VIBRATION MONITORING SYSTEM

General Application Notes

The I.Q. system is designed to be used as a permanent installation to monitor the vibration level of machinery and provide a warning before serious damage occurs.

It measures the average vibration velocity in the frequency range from approx. 10Hz to1kHz and provides a 4-20mA signal proportional to this. It does not provide any form of analysis of the vibration signal, just an overall value.

Our design philosophy is that this equipment be used as an economical method to warn of impending problems. These can then be analysed at greater depth, if necessary, with more sophisticated portable equipment.

The economical pricing of this technique soon pays for itself when compared with the labour costs involved with scheduled maintenance. The saving of only one unnecessary machine overhaul is usually enough to pay for the system.

Scheduled vibration measuring with portable equipment may be less costly initially but the cost of a trained technician to take regular readings will cost more in the long run. Plus the fact that a catastrophic failure could occur between measurements.

Our technique gives even more appreciable cost savings over a permanent installation of sophisticated vibration analysers.

Why Measure Velocity?

There are three vibration parameters which are usually measured for machine protection, acceleration, velocity, and displacement. Each has its use.

Vibration represents wasted energy and the higher the vibration the greater the energy wasted. That energy is being translated into movement which can ultimately cause damage. If we can measure that energy we will have a good idea of the condition of the machine.

From our high school maths we know that the kinetic energy of a moving object is ¹/₂MV² where M is the mass of the object and V is its velocity. As the vibration energy is proportional to the square of the velocity it is easy to see that if the velocity doubles the vibration energy increases by a factor of 4.

Note that this is independent of vibration frequency. There is no such simple mathematical relationship between energy and acceleration or energy and displacement which does not involve frequenct. Measuring the velocity is therefore a logical choice.

System Components

1. Model 2212IV, 2212VR & 2300

This is the heart of the whole vibration system. The vibration transmitter measures vibration in mm/sec (RMS) and converts this signal to an analogue 4-20mA. The 2212VR has a grounded accelerometer for improved noise immunity however this can cause ground loop problems when used with PLCs and should be used with an isolator.

2. Model 1200 or 1210 - These are often called dual alarmers or process trip amplifiers. These units monitor the 4-20mA signal and are used to set alarm and trip points. They also have an auxiliary power supply for the transmitter. The model 1200 has programmable relay action. The model 1210 has programmable open loop monitoring.

3. Model 1250 - Latching relay. This unit provides two functions :

1. It gives a start up delay to prevent spurious tripping caused by excessive vibration while motors come up to speed. Usually preset for 30 or 60 seconds (check the side label).

2. When an alarm or trip point is reached the relay latches out. This is acts as a means of identifying which transmitter caused the alarm or trip and prevents the system from restarting without the operators knowledge.

4. Model 200IMR - Timer. This timer is used on the trip circuit to prevent spurious tripping on very short bursts of high vibration, for example on a change of load. Using too long a setting here could be dangerous and a timer with a maximum range of 6 seconds is the norm.

5. Model 1120 - Signal Isolator. This is a must when vibration transmitters with grounded accelerometers (eg. 2212VR) are used with PLCs or other distributed systems. The isolator powers the transmitter and provides an optically coupled 4-20mA signal which tracks the 4-20mA in the transmitter.

6. Model DI351- 3,5 Digit Indicator. This is an optional unit to provide indication of the vibration level. It is also loop powered which simplifies wiring. A lower cost alternative would be to use a 4-20mA analogue meter instead.

Typical Systems.

1. Using a PLC

The simplest system is to power the model 2212 vibration transmitter from a 24V DC power supply and connect to a 4-20mA input on a PLC. Alarm and trip points can be set in software along with startup and nuisance trip timing. Because of the possibility of ground loops the vibration transmitter should either have an isolated accelerometer or an isolator on the 4-20mA signal.

2. Basic Stand Alone System (2212 & 1200).

The model 1200 is powered by 220VAC or 110VAC and powers the vibration transmitter. The first relay is used for alarm and the second relay to trip a main contactor somewhere in the motor control circuit. Under normal running both relays are energised. A push button is normally required to provide a manual override for the operator during start up. This system is the most cost effective however the addition of an indicator is recommended.

3. Complete system (2212 + 1210 + 1250 + 200IMR + DI351).

This system provides the best protection for unattended machinery. Spurious trips are minimised by the use of the trip delay timer while alarm latching provides an indication of potential problems.

The vibration level may be checked at any time by reading the indicator.

Settings.

A lot of people don't know where they should set their alarm and trip points. This can be critical and where possible this information should be obtained from the manufacturer of the machine being monitored.

The following suggestion may help if the information is not available. Measure the vibration of the machine running under normal conditions. This can be done using a hand held vibration meter, reading the value of an indicator in the 4-20mA loop, or by turning the set point on the alarm setting until the relay contact switches.

Assuming the machine is in good condition the usual alarm point is 50% above this level and the trip point is 100% above.

Eg. If the machine is running at 2mm/sec the alarm would be 3mm/sec and the trip would be 4mm/sec.

