Off center loading compensation for multiple actuator vibration table

Claim 2

A test apparatus comprising a plurality of displacement generating actuators for displacing a test structure for movement in a vibratory path, said structure having at least one axis about which the displacement of the structure is to be controlled, and said structure having a center of mass offset from said one axis, at least some of said plurality of actuators also acting at points spaced from said one axis, means to control said actuators for reciprocating movement in their desired vibratory path, including means connected to each actuator to provide separate signals indicating force applied to said structure by that actuator, means to provide a signal indicating the average force actually applied to said test structure by said actuators, means to separately modify said average force signal for each of said some actuators by a factor which is a function of the distance said center of mass is offset from said axis and the distance that each of said some actuators is from said axis with respect to the distance from said axis of the other of said some actuators, and means to compare the individual force exerted by each of said some actuators with the separately modified average force signal for that actuator to provide a force balance signal in said means to control representing the difference between the force actually applied to the structure by that actuator and the separately modified average force signal for that actuator.

Claim 3

The combination as specified in claim 1 wherein said center of mass is also offset from a second axis mutually perpendicular to the first mentioned axis, and at least one of said actuators is positioned to act on said structure at a position spaced from said second axis, and second means to separately modify the average force signal for each of the actuators spaced from said second axis by a factor which is a function of the distance said mass is from said second axis and the distance said one actuator is from said second axis in relation to the distance of any other actuators from said second axis, means to sum said first modified average force signal for said one actuator with said second modified average force signal, and means to connect said means to sum to said means to compare.

Claim 4

The combination of claim 2 wherein said means to compare comprises a cOmparator amplifier.

Claim 5

The combination of claim 2 wherein said means to provide said average force signal comprises an averaging amplifier connected to receive force signals from each actuator and to provide an output representing the sum of the inputs divided by the number of inputs.

Claim 6

The combination of claim 1 wherein said means to modify comprises means to multiply said average force signal by a constant factor.

Claim 7

A control system for a test structure being vibrated in opposite directions in a vibratory path comprising a plurality of reciprocating actuators connected to said structure in spaced locations for moving said structure in said vibratory path, said structure having at least one axis spaced from some of said actuators by radii of actuator action and about which axis the structure is to be controlled, and having a center of mass spaced from said one axis by a distance equal to x, means to control said actuators for reciprocating movement including servo control means connected to the actuators and providing signals to drive said actuators, and means connected to said servo control means for providing compensation signals for each of said some actuators for balancing the forces exerted by said actuators and for compensating for loading of actuators closest to said center of mass, comprising means connected to each actuator to provide signals representing the force applied to the structure by said actuator, means to provide an average force signal representing the average force actually applied to the structure by each of said actuators, means to weight said average force signal for each of said some actuators by multiplying the average force signal by a quantity substantially equal to (x/Na) where N is the number of actuators controlling the vibratory structure, and a is the distance from the axis of the respective actuator being controlled, and means to provide a force discrepancy signal indicative of the difference between the force actually exerted by the respective actuator and the average force signal modified by said quantity for that actuator.

Claim 8

A control system for a test structure being vibrated in opposite directions in a vibratory path comprising a plurality of reciprocating actuators connected to said structure in spaced locations for moving said structure in said vibratory path, said structure having at least one axis spaced from some of said actuators by radii of actuator action and about which axis the structure is to be controlled, and having a center of mass spaced from said one axis by a distance equal to x, means to control said actuators for reciprocating movement including servo control means connected to the actuators and providing signals to drive said actuators, and means connected to said servo control means for providing compensation signals for each of said some actuators for balancing the forces exerted by said actuators and for compensating for loading of actuators closest to said center of mass, comprising means connected to each actuator to provide signals representing the force applied to the structure by said actuator, means to provide an average force signal representing the average force actually applied to the structure by each of said actuators, means to weight said average force signal for each of said some actuators by multiplying the average force signal by a quantity substantially equal to (x/Na) where N is the number of actuators controlling the vibratory structure, and a is the distance from the axis of the respective actuator being controlled, and means to provide a force discrepancy signal indicative of the difference between the force actually exerted by the respective actuator and the average force signal modified by said quantity for that actuator.

Claim 9

In a test apparatus comprising a structure vibrated in opposite directions with a plurality of reciprocating actuators spaced at separate radii of action from a control axis on said structure and wherein said structure has a center of mass also spaced from said control axis, said apparatus having control means for controlling said actuators including comparison means connected to the actuators to provide a signal representing the difference between the undivided force actually being exerted by each of the actuators, and an average force signal comprising the average of the force exerted by the actuators, the improvement comprising means connected to the comparison means to modify said average force signal for each of said actuators spaced from said axis by a factor relating to the amount of offset of said center of mass from said axis and the respective distances of each of the actuators from said axis before comparing the average force signal with the individual force, including means to receive said average force signal, and to multiply said average force signal by a factor equal to Ki where Ki (x/Na), and where K is the radius of the mass from said axis, a is the radius of the respective actuator from said axis, and N is the number of actuators that are offset from said axis.

Claim 10

In a test apparatus comprising a structure vibrated in opposite directions with a plurality of reciprocating actuators spaced at separate radii of action from a control axis on said structure and wherein said structure has a center of mass also spaced from said control axis, said apparatus having control means for controlling said actuators including comparison means connected to the actuators to provide a signal representing the difference between the undivided force actually being exerted by each of the actuators, and an average force signal comprising the average of the force exerted by the actuators, the improvement comprising means connected to the comparison means to modify said average force signal for each of said actuators spaced from said axis by a factor relating to the amount of offset of said center of mass from said axis and the respective distances of each of the actuators from said axis before comparing the average force signal with the individual force, including means to receive said average force signal, and to multiply said average force signal by a factor equal to Ki where Ki (x/Na), and where K is the radius of the mass from said axis, a is the radius of the respective actuator from said axis, and N is the number of actuators that are offset from said axis.

Claim 11

The combination as specified in claim 6 wherein said structure is controlled about a second axis mutually perpendicular to said first mentioned axis and the center of mass is also offset from said second axis, said means to modify said average force signal including a second means to receive said average force signal and to provide a second signal representing the average force signal multiplied by a factor equal to Gi where Gi (y/Nb), and where y is the radius of said means from said seCond axis, b is the radius of action of the respective actuator from said second axis and N is the number of actuators that are offset from said axis, and said means to modify further includes means to sum the signals from the first mentioned means to multiply and said second means.