Design of a Continuous Passive and Active Motion ...

Design of a Continuous Passive and Active Motion Device for Hand Rehabilitation B. Birch*, E. Haslam, I. Heerah, N. Dechev, Member, IEEE, and E. J. Park, Member, IEEE 

Abstract—This paper presents the design of a novel, portable device for hand rehabilitation. The device provides for CPM (continuous passive motion) and CAM (continuous active motion) hand rehabilitation for patients recovering from damage such as flexor tendon repair and strokes. The device is capable of flexing/extending the MCP (metacarpophalangeal) and PIP (proximal interphalangeal) joints through a range of motion of 0° to 90° for both the joints independently. In this way, typical hand rehabilitation motions such as intrinsic plus, intrinsic minus, and a fist can be achieved without the need of any splints or attachments. The CPM mode is broken into two subgroups. The first mode is the use of preset waypoints for the device to cycle through. The second mode involves motion from a starting position to a final position, but senses the torque from the user during the cycle. Therefore the user can control the ROM by resisting when they are at the end of the desired motion. During the CPM modes the device utilizes a minimum jerk trajectory model under PD control, moving smoothly and accurately between preselected positions. CAM is the final mode where the device will actively resist the movement of the user. The user moves from a start to end position while the device produces a torque to resist the motion. This active resistance motion is a unique ability designed to mimic the benefits of a human therapist. Another unique feature of the device is its ability to independently act on both the MCP and PIP joints. The feedback sensing built into the device makes it capable of offering a wide and flexible range of rehabilitation programs for the hand.

I. INTRODUCTION HIS paper presents a new hand rehabilitation device capable of both continuous passive motion (CPM) and continuous active motion (CAM) of the metacarpophalangeal (MCP) and proximal interphalangeal (PIP) joints in the human hand. Although the usefulness of CPM is debated, it is still widely used as a recovery and preventative tool [1]-[8]. As a rehabilitation aid, CPM is used for recovery from surgery (such as flexor tendon repair), stroke, hemiplegic hand, intra-articular adhesions, and extra-articular contractures. Generally, CPM is most beneficial in the early stages after a surgery. In the later stages of recovery, active resistance motion (i.e. CAM) becomes more important. When the patient has regained their range of motion there is often still a need for strengthening of the muscles in the hand. The addition of the active resistance mode allows for strengthening of the patient’ s muscles in a safe and controlled manner. The torque supplied by the device

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Department of Mechanical Engineering, University of Victoria, PO Box 3055 STN CSN, Victoria, BC, Canada, V8W 3P6 (* corresponding author – phone: (250)721-6292, e-mail: [email protected]).

is controlled to allow the user to progress at the required pace through both CPM and CAM exercises. Both the active and passive motions can be performed over the range of 0° to 90° at the MCP and PIP joints, and the device can be configured to accommodate either hand. Since the MCP and PIP are independently controlled, the device is capable of performing the intrinsic minus, intrinsic plus and fist motions (see in Fig. 1) without the need for any changes to the device or splints. This enables easy changes between the different exercise programs, and allows for fluid MCP and PIP joint motions. This paper discusses the state-of-the-art of current hand rehabilitation devices in the market or under research and development (R&D), and presents the design and functionality of the proposed CPM/CAM device. Background There are currently a number of different hand rehabilitation devices on the market or under R&D [9]-[13]. Many of these devices provide CPM-only and employ a single motor to drive a linkage that attaches to the fingertips. The motor powers the hand to close in a fist-like motion. The use of additional splints for either the MCP or the PIP joint allows these devices to achieve the intrinsic minus and intrinsic plus motions as well –however, the addition of these splints is a time-consuming process. Additionally, these devices are strictly passive in that they take no feedback information from the user’ s hand during operation. There are a wider range of rehabilitation devices available for the larger joints such as the knee. For example, Ho et al. [14] developed a hybrid CPM/CAM knee device that was capable of supplying active resistance to a user by modifying an existing CPM device. Presently most active hand rehabilitation exercises are done with putty, resistance webs, squeeze balls, grip strengtheners and other purely mechanical devices. Noteworthy ‘ active’ devices are the dorsal-mounted CyberGrasp [15] and the palm-mounted Rutgers Master II-ND haptic glove [16], which integrates ‘ sensing gloves’ for force feedback. The CyberGrasp is designed specifically as a haptic interface for the hand, whereas the Rutgers Master II-ND is designed for hand rehabilitation. It is capable of exercising the thumb and first three fingers separately; however, it cannot achieve a fist form, nor does it have the independent control of the MCP and PIP joints. There is currently a need for a cost-effective device for hand rehabilitation clinics that can flex/extend the MCP and PIP joints independently without external attachments (to reduce cost and setup time) and that can perform both CPM and CAM exercises. II. MINIMUM JERK TRAJECTORY MODEL Both the warm-up and CPM modes (described in Sec. V)

