There are limited options for paraplegics when it comes to assisted walking devices and those that are available are priced anywhere from $40,000 - $200,000. This high price range means insurance companies are generally reluctant to pay for such devices, making walking more of a luxury than a necessity. The goal for the ParaWalker is to offer paraplegic patients an assistive walker for an affordable price of $10,000. This report overviews the design and progression of the 2022 ParaWalker project. After noticing the lack of affordable assisted walkers, a patent was made for the ParaWalker and has gone through three iterations. The third and latest iteration has implemented knee actuation, whereas previous models have only achieved hip actuation. The addition of knee movement to this device enabled important patient motions such as sitting to standing, and walking. Other design criteria were listed by the project sponsors such as maintaining a sleek and narrow design and improving patient safety by eliminating sharp edges and pinch points. To enable the device to progress from a sitting to standing position required significant redesign by adding a geared knee, driven by a 775 DC motor. An Arduino microcontroller and RoboClaw motor controllers were implemented to execute complex motions with four motors rather than the previous model’s directly wired toggle switches that actuated two motors. To increase the safety of this device, coverings were added to all moving components and gears. Patient harnessing has been improved by using an EXOFIT X300 lineman harness and leg supports rated to hold more than the maximum patient weight of 300 lb. All bolts and other protruding features of this device have been eliminated or redesigned to be enclosed in the device coverings. The overall width of the device has been increased by 0.25 in, maintaining the sleek and narrow design of past models. With the third design of the ParaWalker, many problems were solved from the second iteration such as holding the weight of the patient, sitting to standing, implementation of safety standards, and was built for under $4000. Since the redesign and addition of standards, the ParaWalker is one major step closer to being on the market and helping paraplegics walk again. There are a few things that need to be looked at before this device can be on the market. The carbon fiber backrest needs to be replaced because it allows for too much lateral flexion of the device, relief cuts in the leg members can decrease the weight of the device, an integrated circuit will increase the reliability of the logic circuit, and replacement of potentiometers with Hall effect rotary encoders would increase sensor longevity and consistency, analysis of proper blood circulation when strapped into the harness is needed for additional patient safety.

Project Scope

This paper documents the design and fabrication of an automated ParaWalker for Eric Skoog, and his partners, Tom Benton, and Lonny Head. Eric Skoog is a retired nurse who worked at the VA hospital for over 20 years. During his time as a practicing nurse, he treated many paraplegic patients which inspired him to create a device to enable his patients to walk again. The client has tasked the 2021-2022 senior engineering design team with enabling the patient to move from a sitting to a standing position, walk on level ground, and increase safety by eliminating safety hazards such as pinch points, sharp edges, extended bolts, and improper patient harnessing.

Technical Review

Industry Background

Wearable devices that assist physical movement have been around for centuries. The earliest model that involved human usage was in 1890 when Nicholas Yagn of St. Petersburg, Russia, patented a device called an “Apparatus for Facilitating Walking” [1]. The device that was worn on the legs was developed in two iterations being powered either with a bow spring or a compressed gas bag [2], since that point in time there have been models adapting the same idea with the modern technology of their time. As of August 2018, well over 100 commercial and research exoskeleton projects have been in development [2]. The industries that benefit from the integration of exoskeletons are the military, consumer, industrial, and medical. This project focuses on bringing this product to both the consumer and medical industries. The primary users are clinically diagnosed with paraplegia or lesser leg functionality. Paraplegia is the loss of muscle function in the lower half of the body, including but not limited to both legs [3]. The name of this project is designated as a “ParaWalker,” intended to represent a walker for paraplegics.

Three models currently on the market are SuitX’s Phoenix, ReWalk’s ReWalk Personal Exoskeleton, EksoBionic’s Exoskeleton. While there are more products, this paper will use three to compare to the project. All the models currently on the market allow paraplegics to walk, this device will compete within the market in a unique niche by being more affordable. The Pheonix is the cheapest model with a price of $40,000 going up to $71,600 and $100,000+ for ReWalk’s and EksoBionic’s, respectively [4][5]. Current models utilize motors that drive pulley systems or directly drive the shaft, both of which accomplish successful mobility. However, each company owns the intellectual rights to designs causing concern to avoid copyright and maintain original design. The project’s design uses pinion-bevel gears to keep the device within the patent attained by the client previously.

