teristics of the IAD is that the wire rope does not become upper-extremity enhancers for industrial applications.The slack if the end-effector is physically constrained from IAD can follow a worker's high-speed maneuvers very moving downward and the end-effector is pushed down- closely during manipulations without impeding the work- ward by the operator.Slack in the wire rope can have er's motion. far more serious consequences than slowing down the workers at their jobs;the slack line could wrap around synchronized perfectly and the load feels substantially the operator's neck or hand,creating serious or even lighter to the operator.With this load-sharing concept, deadly injuries.The control algorithm in the computer the operator has the sense that he or she is lifting the load,of the IAD,employing the information from various but with far less force than would ordinarily be required.sensors,ensures that the wire rope will never become For example,with a 25kg load force (gravity plus ac-slack [33.32]. celeration),the IAD supports 24kg,while the operator Another form of IAD can be seen in Fig.33.7 where supports and feels only I kg.With the assistance of the a sensory glove measures the force the wearer imposes Part D33.3 Fig.33.6a-c The end-effector(a)contains a sensor (b)that measures the force that the operator applies to the handle (c)in the vertical direction
Exoskeletons for Human Performance Augmentation 33.3 Intelligent Assist Device 777 End-effector Controller Fig. 33.5 Intelligent assist device: the simplest form of upper-extremity enhancers for industrial applications. The IAD can follow a worker’s high-speed maneuvers very closely during manipulations without impeding the worker’s motion. synchronized perfectly and the load feels substantially lighter to the operator. With this load-sharing concept, the operator has the sense that he or she is lifting the load, but with far less force than would ordinarily be required. For example, with a 25 kg load force (gravity plus acceleration), the IAD supports 24 kg, while the operator supports and feels only 1 kg. With the assistance of the a) b) c) Fig. 33.6a–c The end-effector (a) contains a sensor (b) that measures the force that the operator applies to the handle (c) in the vertical direction intelligent assist device, a worker can manipulate any object in the same natural way that he/she would manipulate a lightweight object without any assistance. There are no push buttons, keyboards, switches, or valves to control the motion of the intelligent assist device; the user’s natural movements, in conjunction with the device computer, controls the motion of the device and its load. Figure 33.6 shows the end-effector that measures the operator forces at all times even in the presence of loading and unloading shock forces. This robust end-effector also includes a dead-man switch, which is installed on the handle and sends a signal to the controller via a signal cable. If the dead-man switch on the end-effector is not depressed, (i. e., if the operator is not holding onto the handle of the end-effector), the device will be suspended without any motion even if loads are added to or removed from the end-effector. The IAD is engineered with variety of embedded safety features. One of the most important safety characteristics of the IAD is that the wire rope does not become slack if the end-effector is physically constrained from moving downward and the end-effector is pushed downward by the operator. Slack in the wire rope can have far more serious consequences than slowing down the workers at their jobs; the slack line could wrap around the operator’s neck or hand, creating serious or even deadly injuries. The control algorithm in the computer of the IAD, employing the information from various sensors, ensures that the wire rope will never become slack [33.32]. Another form of IAD can be seen in Fig. 33.7 where a sensory glove measures the force the wearer imposes Part D 33.3