Twelve Station Hip Simulator
The AMTI Hip Simulator,
collaboratively developed by AMTI and a team of world renowned arthroplasty
specialists, simulates the physiological motions of the hip. The machine
provides four degrees of freedom (4 DOF) which are in accord with the
physiological function of the hip. The simulator has three axes of
rotation which include: flexion-extension, internal-external rotation and
abduction-adduction. In addition to the rotations the machine provides
vertical load control analogous to loading along the axis of the femur.
The simulators flexible control scheme allows the user to independently program
the motions or forces (some motions and control schemes are coupled, please see
the specification table below for details) for each of the controlled degrees
of freedom to simulate physiological functions such as walking, running and
stair climbing.
Hip simulator features
Long
life servo-hydraulic actuators with hydrostatic bearings and laminar seals.
Continuous operation with expected actuator life greater then 108
cycles.
Twelve
stations arranged in 2 banks of 6, each bank separately controlled.
Vertical
load independently controlled for each bank.
Internal-External
rotation independently controlled for each bank.
Flexion/Extension
independently controlled for each bank.
Adduction-Abduction
rotation independently controlled for each bank.
Each
station equipped with an six component load cell (Fx, Fy, Fz, Mx, My, Mz).
Individual
peristaltic pumps for isolated specimen fluid re-circulation
Specimen
fluid temperature controlled by a heater-chiller system.
PID
controller included, 8 independent programmable actuator control channels.
Data
acquisition system included, 96 channels at 200 Hz, 15 channels at 2000 Hz.
Threshold
event monitor on feedback control signals system and specimen protection.
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. A natural vertical orientation of the prosthetic
device, with the acetabular cup above the femoral component, prevents the
accumulation of wear debris in the cup. A temperature controlled fluid
recirculation system provides isolated bovine serum, saline solution or other
fluid immersion bathes for each station.
The simulator has twelve stations, which accommodate the
simultaneous testing of twelve samples. The stations are organized into
left and right banks of six stations each, which may be independently
programmed to provide a different testing procedure on each bank. Each station
has an independent disconnect, which allows a single station to be disabled
while testing continues on the remainder of the active stations. Thus specimens may be removed from one or
more stations without interrupting the testing process. 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 the natural
anatomic range of motion of the hip joint. The acetabular
flexion-extension range is ±26°; the abduction-adduction range is ±9°, while
the internal-external rotation range is ±20°. Any loading cycle between
zero and 4500 N (1000 lb) can be applied, including the Paul or Bergman type
loading profiles at frequencies up to 2 Hz.
Each station is equipped with its own temperature-controlled
specimen fluid system. Independent
peristaltic pumps are used to circulate bovine serum, saline solution, water,
or other test fluid over the test specimens. Each station may be enclosed
in a flexible plastic bag which allows complete specimen immersion and reduces
fluid evaporation and oxidation of the serum and specimen surfaces. The
fluid systems are completely isolated from their fellow stations, which permits
filtering and 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
(where appropriate). 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 72 channels of force and moment
measurements and 24 channels of temperature and abduction head 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.
< Download Hip simulator PDF brochure
now>
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Hip simulator specifications
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Actuator and station arrangement |
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Item |
Specification |
Comment/condition |
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DOF (per bank) |
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Vertical load |
0-4500N(0-1000lb) |
±2.5 cm (1 in) |
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Flexion |
±26 degrees |
10 N-m (88 in-lb) |
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IE rotation |
±20 degrees |
5 N-m (44 in-lb) |
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Abduction |
±9 degrees |
5 N-m (44 in-lb) |
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Control scheme |
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Vertical load |
Servo-hydraulic |
Single control loop per bank |
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Flexion |
Servo-hydraulic |
Single control loop per bank |
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IE rotation |
Servo-hydraulic |
Single control loop per bank |
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Abduction |
Brushless servo-motor |
Six loops per bank ( Note1) |
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Control feedback |
Typical |
Alternate |
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Vertical load |
Actuator pressure |
Fz load cell |
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Flexion extension |
Position sensor |
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IE rotation |
Position sensor |
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Abduction |
Position sensor |
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Station arrangement |
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Left bank |
6 stations, coupled motion |
Independent of right bank |
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Right bank |
6 stations, coupled motion |
Independent of left bank |
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Dimensions |
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Length |
1.8m (72 inches) |
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Width |
1.0m (39 inches) |
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Height |
1.9 m (75 inches) |
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Note 1: The abduction heads have individual servomotor
drive loops and amplifiers but are all controlled (on each bank) by a single
control signal. |
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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 hydraulic power requirements are shown in the following
table. AMTI manufactures a quiet HPU ideally suited for our simulator
machines, for specifications click here (AMTI HPU Specifications).
