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Project 5: Gait Training Robotic Assist Device Investigators:
David Reinkensmeyer, Ph.D. Summary: This project is to develop a robotic system that can be used by therapists to automate labor intensive aspects of gait therapy. This includes a robotic device for measuring and manipulating pelvic motion during step training on a treadmill and a novel robotic device for measuring and manipulating leg trajectories during step training on a treadmill. "PAM" (Pelvic Assist Manipulator) uses two, three-degree-of-freedom, pneumatic robots to measure and control the pelvis of a person during body weight supported stepping on a treadmill. The device can be used in a passive mode to record pelvic trajectories, either specified manually by a therapist or pre-recorded from unimpaired subjects, then replay these trajectories using a non-linear force control algorithm. Data are presented that demonstrate the ability of the device to record and replay the pelvic motions that occur during normal walking. Background Information: Contrary to what many believed to be a strictly long-term, if not permanent, loss of ability, recent clinical evidence has shown a significant correlation between the regular application of intense physical therapy and the recuperation of functional levels of locomotion in neurologically impaired individuals [1,2]. The mammalian spinal cord has a remarkable capacity to learn, and thus that it may be possible to teach individuals with spinal cord injury to step with appropriate sensory motor training [1]. The key characteristics of this training are thought to be partial body weight support (BWS), and assistance of torso and leg movements during stepping on a treadmill. It is hypothesized that the spinal cord can functionally reorganize itself in response to specific and repetitive patterns of proprioceptive input repetitively provided to it during training. Such locomotor training is currently labor intensive, requiring up to four trainers to assist in leg and pelvic motion and to operate the treadmill and BWS system. Automation makes the precision and reproducibility of movements superior to any attempts made by human hands, making the therapy sessions more accurate and efficient. Recognizing the potential benefits of automating the training, several groups are developing robotic devices that can assist in leg movement. The Lokomat consists of four rotary joints, driven by precision ball screws that are connected to DC motors, which are mounted onto a motorized exoskeleton that manipulate a patient's legs in gait-like trajectories [2]. The Mechanized Gait Trainer (MGT) is comprised of two foot plates connected to a double crank and rocker system that is singly actuated by an induction motor via a planetary gear system and drives a patient's legs in a walking pattern [3]. ARTHuR makes use of a linear motor and a two DOF mechanism to measure and manipulate leg movement during stepping with good backdriveability and force control [4]. Other devices under development include HealthSouth's Autoambulator, and a more sophisticated version of the MGT that can move the footplates along arbitrary three degree-of-freedom (DOF) trajectories. These initial gait-training devices have focused primarily on controlling leg movement.
However, pelvic motion also plays an important role in normal locomotion. During
This paper describes the development of a robotic device that can measure and manipulate naturalistic motions of the pelvis. A key design goal was to create a device that exhibited good backdriveability, defined as low intrinsic endpoint mechanical impedance [7], or accurate reproduction at the input end of a mechanical transmission of a force or motion that is applied at the output end [8]. Good backdriveability offers several important benefits for robotic therapy devices [4], including the ability for the device to act as a passive motion capture device. In such a passive motion capture mode, the patient's movement ability can be quantified, and the therapist can manually specify desired, patient-specific training motions for the device. Methodology: Design Criteria
Our design criteria for a robotic device for assisting in pelvic motion during step training are:
Mechanical Design
Based on these design criteria, we have developed a device called the Pelvic Assist
Manipulator (PAM) (Fig. 1,2). With backdrivability and affordability as two of our main
design criteria, pneumatic actuators were chosen to provide the required backdrivability,
force output, force control, position control, and affordability. Pneumatic devices are
ideal for backdrivability. In a passive state, the backdrivability of a pneumatic cylinder
is excellent as air is easy to move. Another beneficial feature of pneumatics is the high
force-to-weight ratio they offer. A function of bore size and supply pressure, pneumatic
cylinders can provide more than enough force for locomotion training purposes in a relatively
lightweight package. Geared motors providing comparable levels of force are much heavier.