attempt to move the device such that it follows a joint trajectory path that closely approximates natural human hand motion. To achieve this, the natural human hand’ s joint motions were measured. Each of three healthy volunteers was outfitted with six LED markers recognized by a Visualeyez optical motion tracking system (from PhoeniX Technologies Inc.). The markers were placed on the back of the volunteer’ s middle finger as follows: two on the back of the hand, two on the finger between the MCP and PIP and two on the finger between the PIP and DIP. This way each group of two sensors could be used to determine the vector corresponding to that bone or part of the hand and thus the MCP and PIP angles can be calculated. The volunteers were then asked to perform a series of four hand motions typically used during hand rehabilitation, five repetitions for each motion. The data from these tests was compiled and the joint angles of the PIP and MCP were calculated for each time step. This produced the angles for the MCP and the PIP. Data for five consecutive time steps was averaged to account for minor fluctuations in the data. Next, the test data was compared to the minimum Jerk model [17] in order to determine if the model can approximate human hand motion accurately. Jerk, which is the time derivative of acceleration, is a universally accepted quantity of evaluating motor smoothness of human limbs. If we apply minimum jerk theory to the motion of the natural finger joints, each joint should move smoothly from one position to another following a joint trajectory that minimizes the sum of the squared jerk, i.e. 1 tf & & & (1) minimize J   (t ) 2 dt 2 ti & & & where  (t ) is the jerk of the joint trajectory (t ) . If we set the initial and final resting joint positions as θ i and θ f, respectively, and if the joint moves between these two positions in time D (= tf –ti), the minimum jerk trajectory is obtained: 3 4 5  t  t  t  (2) (t )=i (f i )  10 15 D  6 D         D  

representative case, shows that the natural PIP flexion motion indeed follows a very similar trajectory to that of the minimum jerk model (black). Hence, the model was deemed acceptable and subsequently implemented in the proposed device in guiding the passive motions (see Sec. V). The model is adaptable as it can be tailored to match the needs of the patient (i.e. per-user bases) by adjusting the maximum ROM and the duration of the motion. III. MECHANICAL DESIGN The proposed hand rehabilitation device, shown in Fig. 2, is designed as a table-top machine with independent control of the MCP and PIP joints. During use, the patient places his/her hand within the frame of the device so that their MCP is lined up with the axes of rotation of the first motor. The proximal segment of the fingers between the MCP and the PIP joints rests on the first rotating link (proximal). The extension is slid until the PIP joint is aligned with the axes of rotation of the second motor and the remaining segments (i.e. distal and middle) of the finger rest on the second rotating link (distal). This can be seen in Fig. 3. The distance between the center of rotation for the MCP and PIP joints can be changed by sliding the extension (see Fig. 3) to the desired position. Note that reconfigurable stainless steel pins can be adjusted in position, to adapt to various hand sizes. Also, the stainless pins are padded (not shown) to comfortably contact the hand. The pins between the MCP and PIP lock the extension in place so that it cannot slide in and out once the patient’ s hand is set.

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(a)

(b)

(d)

(c)

Figure 2. Prototype CPM/CAM hand rehab device (padding over pins removed for clarity): (a) initial position, (b) intrinsic plus at MCP, (c) fist, (d) intrinsic minus at PIP.

Figure 1. Minimum jerk trajectory model compared to user data in the case of PIP joint.

Next, the above minimum jerk trajectory model was compared to the Visualeyez data by inputting the average duration and average maximum joint angle from the users into Eq. (2) and plotting the results. Fig. 1, which is a

A schematic diagram of the device is illustrated in Fig. 3. The joints are controlled by separate MicroMo DC motors with 246:1 planetary gear heads. The first motor is rigidly constrained to the frame of the device and a coupling connects the motor shaft to the first rotating link. There is a large disk shaped guard attached to the first link, which separates the user from the mechanisms inside the device. The guard rotates with the first link. The motor for the PIP joint is suspended from an extension

(Item D, Fig. 3) by posts. The motor shaft is connected by a coupling to the second rotating link which is constrained in a similar way as the first link, with two thrust bearings and a needle roller bearing. The design is such that the first link, guard, extension and second motor all rotate together about the first motor. Only the second link rotates about the second motor.

A. C. E. G. I. K. M. O.

MicroMo 12V DC motor B. Oldham coupling Frame D. Extension First rotating link F. Second rotating link Safety stop H. Double ball bearings Safety sensor J. Lock nut Needle roller bearing L. Needle thrust bearing Pins N. Force sensor mounting Corner post P. Guard Figure 3. Schematic overview of the hand rehab prototype.

This design allows the device to operate the MCP and the PIP independently, enabling intrinsic minus, intrinsic plus and fist motions (seen in Fig. 2) without attachments. The design has a range of -90° to +90° (to accommodate either hand) for both MCP and PIP. The maximum torque for the device is 4.5 Nm, which is the maximum torque rating on the motor’ s gearhead. The FlexiForce sensors (from Tekscan), which is described in the subsequent section, can read up to 6.74 Nm. Standard rehabilitation grip mechanisms (such as Digi-grip) offer a maximum of 9 lbs for rehabilitation. When this is converted to a torque on the MCP it corresponds to approximately 2 Nm. Therefore the torque range offered by the proposed device should be a sufficient for both the active and passive exercises. The complete device is enclosed in a Plexiglas case where the three input toggles, the on-off toggle and the emergency stop switch are mounted, as shown in Fig. 2. There are two LCD panels which display same information on opposite sides of the device for the patient and the operator. IV. SENSORS AND INTERFACE Each motor has two FlexiForce sensors which can sense the torque acting on the motor case (with respect to the motor shaft). This is done by mounting the motors within double ball bearing sets, thereby constraining them in all directions except rotation about their shafts. The reaction force developed by the motor case presses against FlexiForce sensor, and hence

