Ask the Experts

Occasionally my electronic components return an error "Ch1 grms too low" during a random test. My initial thought is that the sensor is not providing sufficient output but I'm not sure how to confirm this. What should I be considering to find the root cause of this problem?

The low grms message could be caused by several sources, but from our experience an accelerometer failure is a rare cause. Accelerometers are designed to operate in a high vibration environment and unless the sensor experienced a shock which exceeds its limit or there is some connector damage due to misuse the problem most likely is caused by some of the following:

Unless there is a problem with the vibration controller, cables and accelerometer mounting are the most common cause of grms shutdowns.When this occurs, check the cable condition and connections. Make sure the accelerometer is securely mounted. An accelerometer with a stud mount is best along with a thin layer of grease, or oil between the accelerometer and mounting surface (usually the test fixture). Also make certain that the cables have not endured any kinking or bending in excess of their recommended bend radius which could cause internal conductor damage. The cables should also be properly tied to a surface to reduce any triboelectric effects or disconnection from the sensor.

Another checkpoint is to verify that the connectors are clean and that the cables are installed properly. A connector with debris or damage will provide intermittent, reduced or zero output. It is also essential that cables be secured to a surface with proper strain relief in the area of the accelerometer so they do not interfere with measurements. And of course as with anything else, you get what you pay for so be sure to purchase the best quality possible. Paying a little more for a better cable is overall cheaper that having to re-run a cable line that goes bad.

If you are using IEPE (Isotron) accelerometers, it is easy to check the accelerometer/cable system by measuring the bias voltage.Somesignal conditioners have fault lights to indicate if there is an open or shorted cable. In the absence of a built-in indication, one can install an in-line T connection (usually a BNC connector) at the input of the controller. One side of the T will be connected to he accelerometer/cable and the other side of the T is connected to a DC voltmeter. The center of the T is connected to the signal conditioner's input. If the voltage reading is close to zero volts, this indicates a shorted cable. If the voltage is >20%of the accelerometers specified bias voltage, the cable, or internal electronics are open or non-functional. The proper bias voltage can be found of the accelerometer's calibration certificate or the product data sheet.

If you are using a shaker/chamber combination, make sure the chamber temperature does not exceed the temperature rating of the accelerometer. Many chambers are operated above the maximum temperature of IEPE accelerometers. The solution here is to use a charge-mode accelerometer with a Remote Charge Converter located outside of the chamber environment. There are also some high temperature IEPE accelerometers available like our models65HT and7251HT. Generally speaking, when an IEPE accelerometer is operated outside of its temperature range, it will "shut down" until it has had time to "cool off." Excessive temperatures over time will cause irreparable damage to the accelerometer.

If the fault is not found in these common areas it may be a malfunction within the controller. At times, the input amplitude auto range circuit can fail, or the accelerometer power supply will malfunction (very rare). If the controller is in the manual amplitude range control, make sure the amplitude sensitivity is not too low. There are easy ways to check these functions and the controller manufacturer can assist you with troubleshooting these problems.


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