Client Background and Current Process

Eric Skoog is a nursing professor at CMU, retired nurse, and inventor. From his 20 years working with veterans at a VA hospital, he developed a passion to help veterans walk again. Eric’s idea is to make an affordable and slimmer exoskeleton that can fit under clothes, something that current market models do not fulfill. Moving forward he obtained a patent on an exoskeleton in 2011 with Patent NO. US 7998096 [6] and two iterations have been based around the patent (Figure 1).

The project has gone through two iterations of designs from 2012-2013 and 2013-2014. The 2012-2013 ParaWalker consisted of a ridged hip panel and two leg bars (Figure 4). The leg bars were actuated at the hip using DC motors and were used in conjunction with a walker. The lack of bending leg members did not permit the patient to go from a sitting position to a standing position. The 2013-2014 model maintained the same rigid leg structure with lockable knee joints enabling the user to sit while in the device. However, there was still no transition from a sit to a stand position and vice versa. The second iteration (Figure 5) moved from a walker to crutches with rocker switches to directly control motor movement. Unfortunately, the switches caused overshoot while controlling leg position. Each time an overshoot occurred, the error in movement would propagate making the finite movements of sit to stand unobtainable. This model did develop a practical hip joint and backrest to support the lower torso of the user. The current ParaWalker also presented safety concerns such as sharp edges of metal, exposed wiring, exposed gears, and a poorly secured harness.

The 2021-2022 iteration of the project incorporates the same backrest, hip joint, bevel miter gear interface, motors, and crutches developed by the 2013-2014 senior design team.

Design requirements/ Criteria

The project deliverable is a functioning prototype for a third iteration ParaWalker that meets the following requirements.

• Enable the user to stand from a sitting position and sit from a standing position.

• Allows the user to walk forward in a straight line on level ground for one minute at a minimum rate of 30 steps per minute.

• Fits a range of patient heights from 5 ft 8 in to 6 ft 2 in

• Does not have safety hazards: no pinch points per OSHA standard 01-12-002 [7], no edges sharper than a 0.005 in radius, no exposed gears, and no bolts protruding more than 0.25 in.

Overview

The ParaWalker consists of an electrical, structural, and power transfer system. The electrical system governs the movement of the device based on user input. The structural system provides structural support and rotational motion in the hip, knee, and ankle joints. The power transfer system enables the electrical system to actuate the leg members.

Detailed Design Description

Leg members

• Integration to existing hip assembly

• Hip actuation was achieved in the previous model. This assembly will remain the same and the leg members were redesigned.

• Figure 6 pictures the whole assembly of the left leg

• Figures 7 - 12 show various parts of the left leg of the Parawalker

  • Part numbers called out in the figures below pertain to the user manual of this device and will not be discussed on this webpage

Crutches

• Hand control mounting to crutch depicted in Figure 13

• The button mount is a nonstructural component that is meant to house the buttons for user input, LED indicator lights, and a piezoelectric buzzer. The mounting for the hand control buttons is slim adding 1 inch to the handle's width. All edges on the part have a fillet of 0.01 inches or greater to meet the no sharp edges requirement. Using two screws the part can be mounted to the crutch and the screws are cut and rounded to meet the requirement of no sharp edges. This part was 3D printed on Stratasys Fortus 250 using ABS.

Electrical system

The electrical system will start with a basic block diagram that will include the sub-bullets to be talked about more in-depth (See block diagram for the project in Appendix A, Figure A1).

• Microcontroller

The logic system is controlled by an Elegoo Mega 2560 that is based on a 32-bit ARM core microcontroller. The board is identical to the Arduino Mega so the pinout diagram for the Arduino is referenced in Appendix A, Figure A2. The microcontroller has 54 digital input/output pins and 12 analog pins to receive sensor readings and communicate with the systems motor controllers via simple serial. The Arduino Mega can only execute one task at any one time therefore it sends instructions to each microcontroller while it monitors user input and requests position checks from each motor controller.

