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AM-123 Vibration Monitor Limiter

The Model AM-123 provides effective over-test protection for shakers and test specimens in the event of operator errors or system malfunctions which would otherwise cause the vibration level to exceed the desired test level. The AM-123 can be used for both sine and random testing and can operate with any of today's digital vibration control systems.

  • Operates with any Digital Vibration Controller
  • Smooth, transient free compression of controller drive whenever a limit is exceeded
  • Acceleration and Displacement monitored and limited simultaneously
  • Independent adjustment of acceleration and displacement limits
  • Secondary protector removes power amplifier power if limit is exceeded after compression occurs (option)
  • Charge or Voltage input with dial-a-gain sensitivity adjustment
  • Push-button selection of
    • Sine or Random Modes
    • Acceleration or Displacement Readout
    • Meter Full Scale Range

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AM-123 Protection

 The primary function of the AM-123 is to provide specimen over-test protection, particularly in the following situations:

  1. Loss of Control Signal: The AM-123 provides protection during shaker system start-up period where large servo errors occur, and when conventional protection devices must be deactivated. In addition, the AM-123 allows for step changes in multi-level sine tests without "error" shutdown. The following familiar problem areas are eliminated:
    • Failure to connect accelerometer cable
    • Wrong cable attached in multiple accelerometer array
    • Faulty accelerometer
    • Faulty cable
    • Faulty charge amplifier
  2.  Programming Error: If a programming error is made in the digital controller, the AM-123 will typically protect the test specimen since AM-123 limits are programmed independently and in a different manner.
  3. Fault Conditions: Any fault condition in the controls or the power amplifier which results in a continuing overdrive condition at the shaker will initiate AM-123 limiting. Auxiliary contacts in the AM-123 will shut down the Power Amplifier if input signal compression does not remove the overdrive condition.
  4. Incorrect Control Accelerometer Sensitivity: When testing small to intermediate size specimens where single point control is practical, a separate (redundant) accelerometer can be utilized for the AM-123 input, providing a completely independent data channel for monitoring and limiting.

Digital Vibration Meter

Vibration amplitude is displayed in terms of acceleration or displacement on a 3 1/2 digit meter for either sine or random operation. The decimal point is automatically positioned, and the display indicates the selection of acceleration or displacement with the appropriate units. A "push-to-read" momentary switch and potentiometer adjustment  for displacement  and acceleration limits assure simple, convenient programming. Direct display of each limit value on the digital vibration meter provides an unmistakable indication of the safe operating envelope selected. The limiting values may be set before or during system operation and displayed at any time without interruption of operation.

Charge / Voltage Input

The AM-123 includes an internal charge converter and sensitivity normalizing circuit (Dial-A-Gain) permitting direct inputs from accelerometers with 1-100 pC/g sensitivities. The AM-123 will also accept a low impedance voltage input such as a normalized 10 mV/g signal. The internal charge converter allows the AM-123 to be utilized as a convenient, stand alone vibration monitor.

Vibration Limiter

For limit detection, acceleration and displacement signals are full-wave rectified and averaged. Averaging of the detector signal provides low probability of shutdown due to spurious noise spikes. For sine operation, the AM-123 detection time constant and internal compressor speed are varied as a function of operating frequency utilizing an internal frequency-to-DC converter which operates on a fixed amplitude input from the digital sine controller. Thus, the AM-123 response time is always many times faster than the closed loop response of the sine control. When a limit threshold is detected in the AM-123, drive signal compression is performed. 

Also, this interrupt command may be used to trigger an ABORT in the digital controller. With the digital controller in ABORT, resetting the AM-123 limit detectors and compressor cannot result in a drive signal being applied to the Power Amplifier. Both AM-123 limit detectors are latching, and each over-limit condition is indicated by a front panel LED. If the vibration level exceeds the programmed limit(s) for more than 100 msec, auxiliary contacts place the Power Amplifier into READY (optional amplifier interlock required).

For random operation, the input signal to the Power Amplifier is also interconnected through the AM-123 compressor. In this mode the response time is fixed and smooth signal compression is provided.

Loss of Acceleration Signal (low limit)

The AM-123 protects against start up without an acceleration input signal, or loss of signal during a test. An internal comparator circuit continuously monitors the input accelerometer signal versus AM-123 output voltage to the Power Amplifier. If this relationship departs from a preset value, the compressor circuit is triggered and the front panel LOW LIMIT light indicates this fault mode.


The AM-123 is connected between the signal source (typically a digital controller) and the power amplifier input. The AM-123 acts as a unity gain amplifier under normal conditions. Whenever a displacement limit, acceleration limit, or loss of acceleration signal (LOW LIMIT) is detected, the AM-123 internal compressor circuit smoothly attenuates the drive signal.