The compliant characteristic of air, although advantageous with respect to backdrivability, becomes problematic when it comes to controlling it. The highly compressible nature of air causes nonlinearities that are difficult to model. Additional nonlinearities can arise from the friction of the moving piston. Bending moments on the cylinder rod can cause excessive friction within the cylinder, which is not desirable when trying to achieve good control. Bending moments on the cylinder rod also create undesirable stresses that over time can cause failure or malfunction of the cylinder. Proper precautions must be taken to minimize these bending moments. In order to design an appropriate mechanism for assisting in natural pelvic motions, it is necessary to first establish what constitutes a natural pelvic motion. During unconstrained locomotion the pelvis undergoes three translational displacements and three angular displacements. Therefore, PAM should be able to accommodate all six of these DOF’s to be an effective, natural pelvic manipulator. PAM consists of two, three DOF pneumatic robots that attach to the back of an adjustable belt worn by the subject. For each robot, the three pneumatic cylinders are anchored to a support pillar via ball-joints, and attach to a point through their lines of center to a revolute joint on the belt. Two cylinders lie coplanar in the horizontal plane, and the third cylinder lies in an oblique plane to provide upwardforces. The resulting system has five DOF, providing control of three translations (side-to-side, forward-and-back, up-and-down) and two rotations (pelvic rotation about the Z-axis, and pelvic obliquity about the Y axis, Fig. 1). One rotation cannot be controlled - pelvic tilt about the X-axis. As shown schematically in Fig. 1, a separate, pneumatic over-head BWS mechanism partially unloads the patient's weight depending on the desired level of support. Each three-cylinder robot is mounted to an adjustable slide that allows the robots to be moved vertically to accommodate subjects of various hip heights. The mounting of the pneumatic cylinders on ball joints minimizes the moments that can be imparted onto the pistons, potentially damaging the cylinders. The cylinders attach to the belt behind the subject in order to allow for the subject to swing the arm naturally during gait and also to provide an unobstructed view for the subject. The cylinders are also angled in from the sides with sufficient spacing to allow the subject to enter the device via a wheelchair, and to allow the therapist to access the subject from both behind and on the sides, as is necessary for manual assistance of pelvic motion using a "teach-and-replay" strategy. Control Design To begin with, the system needs a feedback system that monitors the movement of the cylinder rods and the pressure entering the cylinders. Linear Resistive Transducers (LRT’s) integrated into the cylinders provide accurate position information. Pairs of pressure sensors are used for each cylinder that output a signal proportional to the absolute pressure supplied to each side of the piston. To record movements, PAM's cylinders can be vented and the device can be used in a passive mode. The cylinders are instrumented with linear potentiometers. The position and orientation of the pelvis can be inferred in real-time from the potentiometer voltage measurements using the forward kinematics of the mechanism. To replay desired movements, a hierarchical control system is used for which the actuator dynamics are separated from the rigid body dynamics of the robot (Fig. 3) [9]. This permits well-established control laws, like those used for motor driven robots, to be used for the pneumatic system. To achieve this hierarchy, we model and control the nonlinear compressible air flow dynamics for each cylinder and servovalve, and use pressure sensors on both sides of the pistons for feedback in order to achieve fast and accurate force control for each cylinder of the system. This transforms the control problem into one that is standard for robotic control designers. The inner-loop force control law is:
Safety
The wellbeing of the both the patient using the system and the therapist operating the system
cannot be compromised. One benefit of conventional training is that a human is in control of
the action at all times and can sense and react to any signs of danger. A robotic device must
safely interact with both patient and therapist to even be considered as a viable means of training.
Our approach toward the overriding concern of safety is to incorporate redundant electrical,
mechanical, and software safety features.
The keystone of the pneumatic safety system is the main valve, which is a normally-closed valve that lies just downstream from the compressed air regulator. Air is allowed to flow through the valve only when a sufficient current is applied to it, otherwise the incoming supply port is plugged and outgoing ports are exhausted to the atmosphere. If the power supplied to the MOSFET ceases for any reason, the main valve will receive no current and the system will no longer receive a compressed air supply. Should the main supply pressure be cut, pressure-actuated safety valves vent both sides of the cylinder, leaving the system in its passive state. Main supply pressure is vented with an electrically controlled valve when an emergency stop button is pressed. Main supply pressure is also vented when software limits on position, velocity, and pressure are exceeded. The core of the mechanical safety features is a hardstop (Fig. 4) that limits the allowable rotation and obliquity angles of the pelvis, as the spherical belt joint allows belt motions that exceed the body’s natural range of motion. As a precaution, all metal corners and edges are rounded to prevent subject or operator injury. Also, pinch points at the universal joints and belt joints will be labeled and covered to prevent anything from getting caught in between them. Present Status: Having successfully created a safe training environment, PAM is near ready to train with spinal cord injury patients. This is the primary goal for the near future. Also, a user-friendly computer interface needs to be implemented to give therapists, or operators, a straightforward method of operating the device from start-up to shutdown. References:
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Last modified: June 20, 2005 |
unconstrained locomotion the pelvis undergoes three translational displacements and
three angular displacements, which are tightly coupled to step rate and stride length
parameters [5]. The Lokomat allows unrestricted movement in the vertical direction but
restricts pelvic rotation, pelvic obliquity, and horizontal translation of the pelvis.
The MGT has taken the simplified approach of moving the torso with a single DOF mechanism
along fixed horizontal and vertical trajectories that approximate those achieved during
normal stepping. Such a fixed trajectory cannot be optimal for every patient. In addition,
this approach requires the same torso motion to be applied regardless of the stage of
recovery of the patient. Thus, both of these devices are incapable of direct control or
recording of pelvic movements. Patient-specific torso motions may be useful for generating
desired gait patterns, as recently demonstrated using dynamic motion optimizationtechniques
[6]. 
