Machine vibration and condition monitoring — sensors, cabling and systems

What makes up a measurement chain

1. Sensor and mounting — an AC (IEPE) accelerometer for vibration analysis or a 4–20 mA transmitter for the control system, fixed on a mounting stud, an adhesive mounting pad or a magnetic mount.

2. Cable — integral with the sensor or connected via a connector, with a sheath suited to the operating conditions (PUR, FR, HFFR, FEP, braided).

3. Connection point — a bracket with a BNC socket at the machine, a connection box or switch box for multiple sensors, or a junction box in which the sensor cables are joined to a multi-core cable.

4. Receiving the signal — a portable data collector or meter, a DIN-rail module, a PLC, a DCS or SCADA system, or an online monitoring system.

It is best to start planning from the end: the device that receives the signal determines the sensor type, and the layout of the machines and control cabinets determines the cable routes and the locations of the connection points.

Periodic (offline) monitoring

In periodic measurements, a vibration analyst walks a measurement route with a portable data collector. The sensor can be attached with a magnet for each measurement, but on many machines a permanently mounted sensor, with its cable run to an accessible location, works better. The measurement is then always taken at the same point and in the same direction, without removing guards and without approaching rotating parts.

• Measuring point connections — brackets with BNC sockets and connection modules for single points.

• Connection boxes — a separate socket for each sensor in one enclosure.

• Switch boxes — channel selection with a switch and readout of many sensors through one connector; there are also versions for triaxial sensors and versions with vibration and temperature outputs.

• Data collector cables — connect the measuring point to the instrument.

For a quick condition check without a permanent installation, there are vibration meter kits — portable meters with a sensor, magnet and probe that measure vibration velocity and bearing condition.

Continuous (online) monitoring

In continuous monitoring, the sensor signal goes to the system permanently.

• 4–20 mA transmitters → PLC, DCS, SCADA — the overall vibration value is shown in the control system, with a trend and with warning and alarm thresholds.

• IEPE accelerometers → online monitoring system — the full dynamic signal for automatic spectrum and envelope analysis.

• IEPE accelerometers → DIN-rail modules — the monitoring systems category includes modules that power IEPE sensors and convert their signal to 4–20 mA or 0–10 V outputs; some of them also have a buffered AC output for data collection.

• Alarm functions — vibration switches with a relay output and display units with a vibration switch function, e.g. for HS-420 and HS-422 series sensors.

Modules are powered from power supply modules and installed in module enclosures. The boxes at the machines are connected to the control cabinet with multi-core cables, and the sensor cables are joined to these in junction boxes.

Combined solutions

Periodic and continuous measurements are not mutually exclusive. A 4–20 mA transmitter with an additional AC output sends the overall value to the controller and the dynamic signal to a socket in a connection box used by the vibration analyst. Some switch boxes have terminals prepared for later expansion with an online system, so the installation can be extended in stages.

Commissioning and checking the measurement chain

After installation, check each measurement chain. An accelerometer tester verifies that the sensor and cable work and checks the bias voltage, and an accelerometer simulator supplies a signal of known level that can be used to check monitors and alarm circuits without running the machine. We describe the rules for correct sensor mounting in our article on mounting vibration sensors.

If you are planning a system for many machines, prepare a list of measuring points with the machine type, rotational speed, operating conditions and distance to the control cabinet — this makes it easier to select the sensors, enclosures and cables. You can send the list using the contact form or add the products straight to a quote request.