The VIVO provides accurate robotic joint motion simulation for knee, hip, shoulder, temporomandibular, elbow, ankle, spinal and other joint motions. The simulator can be programmed with standard or custom force and motion profiles, supporting research and test protocols that range from standard activities of daily living (ADL) tests through extreme sports and even simulation of accidental conditions and injurious activities.

Beside classical displacement or force control for each axis, the unique actuator configuration on the lower stage provides a floating instant center of rotation. In combination with the software-defined virtual axes of the Grood and Suntay coordinate system, many of the joint alignment issues found in legacy test machine designs are eliminated (self-centering).

Command waveforms are generated by independent 1024-point waveform buffers for each axis. The waveform is interpreted as a position (translation or rotation) or force (linear force or moment) command according to the current axis command mode. Switching an axis between position and force command mode is as simple as ticking a box in the setup configuration dialogue. VIVO’s unique combination of speed, range of motion and force capability, programmability and virtual soft tissue models enables testing of real-world implant failure modes, such as adverse edge loading conditions, micro-separation, stem and cup impingement, condylar liftoff, and joint subluxation.

The latest multi-fiber ligament model is the result of a ground-up re-thinking of how to structure a kinematically- and kinetically-accurate virtual force model. Rather than determining ahead of time what constraint force corresponds to every coordinate for each axis, in the multi-fiber ligament model, up to 100 ligament fibers are defined by specifying the (x, y, z) coordinates of their proximal and distal insertion sites on the joint components, their stiffness, and the amount of strain or slack the ligament has when the joint is in the defined reference pose. During operation, the controller calculates in real time the length and line of action of the ligament fiber. The contributing forces and moments of all fibers are summed and applied to the joint. These calculations are performed at every servo tick on the realtime controller. Therefore, they are inherently synchronized with the instantaneous kinematic configuration of the joint. The multi-fiber ligament model enables the development of testing for sensitivity to ligament balance and post-surgical ligament condition while providing the complex inter-axis coupling exhibited by the natural knee. The concepts inspiring the multi-fiber ligament model may be used for future ASTM and ISO testing standards.

The heuristic constraint model in the VIVO simulator is an enhanced version of the constraint model used in AMTI’s classical hip and knee simulators, now independently programmable for any axis in force-control mode, and also able to represent 2-axis cross-coupling using any axis as the secondary axis. During operation, the controller uses the instantaneous axis coordinates and calculates the constraint forces (or moments). Every axis offers a completely independent implementation of this model. The VivoControl user interface offers tools to assist in generating the constraint force tables, such as linear, quadratic and polynomial-fit spring models. Alternatively, the constraint force tables may be generated in a prescribed CSV format by an external application and imported into VivoControl. Among other uses, the heuristic soft tissue constraint model is typically required for tests that conform to today’s ISO and ASTM standards.

The Cartesian Coordinate System mode allows the use of a non-joint-based coordinate system, which makes the VIVO downwards compatible with traditional machines. In this mode, input and output translations and linear forces are resolved along an orthogonal X-Y-Z coordinate system that is fixed with respect to the frame of the machine. Input and output rotations are resolved along rotational axes that coincide with the physical actuator axes of the flexion and ab/adduction actuators, and a virtual Z-rotation actuator. In Cartesian Coordinates mode the four axes of the lower stage can operate in force- or position-command mode. The flexion and ab/adduction axes operate in position mode only. The VIVO is the world’s first simulator to fully support the Grood and Suntay joint coordinate system, accepted by ASTM, ISO and the International Society of Biomechanics (ISB) as a standard for biomechanics joint testing. The user commands forces and motions with respect to software-defined joint-referenced axes, making it unnecessary to consider the complex, pose-dependent, compound-rotation kinematics of the machine’s physical actuators. The virtual coordinate system is also the key enabling element in VIVO’s exclusive new test setup system. The VIVO can sense how the joint or implant is placed in the workspace and relocate the coordinate origin to match the installed pose of the joint. This capability also accommodates mounting variations in the experimental setup. It relaxes the level of accuracy required when mounting joint specimens in the adapters that hold them in the machine, significantly increasing repeatability of testing.