• Motor controllers

RoboClaw made by BasicMicro is rated for 30 Amps and 34 VDC (Figure 14). The motor controllers can operate independently of the microcontroller by running their own PID loop after receiving simple serial instructions from the microcontroller. These instructions contain acceleration, deceleration, speed, and final position. The current position is read from potentiometers in the hip and knee joint and compared to the final position instruction until the error between the two is within an acceptable range of plus/ minus two.

• User interface (HMI) for system input

Two hand control assemblies are located on each crutch. Each assembly consists of 4 tactile push buttons of four different distinguishing colors: yellow, green, red, and black. Table 1 overviews the color of each button and what the program executes when each input is used.

• User feedback

A total of 3 RGB LEDs are used on the system (Figure 14). One RGB LED will provide feedback to the user on high to low battery life by transitioning from green to red. The other two will be in either hand control to blink in different patterns each time a movement is to be executed by the system. A piezoelectric beeper provides a sequence of beeps to accompany each blinking LED in the hand controls. The combination of both components will alert the user to what the microcontroller is executing after receiving their input.

• Battery and backup power system (wiring diagram/ pic of components)

A backup battery system (Figure 16) is in place to ensure a constant voltage to the system in the event the two main batteries fail (G1 and G2). A solid-state relay (T1) is held open by the two main batteries and switches over to a closed position when the batteries reach 3-4 VDC. In the closed position a singular backup battery (G3) will energize the system maintaining a constant voltage. An emergency shutoff is implemented by a SPDT switch that cuts all battery power to the system. The switch will also provide safety before startup by cutting power to the system while the user puts the device on. A total of three Lithium-ion batteries are used to power the system and its circuitry. The batteries are 5 Ah wired in parallel providing 10 Ah for the system. The dimensions of the batteries are length 3.54 inches, width 2.76 inches, and height 3.98 inches.

Power Transfer

• Bevel Miter gear on hip and knee

The mechanical power transfer using gears is stated within the patent held by the client. This design is kept the same as the previous design team for hip actuation. The same design was used for enabling knee actuation and can be seen in Figure 17.

Gearbox

• Reduction of RPM from motor and increase in torque through mechanical advantage (Figure 18). The gearbox provides a reduction of 256:1 for the motor to slow down the RPM of the motors. They can handle a max torque of 60 ft-lbs.

Motor

• There are a total of four motors each located on the right and left hip and knee joints (Figure 19). All motors fall under the classification of a 775-motor capable of operating at 6 – 20 VDC. The motors have a speed of 17350 RPM and a current of 2.6 Amps at no load. The stall current is 102 Amps at a torque of 111.18 oz-in with a peak efficiency of 15047 RPM and 17A.

Use

The user will apply the device to themself and operate it independently. The user will transfer themself to the ParaWalker from a wheelchair in a seated position and apply the safety straps on the legs and upper body. Once all safety straps have been applied properly, the user will decompress the e-stop allowing the motors to be powered. The user will press a button on the crutches to send a signal to the logic controller to initiate the stand function. After the button is pressed the user will receive feedback in the form of a flashing light on the crutches to indicate the stand function will be executed. A three second delay will take place between the user pressing the stand button and the execution of the stand function to allow the user to cancel the function if needed. Once standing the user will have the option to use the button panel to execute the walk function. Once the walk function is initiated, the user will receive feedback in the form of a flashing light to indicate the walk function was initiated. After initiating the walk function, the user will press a button to take a step with the left or right leg. After each step, the ParaWalker will wait for the next user input such as, step with opposite leg or sit down. When the user would like to get out of the ParaWalker and return to their wheelchair, a chair or bench must be placed approximately eight inches behind the user. The user will then press the sit button. Again, there will be a flashing light and a three second delay indicating to the user the sit function will be executed. Once the user is seated, press the e-stop button to disconnect power to the motors and remove the safety straps. The user is now able to transfer themself back to the wheelchair.

