Six Station Knee Simulator

The AMTI Knee Simulator, developed by AMTI with the collaboration of leading implant manufacturers and medical professionals, simulates the physiological motions of the knee. The machine provides four degrees of freedom which are congruous with the physiological function of the knee. The simulator has two axes of rotation which are flexion-extension, internal-external rotation and one translation axis corresponding to anterior posterior displacement. In addition to the rotations and translation, the machine provides vertical load control analogous to loading along the axis of the tibia. The simulators flexible control scheme allows the user to independently program the motions (or forces) for each of the controlled degrees of freedom to simulate physiological functions such as walking, running and stair climbing.

Features

Long life servo-hydraulic actuators with hydrostatic bearings and laminar seals.

Continuous operation, expected actuator life greater then 10⁸ cycles.

Six stations arranged in 2 banks of 3, each bank separately controlled.

Vertical load independently controlled for each bank.

IE rotation independently controlled for each bank.

Flexion/Extension independently controlled for each bank.

AP translation independently controlled for each bank.

Each station includes a six component load cell (Fx, Fy, Fz, Mx, My, Mz).

Independent peristaltic pumps for isolated specimen fluid recirculation.

Specimen fluid temperature controlled by a heater-chiller system.

Eight channel PID controller for flexible actuator control.

Data acquisition system, 48 channels at 200 Hz, 15 channels at 2000 Hz.

Threshold event monitor on feedback control signals.

Automatic interlocks for system and specimen protection.

High pressure hydraulic filter, 6 micron (6 µm) with indicator light.

Mechanical system

The simulator is designed to closely duplicate the conditions of the implant in vivo. Natural mounting, with the femoral component above the tibial component simulates the implant orientation during gait and normal stance. A temperature controlled fluid recirculation system provides isolated bovine serum, saline or other fluid immersion bathes for each station.

The simulator has six stations, which accommodate the simultaneous testing of six samples. The stations are organized into left and right banks of three stations each, which may be independently programmed to provide a different testing procedure on each bank. Each station can be independently disabled which allows testing to be continued after removing specimen from one or more stations. In addition, any station can be utilized to provide a "load soak" control (loading cycle only, no motions) while other stations operate under full test conditions, thus providing a control specimen for comparison to worn samples.

The range of motion of the simulator approximates or exceeds the natural anatomic range of motion of the knee joint. The flexion-extension range is ±100°; the internal-external rotation range is ±20° and the anterior-posterior translation range is ±2.5cm. The vertical load range is 0 to 4500 N (acting on each station) and loading profiles with a frequency up to 2 Hz can be run.

Each station is equipped with its own temperature-controlled specimen fluid system which may be used to circulate bovine serum, saline solution, water, or other fluid over the test specimens. The entire specimen may be enclosed in a flexible plastic bag which permits complete immersion and reduces fluid evaporation and oxidation of the serum and specimen surfaces. Each station’s fluid circulation system is completely isolated from its fellow stations and the fluid may be filtered for the retrieval of wear debris if desired.

Each station is equipped with its own six component load cell. The load cells measure the three orthogonal forces, Fx, Fy, and Fz and the three orthogonal moments, Mx, My, and Mz applied to the test specimens. The force and moment data are useful for understanding the mechanics of the wear process and for the purpose of monitoring and documenting the test progress.

Machine control and data acquisition

The simulator machine is delivered with a complete data acquisition and control system. The control architecture utilizes a DSP (digital signal processor) built into the machine to perform time critical tasks. The onboard DSP provides eight channels of waveform generation and implements eight channels of PID (proportional, integral, and derivative) control to drive the machines actuators. The DSP also manages the acquisition task by multiplexing between 96 channels of analog signals and measurements and by acquiring and assembling data to be delivered to the supervisory PC. The DSP communicates with AMTI’s NetControl software running on the supervisory PC via a simple, single cable Ethernet link.

The PID control loop algorithm is a standard form implementation providing proportional, integral and derivative feedback control with user adjustable parameters Kp, Ti and Td. Each channel is independently adjustable and entire sets of parameters governing all of the channels may be saved as user named files for the purpose of documenting and repeating a particular test or experimental setup.

Control loop feedback signals are digitized at a sample rate of 2000 samples per second per channel providing a control loop bandwidth of better then 100 Hz (hydraulic and mechanical considerations practically limit the band width to about 20 Hz). The digitized control signals are software mapped to the DSP’s eight channel PID loop feedback inputs. This provides the flexibility for the user to choose either force control or displacement control modes of operation by simply selecting the desired feedback channels (certain degrees of freedom are mechanically coupled between stations, see the specification table below for more details). The PID loop’s set point inputs are provided by an eight channel, internal 256 point arbitrary waveform generation and interpolation algorithm. The PID loops calculated control signals are converted to analog signals at a rate of 2000 samples per second per channel and output to the systems servo valves as ±10 volt signals.