Please note that this information is for guidance only. As each installation is unique we cannot give definitive values. Unless you know the machine well, rather err on the side of caution and set lower values initially

The following chart gives a guide of normal vibration levels for different sizes of machine.

Remember to wire in your alarm and trip circuit to the 1200 relay contacts. If you are using a model 1250 keep in mind that on start up it has an automatic start up delay (normally 30 seconds). Nothing will happen for this period.

UNDERSTANDING AND USING THE MODEL 2212 VIBRATION TRANSMITTERS

Principle of Operation

This instrument uses a built in piezoelectric accelerometer which generates a voltage proportional to the acceleration to which it is subjected. This voltage is integrated to provide a velocity related signal which is then rectified, smoothed and converted into a standard 4-20mA transmission signal. The output amplitude of the 4-20mA signal is directly proportional to the total vibration velocity within the frequency range of approx. 10Hz. to 1000Hz.

The 2212 and 2300 are two wire loop powered transmitters and require a supply of between 12 and 32 volts DC unregulated.

The 2212 is designed to be intrinsically safe in Class 1 Division 2 locations. Certificates are available on request.

The model 2212 is available in three versions to customer order. These may be identified by part number as follows:-

2212V Velocity output. Grounded accelerometer in aluminium housing with electronics.

2212IV Velocity output. Isolated accelerometer in aluminium housing with electronics.

2212VR Velocity output. Grounded accelerometer with pre-amp encapsulated in remote stainless steel assembly, main amplifier in aluminium housing. This is the preferred unit for corrosive atmospheres, where mounting space is limited, or mounting surfaces are not perfectly flat.

The model 2300 has been designed as a replacement for the 2212V, IV and VR in most applications. It is smaller, lighter and less expensive.

Using a grounded accelerometer has the advantage that noise pick-up, particularly 50Hz hum, is minimised. This gives the best accuracy. The disadvantage of using a grounded accelerometer is the possibility of ground loops in the installation, particularly when used with distributed systems such as PLCs. The accelerometer casing is likely to have a potential difference with reference to the system earth and if the transmitter casing is connected to system earth a large error will result. In this case a signal isolator should be used between the 2212V and the receiving instrument.

Using a 2212IV will avoid ground loop problems, however the isolated accelerometer is more likely to pick up background electromagnetic radiation, mainly 50Hz hum, which can lead to linearity errors. A good system ground is essential with the 2212IV.

In general use the following rules to make a choice.

Use the 2212V & 2212VR for single system applications where there is no system ground, or in multiple applications with 4-20mA isolators.

Use the 2212IV when working with distributed systems if a system ground is used and cost is a major consideration.

Although using a grounded accelerometer system with an isolator is more expensive the system can always be guaranteed to give the best results.

The IMC. Quanta Instruments model 1120 isolator has been designed with this application in mind. It is a mains powered 11 pin plug-in module which can be mounted in the control panel. It incorporates a 24V DC power supply to drive the transmitter and a separate 24V supply to source the 4-20mA signal to the receiving instrument. It can also work as a current sink on input and/or output if required.

Installation & Electrical Connection

The vibration transmitter should be mounted as close to the excitation point as possible.

In order to ensure that the best possible results are obtained, the instrument should be mounted on a solid part of the machine with a clean flat surface. Brackets which may themselves be subject to resonant vibration can induce considerable errors in the measurement. Mounting on curved surfaces may introduce errors by causing stress on the internal accelerometer.

The remote pickup assembly of the model 2212VR is in two parts. The threaded section should be fitted to the machine and tightened securely before attaching the housing containing the accelerometer. This eliminate stress on the accelerometer for the most accurate results.

Electrical connections should be by means of flexible cables as armoured cable may create stresses which may result in errors of measurement.

The electrical connection is by a 2 pin non-reversible weather-proof connector. With the unit sitting on a desk with the connector towards you, the left hand pin is positive. Screened cable is not necessary for the 4-20mA signal however the cable should be kept away from mains power leads where possible to minimise interference.

A polarity protection diode is fitted to protect against power supply reversal.

On power-up the output will drive up to approximately 25mA for two or three seconds before settling back to zero.

Maintenance

Since the instrument is of solid state electronic construction, preventive and routine maintenance is reduced to an absolute minimum. Inspection should be carried out from time to time, dependent on operating conditions, to ensure that build up of material on the unit does not effect the response, or that corrosion is not likely to do any damage. In either event appropriate action should be taken depending on the severity of the problem.

From time to time a check measurement should be made with a portable test instrument to ensure that calibration is unchanged. When carrying out this check it is necessary to remember that the accuracy to which vibration severity is measured is not as high as most process measurements (5% is usually regarded as very good) and that the positioning and handling of the portable probe will in itself affect the accuracy. Differences in filter characteristics between different types of instrument will also affect comparisons. Note that all model 2212V units are calibrated to RMS values.

Full calibration of these instruments can only be carried out in the factory using a vibrating table and calibrated test instrument. Fine adjustments to the zero setting may be done when the unit is mounted in its normal operating position if necessary.

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