CONTENTS

13.1 Introduction

13.2 Design of Multifunctional Actuator

13.2.1 Motor Design

13.2.2 Clutch/Brake Design

13.3 Analysis of Multifunctional Actuator

13.4 Modeling of Multifunctional Actuator

13.5 Prototype Testing

13.6 Conclusion

Acknowledgments

References

13.1 Introduction

With aging comes various types of physical deterioration, which often affects mobility The muscular strength of older people may decrease and they may be unable to walk or lose their stability during walking Without appropriate exercise and rehabilitation, their muscles will further deteriorate and they may become bedridden. It has been found that exercise training can increase strength and may improve motor activity in people with cerebral palsy (CP) without adverse effects (Damiano et al 2000) It was also demonstrated that exercise increases the strength of affected major muscle groups in stroke survivors (Teixeira-Salmela et al 1999) Therefore, an effective way to relieve these problems and enable older people to fulfill their activities of daily liv-ing is to provide a means for them to be able to continue walking

Assistive knee braces are a species of wearable lower extremity exoskeletons Such assistive equipment can enhance people's strength and provide desired locomotion and have advantages over wheelchairs, which are commonly used for patients with mobility disorders For example, assistive knee braces could help the wearer walk on his or her own legs and therefore exercise the own lower body Moreover, it is possible to use this kind of lower extremity exoskel-eton to assist older or disabled people to improve their mobility in order to solve many daily life problems, such as going up and down stairs and over obstacles

Some research groups have developed several wearable assistive knee braces for walking support. The Berkeley Lower Extremity Exoskeleton (BLEEX) was developed (Kazerooni and Steger 2006) to support a human's walking while carrying a heavy load on his or her back The Hybrid Assistive Limb (HAL; Kawamoto and Sankai 2002) was developed to help people walk, climb stairs, and carry things around The RoboKnee (Pratt et al 2004) provides assistance in climbing stairs and bending the knees while carrying a heavy load The Wearable Walking Helper (WWH; Nakamura, Saito, and Kosuge 2005) and Walking Power Assist Leg (WPAL; F Chen et al 2007) were designed to augment human power during walking based on human-robot interactions. Some companies, including Honda, have also developed assistive walking devices to support bodyweight and reduce the load on the wearer's legs while walking, climbing stairs, and in a semi-crouching position (Honda 2008)

All of the above assistive knee braces use powerful actuation devices to provide adequate supporting torque as well as smooth locomotion To assist the wearer in various postures and prevent knee braces from exceeding the restricted motion, actuators that function as a brake/clutch combined with the ability to safely interlock are desirable Power consumption by the actuation devices is another consideration in lengthening the working time of batteries after they are fully charged Therefore, well-designed actuators would be the key component for assistive knee braces in terms of performance and safety

Generally, actuation devices in assistive knee braces can be classified as active and semi-active actuators The most widely used active actuators are electric DC motors in rotary or linear forms In addition to electric motors, hydraulic and pneumatic actuators are other active actuators used in assis-tive knee braces and exoskeletons

For active actuators, especially for electric DC motors, brake function requires a large amount of power to maintain any posture and might cause safety problems. Some researchers have adopted smart fluids in actuation mechanisms; for instance, a rehabilitative knee orthosis equipped with electrorheological (ER) fluids-based actuators (Nikitczuk, Weinberg, and Mavroidis 2005). An orthopedic active knee brace using a magnetorheological (MR) fluids-based shear damper was developed to make the knee brace have controllable resistance (Ahmadkhanlou, Zite, and Washington 2007). All of the knee braces developed using smart fluids provide controllable torque by passive and semi-active means while consuming little power Furthermore, according to clinical gait analysis (CGA), the knee joint dissipates power during walking (Zoss and Kazerooni 2006) Hence, the knee joint dynamics could be closely matched by a controlled energy-dissipative device; for example, smart fluids-based actuators However, in some situations, such as going upstairs or stepping over obstacles, such knee braces do not help in active ways

To combine the advantages of active and semi-active actuation devices, like electric motors and smart fluids-based actuators, a hybrid assistive knee brace that integrates an MR actuator with an electric motor was developed (J. Z. Chen and Liao 2010). The MR actuator can function as a brake when adjustable torque is preferred and requires low power consumption or it can work as a clutch to transfer torque from the motor to the brace With adaptive control, the actuation system worked well and could provide the desired torque with better safety and energy efficiency. However, the actuator seemed a bit bulky to be used on the human body and could not fulfill the tasks of bidirectional rotation. Therefore, designing a more compact, multifunctional actuator is desired for assistive knee braces

The objective of this research is to develop a novel actuator with multiple functions such as motor, clutch, and brake for assistive knee braces. Utilizing MR fluids, the actuator provides torque safely as a motor and clutch and produces controllable torque with little power consumption for braking. Moreover, the braking torque control of the actuator could be easily implemented

13.2 Design of Multifunctional Actuator

The actuator in this research is comprised of two main components, the motor and the clutch/brake, and each component is associated with cor-responding coils. The motor and clutch/brake are filled with MR fluids.