the torque can be determined. The FlexiForce sensors are piezoresistive sensors that decrease in resistance with the force applied to them. The motor current is also measured to obtain an independent torque estimate. When combined with the position measurement from an integrated magnetic encoder, the position and torque of each motor can be monitored and used to control the device. A microcontroller running C code controls the device. The user interface consists of three rocker switches and a LCD display. Entering values such as a time or a desired torque is accomplished by using one switch for enter and back, one for selecting the digit and one to increment and decrement the number. The selection of the starting and ending points for an exercise is handled by a ‘ motion teaching’method. This method involves specifying a starting position, by means of a button controlling each direction on the two motors, until the hand is in the desired position. Then the user presses enter and the encoder value for that position is stored in memory as the first position. The end positions are handled similarly. In this way the range of motion (ROM) is clearly apparent to both the therapist and the patient. There should be no concern that the position may have been entered incorrectly as the ROM has been visually confirmed. This ensures the machine operates with the correct hand and does not bend the wrong direction. Additional onboard safety features ensure that the mechanism is operating for the correct hand. A mechanical stop is placed in the guard, stopping the first and second rotating links from going beyond a neutral position (i.e. hyper extending the user’ s hand). This safety stop must be in place for the machine to operate and there is an optical switch to ensure it is being used and to determine which side it is on so that motors do not drive into it. Thus there are multiple safety features in place to ensure the user is not harmed. V. INTENDED MODES OF OPERATION The device functions through three main modes which are: warm-up, CPM, and CAM. These modes were chosen based on both an assessment of current established rehabilitation techniques and the advice of hand therapists concerning what they deem most important for future products. A. Warm-Up The warm-up mode is a version of the CPM mode where the device performs an ever increasing ROM. The user selects a start and end point for the ROM, duration for the exercise, maximum torque, and a time for a single cycle (used to determine the motion speed). The device will then travel to an increasing proportion of the selected ROM. It starts at 50% of the full ROM and by the last cycle travels 100%. The motion will follow the minimum jerk trajectory model, discussed in Sec. III. B. CPM This mode passively (no force from the user) move the user’ s hand through a preset ROM following a minimum jerk trajectory model. There are two major differences between this and the warm-up mode. The first is that the entire duration of the exercise will run at 100% of the selected ROM. The

second difference is that the motion is not simply between a single start and end point. The ROM used in this mode is set when the user chooses a series of target positions that the machine will cycle through sequentially. The device will move in order through the chosen positions following the minimum jerk path. If the user picks multiple points, for example four points, then the device will move from one to two, from two to three, from three to four, and then from four back to one again. In this way the different motions can be incorporated into one fluid motion. The user could move from a neutral position, to an intrinsic minus position, to a closed fist, to an intrinsic plus position and then back to neutral. This will allow for utilizing the full capabilities of the two-motor design. C. CAM The active motion mode of the device will create a predetermined resistance to motion against the user’ s hand. The user sets the start and end points of the exercise and the desired resistance as a torque value. Then the user attempts to move his/her hand from the start to the end point with the device displaying a count of the number of repetitions achieved. During the motion, the sensors will measure the torque produced by the user and compare it to the desired torque, thereby controlling the resistance. This will enable the device to produce a steady torque to resist the movement of the user from the start to the end positions. The torque will be scaled up from zero to the full value over the first small deviation from the start position (10° or so) so as not to force the user back further than the start point, and to ensure that a large jerk is not applied when the user first crosses the start point. This mode is a new approach and is designed to enable the user access to a technique which usually requires a human therapist to apply resistance or a more limited mechanical ball, grip strengthener, or web. The device is capable of resisting both the closing or opening of the hand. This allows access to eccentric motion exercises.

of control over the motions and forces applied to a user. This allows for both CPM and CAM to be performed on the MCP and PIP separately. This ability to isolate either the MCP or the PIP without any physical additions or alterations (splints to inhibit one to the joints) to the device is an improvement over existing designs. The next step for this research is for clinical evaluation of the device, with rehabilitation professionals. ACKNOWLEDGMENT We would like to thank Clare Faukner and her therapists at the Island Hand Therapy Clinic for their valuable comments and suggestions. REFERENCES [1]

[2]

[3] [4] [5] [6] [7] [8] [9] [10]

Pins

[11] Guard

[12]

Rotating Links Electronics LCD Display Toggles

Motor 1 (MCP)

[13] [14] [15] [16]

Motor 2 (PIP) [17] Figure 4. Close-up picture of the hand rehab prototype.

VI. CONCLUSION A new hand rehabilitation device capable of both CPM and CAM for the human hand has been presented. The mechanical and electrical design of the device allows for a greater amount

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