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Hydraulic power unit requirements (not
included) |
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Item |
Specification |
Comment/condition |
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Pressure |
5 MPa
(800psi) |
@ rated
flow |
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Flow rate |
1 L/s
(15gpm) |
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Return |
Gravity
drain |
No line restriction |
Hip simulator options
Hydraulic Power Unit (HPU)
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.
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SHS-15-800 Hydraulic
power unit specifications |
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Item |
Specification |
Comment/condition |
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Pressure side |
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Pressure |
5 MPa (800 psi) |
@ rated flow |
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Flow rate |
2L/s (30 gpm) |
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Outlets |
2 |
SAE size 16 |
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Return side |
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Inlets |
2 |
2.5 inch hose |
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Method |
Gravity drain |
No line restriction |
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Electrical |
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Power |
10 HP |
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Voltage |
208/230/460 |
Customer specified |
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Amperage |
60 amp |
@230 volts |
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Phase |
3 phase |
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Cooling |
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Capacity |
15 HP |
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Water inlet |
50-70 ºF |
20-40 psi |
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Controls |
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Temperature |
Oil high limit (125 ºF) |
Indicator light, shut off |
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Oil level |
Low oil level |
Indicator light, shut off |
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Start |
Push button |
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Stop |
Push button |
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Security |
Key interlock |
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Hip Load Soak
Station (model HLS)
This option adds four test stations which provide vertical
loading of the specimens. These stations are meant to provide load soak/weight
gain compensation. Two stations are located on the end of each bank and their vertical
load control follows the standard bank commands. No flexion, abduction, or
rotation is provided. Individual recirculating fluid loops are provided for
each station along with fluid temperature controlled by an electronic
controller. No load cells are provided. The upper and lower specimen holders
are each configured to hold ball specimens and a pair of back to back
acetabular cups allows two pairs of specimens to be tested at each station. A
polycarbonate enclosure is provided.
Hip Simulator Vertical
Position Sensor (model VPS-12)
This option provides the 12 stations with a measurement of the
vertical position of the specimen holder. The VPS sensors are of a
non-contacting magnetic design and provide an output of 10 V/in. (0.4
V/mm). The measurement resolution is less than .001 inches
(.025mm). All outputs are sampled as part of the data acquisition
routine for the 12 stations. The sensor output can be used to indicate
the misalignment of the cup along the flexion and abduction axes. Due to its high
resolution, the sensor output can be used to monitor wear of the plastic cup
component.
Hip Abduction Shoes (model ABDS)
Twelve Hip Abduction Shoes are supplied with the
simulator. They are part of the upper specimen holders and are necessary
to fixture the specimen holders to the hip simulator. Users may prefer to
purchase additional shoes from AMTI. Our parts are made of heat treated high
quality M4 high speed steel. A print is available should users prefer to
manufacture their own.
Hip Upper Specimen Holder (model
ACE)
The specimen holder for the hip simulator is user
supplied. A universal Hip Upper Specimen Holder may be purchased from
AMTI. Our holder is machined from 316L stainless steel. A print is available.
Abduction Heads (model ABDH)
Complete calibrated abduction heads are available for customers
who would like to keep spares on hand. Individual parts, factory repair, and
calibration are also available.
Dummy Abduction Heads (model xxxxxx)
Dummy abduction heads can be used on any of the 12 stations to
provide load soak stations for control specimens. These are inexpensive parts that can free up
abduction heads to be used as spares.
Hip Machine Load Cell Calibration Fixtures
(model HLCC)
Users can calibrate their six component load cells annually
(recommended) with the hardware and software provided in this package. A single
axis calibration cell is used to calibrate the Fz load up to full scale. Dead
weight loading is used to calibrate the Mx, My, Fx, Fy, and Mz channels.
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