The motor converts electric power into mechanical power to provide active torque. Utilizing MR fluids, the clutch/brake transfers the torque generated from the motor to the outside as a clutch or provides controllable semi-active torque as a brake Multiple functions can be achieved by applying current on different coils. Figure 13.1 shows a schematic of the multifunctional actuator.

When current is applied to the outer coil of the motor, the induced electromagnetic field drives the rotor to rotate and then provide active torque. If current is applied to both outer and inner coils simultaneously, the MR fluids produce shear stress under the electromagnetic field induced from the inner coil. As a result, the clutch/brake transfers the torque from the motor to the outside as a clutch When current is applied only to the inner coil, the actuator functions as a brake By adjusting the current, the actuator produces controllable torque In this situation, with no current applied on the outer coil, the rotor will not rotate because of the magnetic interaction force between the stator and permanent magnets The advantage of this design is that it can deal with the tradeoff between the brake function and bidirectional rotation

Figure 13.2 shows a person moving through a normal gait cycle and the location of each walking state (Wikenfeld and Herr 2003) According to the gait cycle shown, in the state of stance flexion and extension, the actuator works as brake; in the state of preswing and swing extension, the actuator works as a motor and clutch; in the state of swing flexion, the clutch is off.

Thus, the multifunctional actuator developed meets any motion required of an assistive knee brace.

The configuration of the knee brace that utilizes the multifunctional actuator is shown in Figure 13.3, where the multifunctional actuator provides active and semi-active torque to help the wearer achieve better mobility.

13.3 Analysis of Multifunctional Actuator

The actuator is modeled and analyzed using a finite element method (FEM). A three-dimensional model was built to analyze the influence of permanent magnets on MR fluids and the electromagnetic torque between the stator and the permanent magnets in the motor A two-dimensional model was built to analyze the electromagnetic flux from the inner coil in MR fluids and the yield stress produced in the clutch/brake. Simulations were carried out using ANSYS (Canonsburg, PA, USA).

Figure 13.8 shows the contour plot of the magnetic flux density in the motor when no current is applied on the outer coil The torque between the stator and permanent magnets can be calculated based on this flux density distribution The result of the simulation was about 0 733 Nm This magnetic interaction torque between the stator and permanent magnets can be used to hold the rotor static and plays a role in the operation as a brake

Figure 13.9 shows the distribution of magnetic flux in the rotor. The magnetic flux from the permanent magnets does not enter the inside of the rotor. Therefore, the MR fluids will not be affected by the permanent magnets.

An FEM was also utilized to determine whether the magnetic flux in MR fluids is perpendicular to the input and output plates so that the maximum shear stress could be produced It was illustrated that the maximum stress occurred when the direction of the magnetic field was perpendicular to the shear motion of the MR fluids (Kordonsky et al. 1990). The simulation result is shown in Figure 13.10. The direction of the magnetic flux in MR fluids was along the normal direction of the plates so that the maximum yield shear stress was obtained

The torque generated from the clutch/brake was estimated using an FEM. Figure 13.11 shows the contour of electromagnetic flux density in the clutch/ brake for an input current of 1.5 A. According to the properties of MRF-132DG and its relationship between the flux density and the yield shear stress, the output torque from the clutch/brake can be obtained as 0. 23 Nm using Equation (13.12). It can be found that the output torque is proportional to the current applied on the inner coil This indicates that the clutch torque transferred from the motor and the brake torque provided by the clutch/brake depend on the value of current applied to the inner coil. Therefore, the torque control of the multifunctional actuator in brake function would be straightforward

13.4 Modeling of Multifunctional Actuator

Because the actuator has multiple functions, modeling of the actuator can be illustrated for different functions For motor function, the model is similar to the conventional DC motor. Equations (13.13)-(13.16) are used for the dynamic model of the motor function:

VM=RMIM+LMdIMdt+E (13.13)

E=Ke(fo (13.14)

TM=KtIMw (13.15)

TM=JMd(f)dt+D(f)+TL (13.16)

where V_M is the supply voltage on the outer coil, R_M is the resistance of the outer coil, J_M is the current, L_M is the inductance, E is the back-EMF, co is the angular velocity, T_M is the motor torque, /_M is the moment of inertia, T_L is the load torque including the external load and the friction torque, D is the viscous damping coefficient, and K_e and K_t are EMF constant and torque

constant, respectively The model of the motor function then can be expressed in state-space form as follows:

{x.=Ax+Buy=Cx+Du

where

A={−DJMKiJM−KeLMRMLM}, B=[−1JM0 1LM0], C=[10 D=[00] (13.17)

x={(f)IM}' y=(fΩ,⋅ u={TLVM}

For the brake function where the motor function is off, the model can be derived. According to the properties of MR fluid and its relationship between the flux density and the yield shear stress, the brake torque can be represented as

TB=KHImr+K(iJ(f) (13.18)

where K_H is the coefficient due to the electromagnetic field, and K_m is the coefficient relating to the viscosity. It should be noted that these two coefficients are nonlinear. The dynamic equation for brake function can be expressed as follows:

TL−TB=JLd(j)dt (13.19)

where J_L is the equivalent moment of inertia of the load. The model of the brake function can then be derived in state-space form as

{x.=Ax+Buy=Cx+Du

where

A=[−Keo0JL 00]' B=[1JL0 −KH0JL]' C=[0 −1Keo]' D=[0 −KH],⋅

χ={(f)TB},⋅ y=(f) , u={TLImr} (13.20)

For the clutch function, two cases should be considered. If the current applied on the inner coil is large enough, no slipping occurs between the actuator and load. In this state, the clutch will transfer the exact torque and angular velocity from the motor to the load. Therefore, the model in this case is the same as the motor function in Equation (13.17), provided that

TC=TM≤KHImr (13.21)

On the other hand, if the current is not large enough and cannot transfer synchronous velocity, slipping occurs The model of the clutch function is then the same as the brake function as in Equation (13.20) and the prerequisite is

Tc=KHImr+K(j)(JΩ<TM (13.22)

Control of the multifunctional actuator is easy to implement. When positive power is required, the motor function is on; when negative power is required, the brake function is on; the clutch function works as a switch between these two functions. By adjusting the current on the inner coil, the output torque is controllable

13.5 Prototype Testing

A prototype as shown in Figure 13.12 was fabricated according to the above design and analysis Experiments were conducted to investigate each func-tion of the actuator and torque tracking in the brake function The main specifications of the prototype are given in Table 13.2. The experimental setup is shown in Figure 13.13. A dynamic torque sensor (Model RST-C4A-30-1-A, RSTSensor Inc., Shenzen, China) was utilized to measure the output torque produced by the prototype By changing the payload for motor function, the output torque versus applied stator current and the output torque versus output speed were investigated If the output torque of the payload is kept constant, the rotor is driven by the motor at a constant speed Therefore, by changing the current on the inner coil, the output torque of the brake function was measured In this case, if the current in the inner coil was input as a

step signal, the response of the clutch function was then tested. In the experiments, the signals were processed by the dSPACE system (DS 1104, dSPACE Inc., Paderborn, Germany), which also commands control voltage signal to drive the actuator.

For motor function, the output torque versus applied stator current and the output torque versus output speed are the most important characteristics. Figure 13.14 shows the measured torque at different currents and speeds. The torque/current as well as torque/speed curves are nearly straight lines. Figure 13.15 shows the output power and power efficiency of the motor part. The efficiency was about 74. 3%.

For the brake function, the torque generated from the clutch/brake with an interior inner coil is shown in Figure 13.16. The rotor was rotated at a speed of 600 rpm and step current was applied to the inner coil gradually. With the current augmented from 0 to 2.5 A, the measured torque increased until it reached a maximum value of 0.48 Nm. Although the torque was not sufficient to be applied to the human body directly, it is promising for use in assistive knee braces with a transmission mechanism As shown in Figure 13.16, in the range of 0.5 to 2.5 A, the output torque was approximately linear with respect to the applied current Therefore, the model of the brake function can be expressed as a first-order single-input, single-output linear plant.

The response of the clutch function was investigated. Figure 13.17 shows the response of the clutch/brake under a pulse signal. The response time was about 0 1 second, compared with the reaction time of an average person (0.15-0.4 seconds), so it is capable of stopping the torque transfer from the motor to the assistive knee brace in case of emergency

Motor braking is an issue for motor control The commonly used methods for motor braking consume large amounts of power and can damage the device with prolonged usage For normal walking, the knee power is mainly passive and usually occurs during the bending of the knee joint. As discussed above, with MR fluids, output torque in the brake function of the actuator is approximately proportional to the current applied to the inner coil. By adjusting the current, the braking torque control is easy to implement Experiments on torque tracking of the brake function were conducted, and the results are shown in Figure 13.18. It can be seen that the actuator can track the reference well in the brake function

13.6 Conclusion

In this research, an MR fluids-based multifunctional actuator for assis-tive knee braces was designed To decrease the dimension of the actuation device while enhancing its performance, a motor and MR fluids were integrated into a single device By applying current on different coils on each part, the actuator has multiple functions as a motor, clutch, and brake Design details of the motor and clutch/brake were considered. The motor was designed based on an interior rotor BLPM DC motor, and the clutch/ brake was in the form of input-output plates. Possible combinations of these two parts were discussed Because the actuator was composed of permanent magnets and MR fluids, the magnetic interaction force, influence of the permanent magnet on MR fluids, and magnetic flux distribution were analyzed using FEM. Modeling of the actuator for different functions was developed A prototype was fabricated and tested, and characteristics of each function were investigated Torque tracking in the brake function was investigated Although the measured torque was not high, by adding transmission mechanisms, the torque can be increased while the speed is reduced Therefore, the new multifunctional actuator is promising for assistive knee braces.

Acknowledgments

The work described in this chapter was supported by grants from the Innovation and Technology Commission (Project No. ITS/308/09) and Research Grants Council (Project No. CUHK 414810) of the Hong Kong Special Administrative Region, China

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