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A GUIDE TO COMMISSIONING
COMMISSIONING GUIDE
7
VARIABLE SPEED SYSTEMS
On systems employing variable speed conveyors, special hardware is required to drive the CTB
(conveyor time base) input on the micro control card. Conveyor systems supplied by LOMA will
already have this hardware fitted and commissioned, but on head only sales the following information
should be observed when using vari-speed.
Overview
In order for the reject timing to work correctly, it must be supplied with information about how far the
contaminant has travelled between the head and point of reject. This is achieved via a shaft encoder
which supplies pulses to the CTB input of the micro control board - refer to the supplied wiring
diagrams.
The delay information, which is entered in the Product Setup menu, then counts the CTB pulses out to
time the contaminated product to the reject point. For this reason the delay is measured in distance,
and should be set at the fastest conveyor speed in order for the reject device to actuate in time. The
dwell, on the other hand, should be set at the slowest belt speed to guarantee accurate rejection at all
speeds. The dwell can be in units of time or distance depending upon the type of reject device.
CTB Constant
This parameter is the number of pulses per metre/foot of belt travel. For systems fitted with a photo-
eye it can be set between 1 and 999 pulses per metre/foot, whereas for non photo-eye systems it can
be set between 1000/coil_gap and 999 pulses per metre/foot. The shaft encoder for the system should
be chosen so that it gives sufficient resolution for accurate rejection, but not so much that the CTB
rate will exceed a frequency of 100Hz at the maximum belt speed. An absolute minimum of 4 pulses
must occur over the coil gap distance. The coil gap distance can be found on the machine details
plate (inside control box).
Vari-Speed Errors
Because the detector can only work within a pre-determined speed range it is possible for belt speed
to move outside the limit of reliable operation of the machine. In these cases, speed logging problems
can be turned on in the configuration menu which will generate either SPEED TOO SLOW or SPEED
TOO FAST run errors.
LOMA Superscan Micro ISC
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