Overview

The design requirements discussed with the client in Section 1.4 Design Requirements/ Criteria were tested through user input, experimental testing for walking rates and sit to stand, having different users try the ParaWalker and report on comfort. There are multiple systems, components, and design criterion to discuss. Each section is labeled with the portion of the design being evaluated and each evaluation consists of a purpose statement, test methodology, and results and discussion. The design evaluation section of this report is relevant to following design teams for this project because, next steps and recommendations for improvements are discussed after each part is evaluated.

Evaluation

Logic Controls

Purpose

The logic system was evaluated for repeatability and accuracy of movement. The purpose of testing is to ensure each instruction sent to the motor controllers from the microcontroller is executed within a tolerance of +- 0.5°. If there is a large error in movement then the error will compound on itself as the system executes more commands. The logic system must be able to boot up from any position and read the degree of the leg correctly 15 times. This test is important because there are emergency shutoff buttons to ensure user safety, when they are engaged the system will lose power. Once the user restarts the ParaWalker, the Arduino needs to know the position of the legs instead of making a new zero point. In any scenario, sitting or standing, the device must be able to read what degree of rotation each leg member is at before it executes any movement commands.

Test methods

The Para-Walker is loaded onto the testing stand without an occupant. Each joint is commanded to move to a predetermined position it will come to rest and be checked by a goniometer. For position and potentiometer reference values see Table 2 and Figure 20. Positions are all measured from the front of the walker refer to proximal body member for positioning. The movement must reach each position three times with an accuracy of +- 0.5° (Table 1). At 15 intermediary positions, the position readout was recorded, and the system's power was cut and remained off for 2 minutes. Once the time has passed the system is powered back on and the position readout is recorded and compared to the readings before shutdown. The error is the difference between the readout before shutoff and after bootup. The error is allowed to be plus or minus one (unitless quantity) (Table 1).

Results and Discussion

The results of this experiment were successful when each joint was tested individually. There were more complications with accuracy when two joints on one leg were tested in the same manner. Some of the complications that occurred was a need for new PID constants now that the system changed from one joint movement to two. There was also a problem in reading position values in the knee because the encoder that is enclosed in the joint became damaged causing the value readouts to jump upwards of 200 from the actual position and return when the leg was no longer in the damaged portion. Another problem occurred when there was side to side deflection of the knee which would change the compression of the potentiometer’s wiper causing the values to jump up and down by 10-20. These values recorded by the motor controllers have no units but are used in a proportional manner to the degrees of rotation at each joint.

Knee Joint

Purpose

The knee joint falls under several different design requirements such as being adjustable to a range of patients, eliminating all pinch points and exposed gears, and allowing the patient to sit and stand. The knee assembly must support the patient’s weight while also keeping the overall width of the leg members to a minimum. The knee joint was primarily designed using Solidworks software to simulate the range of motion and strength requirements. Strength requirements were also calculated by hand for verification of CAD model estimations. For the range of motion verification, component orientation on the knee, and design of the adjustment rods, a cardboard model was assembled for visual aid.

Test methods

To verify the design is structurally safe for patient use, a 200 lb mass was suspended from the patient harness with the legs fully extended. The range of motion was tested by moving the knee joint to a maximum of 120⁰ in flexion and back to full extension. The motion was repeated 5 times with a variation of less than 1⁰. Positions of the knee joint were measured using the SoftPot encoder installed into the knee assembly. Pinch points were not removed from the knee joint assembly; however, a gear cover was designed to prevent objects from being caught inside the gears.

Results

No deflection was observed in the knee joint resulting in a successful test result when loading 200 lbs in the harness.

Hand Controls

Purpose

The hand controls were designed for comfort and ease of use. The hand controls must be accessible to the patient when holding the crutches while also maintaining control of the crutches. The design must also allow for four buttons per side.