The VIVO is the world’s first simulator to fully support the Grood and Suntay joint coordinate system, accepted by ASTM, ISO, and the International Society of Biomechanics (ISB) as a standard for biomechanics joint testing. The user commands forces and motions with respect to software-defined joint-referenced axes, making it unnecessary to consider the complex, pose-dependent, compound-rotation kinematics of the machine’s physical actuators. The virtual coordinate system is also the key enabling element in VIVO’s exclusive new test setup system. The VIVO can sense how the joint or implant is placed in the workspace and relocate the coordinate origin to match the installed pose of the joint. This capability also accommodates mounting variations in the experimental setup. It relaxes the level of accuracy required when mounting joint specimens in the adapters that hold them in the machine, significantly increasing repeatability of testing.

The VIVO also supports AMTI’s patented iterative learning control system to improve the accuracy of the actual forces and positions generated by the simulator. Based on modern control theory, this feed-forward, self-tuning frequency-domain implementation requires less user configuration than ever before, yet provides the best stability and fastest convergence available in any AMTI simulator. This approach produces a feed-forward compensation that, in theory, is capable of driving the error to exactly zero over time. While non-repetitive disturbances in any real system will prevent true zero error, in practical applications the new system usually reduces error to well below 1% of command. The new system includes a pre-run phase we call “haptic mapping.” Before starting a new test, a few cycles of the waveform are run at greatly reduced frequency. This reduced speed allows the basic P+I control system to operate with relatively low error, in effect measuring the pose-dependent compliance of the joint. The ILC system can be disabled for low-speed or non-repetitive testing. In these cases, VIVO’s P+I axis controllers provide the performance of a standard servo-hydraulic control system.

Each station has a temperature-controlled serum containment and circulation system for tests that are conducted in a fluid environment. The thermal plate can heat or cool the serum to achieve setpoints between 10°C and 45°C.

A VIVO system consists of one to three test stations assembled and shipped as a unit. Each station is equipped with six servo-hydraulic actuators. Acquisition and installation costs are optimized by sharing a single electrical power connection, real-time controller, hydraulic pressure supply, and hydraulic return for all stations in a frame. Although there is a single real-time controller, it executes independent control loops for each of the stations. Therefore the stations are programmed and operate independently. However, the VivoControl UI supports one-step copying of programs and setups between stations so that the same test protocol can be executed for multiple samples.

VivoSim is an optional software package that aids understanding by displaying an accurate 3-D model of the joint components and the multi-fiber ligament model as they move in space. While the VIVO by itself is a completely functional, stand-alone simulation system, VivoSim provides an enhanced ability to look inside the multi-fiber ligament model and examine the strain, tension, and resolved force components individually for each fiber. VivoSim also has a near-real-time stand-alone modeling capability.

For compactness and maximum service life, VIVO utilizes an all hydraulic actuator design. The main bearing of the system is hydrostatically supported to provide low friction, more accurate control, and long life. AMTI’s unique hydrodynamic-seal actuators greatly reduce maintenance downtime and are designed to last for hundreds of millions of cycles. A separate hydraulic power unit is required for operation. AMTI can provide hydraulic power units sized for one-, two- or three-station VIVO systems. Customers can use their own infrastructure if preferred. Please consult with AMTI regarding requirements.

The VIVO system sets a new standard of protecting your valuable test investment. Every physical quantity is continuously monitored. In the event of a fault or an overload, the controller responds to protect the machine and the sample. User-defined limits and a programmed action can also be created for many dynamic variables. This rapid response prevents inadvertent specimen damage during setup as well as during operation of the machine.

In addition, operator safety is enhanced with a light curtain safety interlock that encloses the working volume of the machine. The light curtain may be disabled with a key switch for maintenance or setup operations that require the machine to be energized.

Among others, the VIVO is capable of performing tests to ISO 14242-1, ISO 14243-1, ISO/CD 14243-3, ISO 14879-1, ISO 16402, ISO 18192-1, ISO/TR 22676, ISO 7206-4, ASTM F1223-08, ASTM F2790-10, ASTM F2694-07, ASTM F2777-10, ASTM F2028-08, and ASTM F1829-98.