CA2308502A1 - Motion platform - Google Patents

Description

AFTS Project Patent Description: 05/11/00 Motion Platform the Base Plate 7. In one embodiment of the current invention Joint 4 and Mounting Adapter 3 on the Strike Plate 2 are identical to Joint 6 and Mounting Adapter 5 on Base Plate 7 respectively. However, Mounting Adapter 3 and Joint 4 on the Strike Plate 2 need not be identical to their counterparts on the Base Plate 7 so long as they satisfy the configuration requirements for the Motion Platform 1 and are suitable for the intended application.
Strike Plate 2 provides a surface on which the object to be moved can be mounted. Examples of such object are cameras, communication antennas, people and articles to be tested through a motion sequence.
In one embodiment of the current invention Strike Plate 2 serves as a mounting surface for a carpet material, which is then moved to strike a stationary shoe in a manner that replicates the kinematics of human running. For this application Strike Plate 2, Mounting Adapter 3 and Joint 4 are all manufactured from aluminum to minimize the 'flying' weight of Motion Platform 1. These components can be made from materials other than aluminum depending on the needs of the intended application.
The placement position for Mounting Adapter 3 on Strike Plate 2 are governed by a set of relationships and methodologies that relate machine configuration to the desired working volume and range of movement of the Motion Platform 1. These are the same relationships and methodologies that determine the design of Mounting Adapters 3 and 5 and determine the placement location of Mounting Adapter 3 on Base Plate 7.
The location of the center of rotation of Joint 6 on Base Plate 7 is governed by three independent parameters that are measured from the geometric center of Base Plate 7 as illustrated in figure 5:
PMP_X - Primary Mounting Point X value PMP_Y - Primary Mounting Point Y value PMP Z - Primary Mounting Point Z value The location of the center of rotation of Joint 4 on Strike Plate 2 is governed by three independent scaling factors that are applied respectively to PMP X, PMP Y and PMP-Z as illustrated in figure 6:
X_DFF - X Dimension Form Factor Y_DFF - Y Dimension Form Factor Z DFF - Z Dimension Form Factor The location of the center of rotation of Joint 4 on Strike Plate 2 is measured from the geometric center of Strike Plate 2. The geometric center of Strike Plate 2 is itself centered on the geometric center of Base Plate 7 with an offset in the Z (vertical) direction.
The length of Linear Actuator 8 determines the linear distance between the center of rotation of Joint 6 on Base Plate 7 and the center of rotation of Joint 4 on Strike Plate 2. Once the value for the length of the Linear Actuator is established and values are set for PMP_X, PMP Y, PMP_Z, X
DFF, Y_DFF and Z DFF the configuration of Motion Platform 1 is calculated and its corresponding range of motion and working volume is established. The azimuth (B) and altitude (ø) angles (see figure 7) of Mounting Adapters 3 and 5 are calculated using the following relationships:
B = ATAN2[((PMP X/2-J3*PMP Y/2)+X DFF*PMP X),(Y DFF*PMP Y-(d3*PMP-X/2+PMP
Y/2))]
~= ACOS[L/S]
where:
S = the neutral or homed length of Linear Actuator XX; and L=( Sz - ((PMP X/2-J3*PMP Y/2)+X DFF*PMP X)z -(Y DFF*PMP Y-(d3*PMP_X/2+PMP
Y/2))z)uz This design insures that Linear Actuator 8 and its corresponding Joints are held inline when the Linear Actuator 8 is in its "homed" or neutral position (as defined by the length value for the Linear Actuator used in the above calculations). By maintaining this 'inline" configuration the maximum working volume for Motion Platform 1 is realized with a given Joints 4 and 6.

