BRUEL & KJAER 1623 - Magnetic Pickup MM 0002

4.2.1. Photoelectric Tachometer Probe MM 0012
The probe can be directly connected to the TACHOMETER INPUT socket via the 3m long
cable
AO 0158 supplied with the probe. The outer shield of the cable and the centre
conductor carry the signal and the inner shield carries the power supply to the probe
(+ 7,5 V DC). The probe consists of a combined infra-red light source and photo-sensor
fitted with a red filter to reduce its sensitivity to artificial light. When projected onto a
matt black surface the output is almost 0 V, and onto a highly reflective white surface
close to the probe tip the output will rise to a positive level depending on the proximity
and reflectivity of the surface. In practical applications a signal suitable for tuning the
1 623 can be obtained with a distance up to 1 5 mm from a rotating object with a piece
of black adhesive tape fixed so that it passes the projection path of the MM 0012. If the
background is a dark colour a piece of white tape will give a stronger signal. With a
white stripe on a black background a positive going pulse is obtained, see Fig.4.3. The
TRIGGER LEVEL knob should be set on the + side of the zero point (marked by a vertical
line). For a black stripe on a white background the opposite applies. The MM 0012 will
often give a satisfactory signal merely by projecting it onto gear teeth, fan blades etc. A
black felt-tip pen (spirit based) is very useful for marking (wider than 5 mm) stripes on ro-
tating parts instead of using tape. Note that some metallic surfaces, for example ano-
dized aluminium, even though black in colour are very reflective.
+ V
+
t
1623
0
Trigger
1 Level
DC t
Output I
Time-----..
Time----.
770307
Fig.4 .3. Typical output signal from Photoelectric Tachometer Probe
MM 00 1 2. (A) from a white stripe on a black background
and (8) from a black stripe on a white background
4.2.2. Magnetic Pickup MM 0002
18
Magnetic Pickup MM 0002 produces an output signal which reflects disturbances in
the magnetic field surrounding it . It can be connected directly to the TACHOMETER IN-
PUT socket of the 1623 via a microplug to BNC adaptor (JP 0145). It will generate a sui-
table signal for tuning the 1 623 when placed close (less than 1 ,5 mm) to rotating fan or
turbine blades and gear teeth for example. The rotating parts must be of a "magnetic"
material, if this is not the case or where the part is regular, a flywheel for example, one
of the high permeability discs supplied with the transducer can be glued onto the rotat-
ing part. (Check that the 1, 7 gr mass will not upset the balance of the member.) With
the MM 0002 mounted at a distance of 1 ,5 mm from a 60 mm dia. brass rotating part,
and with a high permeability disc glued on the end of the part (e.g. edgewise oriented) a
satisfactory tuning signal could be obtained at rotation speeds above about 500 rpm
("' 8,5 Hz) . Stronger signals could be obtained by reducing the distance between the rotat-
ing part and the MM 0002 and by affixing the high J1 disc on the circumference of the
part.