Test Methods

Hand controls were tested for comfort and functionality. The controls must meet safety requirements of no sharp edges or corners. They were tests were given a pass/fail from each user. The test used was to have different people try to use the crutch with the control on it and give the control a pass/fail rating. The rating was whether they would use the product. This test requires at least 20 people to test the product.

Results

The client wanted a product that users would like. The results from this test confirm that the hand controls are comfortable for the person using the ParaWalker. After 20 people used the controls and gave feedback on the hand controls, they recommended that the controls be smaller for smaller hands. They also said controls don’t allow for a full grip of the crutch handle, which leads to slight discomfort.

Conclusions

The new design meets the customer requirements as stated above. Through testing, the ParaWalker was able to pass most tests. The overall design works great in its current state, however, there is more testing and design to be performed before this prototype is finished. A few of the systems need redesigns such as the logic control boards and backrest.

Next Steps

The logic system meets requirements but has a limited life due to the potentiometers in the hips and knees. The potentiometers are not solid-state; therefore, they wear with every movement and will eventually need to be replaced. A better solution for the encoders is to use hollow shaft encoders that utilize lens law properties to readout rotational position. Hollow shaft encoders are used in many industrial applications and are robust enough to last the lifetime of the walker. Some design changes are required if hollow shaft encoders are utilized, and the overall width of the Para-Walker will increase as a result.

The structural leg members meet functionality requirements but are exceptionally heavy and could be reduced with material removal. Creating relief cuts to leg members in areas of low stress can reduce the overall weight of the Para-Walker. Leg members that may need relief cuts include but are not limited to PN 001, 002, 005a, 009, 010, 014, and 015.

The hip joint was inherited from previous design teams and was not evaluated further for a 300lb patient. The current steel member that the leg assembly connects to the hip would fail before any other structural member within the leg due to poor geometry and limited cross-section of the material. A solution to this problem would be to follow the same design considerations taken for the knee and thicken the member or select better materials such as 7075 Aluminum.

The carbon fiber backrest was also kept from the previous design and after testing began the back rest had major deflection and twist up to 1 inch. Since the backrest is carbon fiber it's also not supposed to be drilled into resulting in splintering and structural weakness in the rest. A new backrest made from aluminum or carbon fiber with a Twill 2 x 2 weave and aluminum, or steel structure would fix the deflection issue.

[1] N. Yagn, Apparatus for facilitating walking, US420179A, 1890. Google Scholar

[2] Arun Jayaraman, Borislav Marinov, Yashna Singh, Sheila Burt, William Zev Rymer, Chapter 15 - Current Evidence for Use of Robotic Exoskeletons in Rehabilitation, Wearable Robotics, Academic Press, 2020, Pages 301-310, ISBN 9780128146590, https://doi.org/10.1016/B978-0- 12-814659-0.00015-1.

[3] Mayo Clinic Staff. “Spinal Cord Injury.” Mayo Clinic, Mayo Foundation for Medical Education and Research, 2 Oct. 2021, https://www.mayoclinic.org/diseases-conditions/spinal-cordinjury/symptoms-causes/syc20377890?utm_source=Google&utm_medium=abstract&utm_content=Paraplegia&utm_campaig n=Knowledge-panel.

[4] Brewster, Signe. “This $40,000 Robotic Exoskeleton Lets the Paralyzed Walk.” MIT Technology Review, MIT Technology Review, 2 Apr. 2020, https://www.technologyreview.com/2016/02/01/163493/this-40000-robotic-exoskeleton-letsthe-paralyzed-walk/.

[5] J. Murtagh, “ReWalk: Robotic Exoskeletons for Spinal Cord Injury,” CADTH issues in emerging health technologies, no. 141, Sep. 2015. ISSN: 1488-6324

[6] Eric Skoog, 2011, “Paraplegic controlled, concealed mechanized walking device” United States, Patent No. US 7998096.

[7] Occupational Safety and Health Administration. “Guidelines for Robotics Safety” U.S. Department of Labor, Occupational Safety and Health Administration (Standard No.01-12-002). OSHA, 1987. Retrieved from https://www.osha.gov/enforcement/directives/std-01-12-002