AFTS Project Patent Description: 05/11/00 Motion Platform Having a desired trajectory for Strike Plate 2 allows the calculation of the corresponding movements that each of the six Linear Actuators 8 must make in order to achieve this motion.
The desired trajectory is represented mathematically by a matrix of 4 x 4 transforms that represent the position and orientation (i.e.
the 'pose') of Strike Plate 7 through the duration of the trajectory. Each element of the trajectory matrix is a 4 x 4 transform that represents the pose of Strike Plate 7 at a moment in time. The first element of the trajectory matrix is the pose of Strike Plate 7 at the beginning of the trajectory. Each subsequent element of the trajectory matrix represents the pose of Strike Plate 7 at a specific interval of time after the beginning of the trajectory. The last element of the trajectory matrix represents the pose of Strike Plate 7 at the end of the trajectory. The positions of the centers of rotation of each of the Joints 4 can be calculated using the following formula:
Pt=i = T =r ' Pr=o Where: p = a 6x4 array representing the positions of the centers of rotation of Joints 4 in Cartesian coordinates T = the trajectory matrix t = time i = a time interval with the trajectory matrix 't=0' represents the beginning of the trajectory The length of each of the Linear Actutators 8 at each time interval along trajectory can then be calculated by subtracting the position of the center of rotation of Joint 6 from the position of the center of rotation of Joint 4 attached to each of the 6 Linear Actuators 8. The formula for this calculation is:
1 n at t=i - ~~ at t=i Bn ' ~~ at t=i Bn Where: I" at t=i - is the length of 'n'th Linear Actuator 8, at the 'i'th time interval along the trajectory and 'n' ranges from 1 to 6 for each of the six Linear Actuators 8 P - is the position of the center of rotation of Joint 4 in Cartesian coordinates on the 'n'th Linear Actuator 8 at the 'i'th time interval B - is the position of the center of rotation of Joint 6 in Cartesian coordinates on the 'n'th Linear Actuator 8 at the 'i'th time interval Having established the movements for each of the Linear Actuators 8 it can then be determined if the Joints 4 and 6 can accommodate this motion. A mathematical model of the Motion Platform is used to determine the corresponding motions of Joints 4 and 6. In one embodiment of the current invention a software package called "Working Model" (The MacNeal-Schwendler Corp. of San Mateo, California) was used to build the mathematical model of Motion Platform land carry out the analysis.
Other means of numerical analysis could be used to gain the same result. Depending on the outcome of the analysis on the movements of Joints 4 and 6 the parameters PMP_X, PMP_Y, PMP Z, X DFF, Y DFF
and Z_DFF, as well as the length of Linear Actuator 8, can be adjusted until an appropriate configuration is found to suit the desired trajectory(s).
In one embodiment of the current invention the configuration of Motion Platform 1 has been optimized to follow trajectories for human running motions. The configuration can be readily adapted to a wide variety of other applications.

