Introducing The Vibration Controller
The digital Vibration Control System (VCS) is a computer system that can conduct close-loop control for vibration shaker table systems. It generates an electronic signal that drives the amplifier which then provides the drive signal to either a hydraulic or electro-dynamic (ED) shaker, or an acoustic driver. The response on the UUT (Unit Under Test) is fed back to the VCS as a feedback control signal.
The response is usually measured with one or more accelerometers. In the close-loop control environment, the control signal must follow certain pre-specified characteristics in the time or frequency domain. These have been defined as Sine, Random, Sine-On-Random, Random-on-Random, Classical Shock, SRS (shock response spectrum), Road Simulation and other forms of control.
Most tests use a single shaker to excite one axis of the structure. More sophisticated tests use multiple shakers or multiple acoustic drivers to excite the structure in multiple directions. When multiple exciters are used, the control system will involve MIMO (multiple-input/multiple-output) cross channel calculations. Since multiple-exciter control is much more sophisticated than single axis control, this paper addresses mainly single axis control.
The control signal refers to one or multiple signals measured from the UUT. If the control signal is not the desired testing profile, adjustments are made to the drive signal until the control signal converges on the desired profile. The control system continuously, in real-time, corrects for the dynamics of the shaker and UUT dynamics to maintain accurate control. Safety checking is enhanced by a distributed processing architecture that handles the control loop independent of the PC host computer. The block diagram below shows the closed-loop control process. Sensors such as accelerometers are used to measure the response of the UUT and provide the control signal.
A random controller will continuously output a random drive signal so that the power spectral density of the control signal has the pre-defined spectrum shape. A pre-defined spectrum shape is called the profile spectrum. Likewise, a pre-defined time waveform can also be used.
A sine controller will continuously output a swept sine signal at a certain voltage so that the control signal, which is also a sine-like signal, will follow the pre-defined amplitude spectrum.
Classical shock controllers use a pre-defined time domain profile. SRS (shock response spectrum) control uses a pre-defined SRS spectrum. A Road Simulation controller uses a very long pre-defined time signal as the control target.
Sine on Random or Random on Random control are also called mixed mode control. Each combines random control with another mode of control, and therefore their testing setup is more complex.
It is estimated that nearly 100% of the controllers on the market provide Random and Sine control. Roughly 50% also contain Classic Shock control. Mixed mode, SRS, Transient History and Road Simulation are less common, and are used in specialized applications. http://www.dtc-techs.com
Even with one excitation source, there are reasons to measure the response at many points on the UUT. Multiple measurement points are used for several purposes, as indicated below:
With multiple control channels, the user will choose different control strategies, such as Average, Maximum or Minimum. For example, an average-strategy might average multiple control signals together in the frequency domain with different weighting factors. A typical VCS might also include additional response channels to be used to monitor critical response points or parameters of the UUT.
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