In addition to the 15 channels of high speed data acquisition used for the feedback control system a 200 sample per second data acquisition system is implemented to monitor the machines 36 channels of force and moment measurements and 12 channels of temperature and vertical position signals. This acquisition scheme is accomplished on a multiplexed per station basis. Each station is periodically sampled for a user specified duration at a rate of 200 samples per second per channel. The digitized data is transferred to the PC via the Ethernet link and subsequently stored in files at the user’s discretion.

The NetControl software running on the supervisory PC provides the user with a complete operating interface for the simulator machine as well as many powerful motion programming and data acquisition tools.

Several different “control panels” are available which facilitate different operating tasks. A “tuning” panel allows the user to adjust the PID control parameters to optimize performance while running different user supplied or canned waveforms. This allows the user to optimize the machines performance to achieve very close agreement between the driving waveforms and the resulting force and motion profiles. A “manual” panel allows the user to jog the machines actuators for the purposes of setup and sample insertion and extraction. In this mode canned (sine, square, triangle, ramp) and arbitrary waveforms may be downloaded and run for evaluation (and testing) without creating a full control program. NetControl’s “simulator” control panel allows the user to select and run preconfigured programs which automatically schedule waveform activities, data acquisition activities and present on-screen gages and controls to monitor the machines activities.

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Knee simulator specifications

Actuator and station arrangement
Item	Specification	Comment/condition
Controlled DOF
Vertical load	0-4500N(0-1000lb)	±2.5 cm
Flexion extension	±100 degrees	10 N-m (88 in-lb)
IE rotation	±20 degrees	5 N-m (44 in-lb)
AP Translation	±25 mm (±1 inch)
Free DOF
ML Translation	±10 mm	Optional tibial tray
Valgus rotation	±3 degrees	Optional tibial tray
Control scheme
Vertical load	Servo-hydraulic	Single control loop per bank
Flexion extension	Servo-hydraulic	Single control loop per bank
IE rotation	Servo-hydraulic	Single control loop per bank
AP Translation	Servo-hydraulic	Single control loop per bank
Control feedback	Typical	Alternate
Vertical load	Actuator pressure	Fz load cell
Flexion extension	Position sensor
IE rotation	Position sensor	Mz load cell
AP Translation	Position sensor	Fy load cell
Station arrangement
Left bank	6 stations, coupled motion	Independent of right bank
Right bank	6 stations, coupled motion	Independent of left bank
Dimensions
Length	1.8m (72 inches)
Width	1.0m (39 inches)
Height	1.9 m (75 inches)

Hydraulic power requirements

As many of our customers choose to power the simulator machine from a central hydraulic system the hydraulic power unit is available as a separate module. The power unit requirements are shown in the following table. AMTI manufactures a quite HPU ideally suited for our simulator machines, for specifications click here (AMTI HPU Specifications).

Hydraulic power unit requirements (not included)
Item	Specification	Comment/condition
Pressure	5 MPa (800psi)	@ rated flow
Flow rate	1.7 L/s (25gpm)
Return	Gravity drain	No line restriction

Knee simulator options

Hydraulic Power Unit (HPU)

The AMTI SHS-15-800 Hydraulic Power Unit is designed to power the AMTI-Boston Hip Simulator HS2-12-1000 or the AMTI-Boston Knee Simulator KS2-6-1000. The Hydraulic Power Unit meets all the requirements of the servo-hydraulic simulators and can power either 2 Hip Simulators or 1 Knee Simulator (consult AMTI for other possible combinations).

The Hydraulic Power Unit is an integral, motor, pump, reservoir, and heat exchanger system equipped with all the necessary valves and plumbing. The heat exchanger system is an oil-to-water system which includes a water (coolant) modulating valve, a shutoff solenoid valve and temperature gage. It is recommended that the heat exchanger be connected to a closed loop circulating water cooling system.

Low and high level float switches are provided to shut motor down in the event of low and high oil conditions. An additional temperature sensor provides protection against over-temperature conditions. The system is delivered with all of the controls including a remote enclosure with On-Off controls, fluid level light, over temperature light and safety key interlock.

SHS-15-800 Hydraulic power unit specifications
Item	Specification	Comment/condition
Pressure side
Pressure	5 MPa (800 psi)	@ rated flow
Flow rate	2L/s (30 gpm)
Outlets	2	SAE size 16
Return side
Inlets	2	2.5 inch hose
Method	Gravity drain	No line restriction
Electrical
Power	10 HP
Voltage	208/230/460	Customer specified
Amperage	60 amp	@230 volts
Phase	3 phase
Cooling
Capacity	15 HP
Water inlet	50-70 ºF	20-40 psi
Controls
Temperature	Oil high limit (125 ºF)	Indicator light, shut off
Oil level	Low oil level	Indicator light, shut off
Start	Push button
Stop	Push button
Security	Key interlock

Knee Load Soak Station (model KLS)