AFTS Project Patent Description: 05109/00 Linear Electric Jack Linear Electric Jack Description Refernng to figure 2, the linear motor assembly 8 is composed of a number of key sub-components: a commercially available linear electric motor block 9 and magnetic rod 19;
bearing assemblies 10;
aluminum side rails 29 30 and bottom plate 26; rod end clamps 31; encoder 17, encoder mounts 15 and 16;
and encoder connecting rod 20.
The linear electric jack relies on a commercial linear electric motor, composed of a powered linear thrust block and a thrust rod 19, a stainless steel tube packed with magnets. In one embodiment of the linear electric jack the LD3808 linear electric motor block was used in conjunction with a 38mm thrust tube both commercially available from Linear Drives of Basildon, Essex, UK
The motor housing is composed of two supporting rails 29 30 and an end plate 26. This open structure permits internal cooling within the motors and, with appropriate material selection, a strong load path that supports anticipated loads, the motor, bearing housings 10, linear measurement system 17, and any other external mounting requirements. The end plate 26 completes the structure and provides an additional coupling surface for externally mounted components. In an embodiment of the present invention gauge aluminum plate was used to make the rails 29 30 and end plate 26. If appropriately dimensioned for strength and cooling, other materials including plastic, steel or composites (such as fiberglass) could meet the requirements of this linear electric jack.
The magnetic rod 19 is driven linearly through the hollow motor block 9 when appropriately commutated current is applied to the motor. The rod 19 is held clear of the motor block's inner wall by two bearing assemblies 10 on either side of the thrust block. Referring to figure 3, the bearings 13 are held rigidly by bearing housings 11 12 and a bearing retaining ring 14. For linear electric motors these bearings should be dry (unlubricated) and made of a material with properties such as low back electromotive force (EMF) and low friction (typical examples include as carbon or plastic). The bearing housings, too, should be manufactured using material with low back EMF properties such as plastic or brass. Both the bearings 13 and the bearing housings 11 12 can be split to facilitate in situ removal and/or replacement of bearing components without disassembly of the linear electric jack. In one embodiment of such split bearing assemblies, the bearings 13 were composed of a Carbon/Resin composite, a dry running low friction material, and custom manufactured by Advanced Carbon Products of Hayward, California, USA. In this same embodiment, the bearing housings were made from Ertalite, a light, strong, plastic with no back EMF
characteristics manufactured by DSM Engineering Plastic Products of Reading, Pennsylvania, USA.
Rod end clamps 20 31 permit the attachment of payloads to the thrust rod 19 and, for the interior rod end clamp 20, prevents the thrust rod 19 from exiting the motor assembly. Refernng to figure 4, the rod end clamps are designed to provide solid attachment points to the rod without machining the rod surface or end components and to provide a solid impact surface during overstrokes. The clamps are composed of a split clamp ring 21 a clamp collar 22, and a clamp compression plate 34. The thrust rod 19 is inserted into the clamp collar 22 and clamp ring 21 such that the wedge profile of the ring fits snugly into the collar. The compression plate 34 is then fastened to the clamp collar using a number of appropriately sized industrial fasteners. As these fasteners are tightened, the clamp collar 22 and clamp ring 21 are pulled against the compression plate 34. The wedge profile of the clamp ring 21 and collar 22 drives the clamp ring 21 against the rod 19 surface. As the clamp ring 21 is compressed, the compression plate 34 is pulled against the rod 19 end. In this way the compression plate 34 provides both a firm payload mount and, important for precision motion, a reliable geometric reference relative to the rod end 19 important for precision motion.
The angle of the wedgeA is related to clamping force, Nb, and the coefficient of static friction, through an explicit relationship expressed below:
N - Fb (cosA - ~Q sin6 ) sine + ~a cos9 To provide closed loop control of the motor, the control system must receive the motor position as feedback. While any linear measurement system of appropriate resolution could be used for this task, commercially available encoder 15 should be selected to avoid problems common to other linear AFTS Project Patent Description: 05/09/00 Linear Electric Jack measurement systems requiring moving power and signal connections. In one embodiment of the current device a commercially available magneto-strictive encoder was used. These devices are typically composed of a passively magnetic encoder carriage 18 and an encoder body 17, containing magnetically sensitive material through which the time-of transit of an ultrasonic pulse is measured and converted into displacement by means of internal electronics. In one embodiment of the present invention the LP-38 ISI
magneto-strictive encoder from TRelectronic GmbH of Trossingen, Germany was selected to provide A
quad B incremental output to the system's control electronics. The control electronics in this embodiment of the present invention was an IDC B8001 servodrive from Industrial Devices Corporation of Petaluma, California, and a Delta Tau PMAC 1 by Delta Tau Data Systems of Northridge, California, USA.
An encoder carnage 18 limited to strictly linear motion can be connected to the interior rod end 19 through an encoder connector assembly 20. The encoder connector assembly is composed of a modified rod end compression plate 23 with a short shaft, a bearing 24, and a connector rod 25.
The connector rod 25,bearing 24 and compression plate shaft 23 are joined (e.g. through press fitting) into a single assembly that permits free rotation of the connector rod about the shaft of the compression plate 23. Therefore, the encoder connector assembly permits the thrust rod 19 to rotate freely about the rod axis without applying detrimental forces to the encoder while communicating rod linear motion rigidly to the linear encoder 17.
Encoder mounting plates 15 16 provide an offset distance for the linear encoder, typically dependent on the encoder type and EM sensitivity properties of the encoder. Other linear measurement systems such as LVDTs, Laser Interferometers, and glass scale encoders might also be applied to this system and, depending on the placement and measurement principle, might or might not require specific rod end couplers.
Unlike some rotary motors, linear motors are capable of exceeding mechanical limits and damaging components in the process, an event called "overstroke". To prevent this possibility, the Jack must use positive and negative limit switches 28 that disable the motor when triggered by the entry of the rod into a dangerous region. In one embodiment of the present invention, Banner D12 optical switches by Banner Engineering Corporation of Minneapolis, Minnesota, monitor enforce the Linear Electric Jack's interior geometric limits and disable the amplifiers on overstroke. Such switches may be further used to implement homing triggers for precise calibration of the motor position.
The damage from an overstroke event is further reduced through the selection of appropriate end plate bumper 27 and rod end ring bumpers. In one instance of the present invention, the rod end ring bumper was manufactured of neoprene while the lower bumper was made of 40 duro gum rubber . The rod end ring bumper can be fixed either to the bearing housing 10 or rod end clamp collar 22 as convenient.

Claims