There are two types of speed sensors: magnetic pickups and proximity switches. Although they look similar there are significant differences between the operating principles and troubleshooting approach. This section is intended to explain the basic differences.
Electromagnetic Speed Sensors
Electromagnetic speed sensors are also known as Magnetic Pickups (MPUs). The operating principle is based on the fact that as the ferromagnetic pole wheel passes, the speed sensor head will alter the magnetic field in a magnetically biased coil (Figure 18.27). Based on the law of induction, an AC output voltage is generated, with a frequency and amplitude proportional to the speed of the pole wheel.
The output voltage of the magnetic pickup can be affected by the following factors:
- Voltage increases as the surface speed of the monitored magnetic material increases
- voltage decreases as the air gap between the magnetic head and the surface of the gear tooth is increased
- voltage waveform is determined by the size and shape of the gear tooth in relation to the size and shape of the pole piece.
Large engines usually have a larger air gap between the speed pickup probe and the monitored gear.
Ferromagnetic Speed Sensors with Line Amplifier
Proximity speed switches operate well on large engines as they can operate with a large air gap and low surface speeds.
The principle of operation is based on the fact that
a ferromagnetic pole wheel passing the sensor
head influences the voltage in an integrated HAL element magnetic sensor (Figure 18.28). The
voltage is amplified to a square wave signal with
the frequency dependent on the pole wheel speed. When a flywheel tooth is within the sensing range of a proximity switch probe, the output of the switch is nearly equal to the supply voltage. On passing the tooth, the sensor output switches to zero volts until another tooth comes into proximity of the sensor head. The amplitude approximates to the supply voltage and is independent of speed.
Proximity switches require a constant external DC voltage power supply. Depending on the proximity switch probe manufacturer, it can be within a range between 10-30 V and is always applied even when an engine is stationary. This may be from the same supply source as the power for the engine control and safety system. The maximum current rating for proximity switches is 80 mA, but the actual current is much less and depends on the external load.
Speed Pickups’ Diagram Definition and Troubleshooting
There are various symbols to define speed pickups in diagrams (Figure 18.29), they are often represented as RPM-to-Frequency (n/f) converters (Figure 18.30).
The difference between the two types of speed sensors, magnetic pickup, and proximity, are as follows:
Magnetic Pickup (MPU)
- Small gears and high surface speeds
- high frequencies 1kHz and more
- no power supply needed
- sensor output signal +/-1.5 V AC (RMS).
Proximity sensor (PU)
- Large gears and slow surface speeds
- max switching frequency is 500 Hz
- needs a power supply 10-30 V DC.
As both types of speed sensors are called pickups, it is important to determine which type it is.
If it cannot be determined for example by the manufacturer’s manual, catalogs, etc, the easiest way is to count the number of conductors connected to the sensor on the diagram (Figure 18.30):
- 3-wires – Proximity speed sensor (173.SE1)
- 2-wires – Magnetic pickup (167.SE1).
As an MPU has a permanent magnetic field, it can be also checked by bringing magnetic material in proximity to the magnetic head.
When determining the type of speed probe for installations, the manufacturer’s data tables are helpful. An example of a table is given in Table 18.9 for Wartsila L38 engines.
On observing a deficiency in speed monitoring
and to establish if an employed proximity speed sensor is failed and has to be replaced, the power supply across 23.1 (+) and 23.2(-), and the frequency across 23.3 (S) and 23.2 (-), should be measured (Figure 18.30).
If no voltage is detected or the voltage level
has dropped below the limits defined by the manufacturer, eg an open conductor or a power supply earth failure, the frequency would be nil or the signal would become erratic.
The procedure for checking the proximity speed switch is illustrated in Figures 18.31 and 18.32.
To confirm that an MPU is in order, Figure 18.BO, both the output voltage and frequency should be measured from the same terminals 24.1 and 24.2.
For both types of speed probes, if the frequency-carrying conductor becomes short-circuited to ground, some PLC-based applications have been known to interpret this condition as being related to over speed and can shut the engine down. Restarting became impossible until the fault was rectified.
Speed Monitoring Relays
The purpose of a speed monitoring relay is to measure the frequency taken from the speed pickup, compare it with the adjusted set point (0-100% of the relay’s frequency range) then produce an output by using the relay’s internal contacts in the event of reaching a set point.
The example given in Figure 18.33 illustrates the speed control for a Piller auxiliary engine. The frequency from one of its speed proximity sensors is shared in parallel by two-speed monitoring relays (K1 and K14). One relay acts as the ignition speed relay to cut off the starting air supply as soon as the cranking speed is reached. The safety system is engaged after the engine has reached a predetermined RPM. The second relay is used for an overspeed-related safety function. As a consequence, the relay set points are different.
Frequency to Current (F/l) Converters
The example in Figure 18.34 represents an extract from a MAN B&W engine speed monitoring diagram. U1 and U2 are frequency-to-current converters (Figure 18.35) that convert the frequency signals received from proximity speed switches into a 4-20 mA signal (required as an analog input for ABB PLC).
