Transcript Application: Bus Health Test using Fluke 225C

Application
Industrial Bus Health Test
using
Fluke 125 ScopeMeter
Application: Bus Health Test using Fluke 125
Elevator maintenance and Bus Health Test
at Woonstede
• Woonstede owns and maintains over 11000 homes and
appartments in the center of the Netherlands, around the
city of Ede
• Their property includes a number of accomodations and
appartment buildings for the elderly.
• Woonstede has an in-house service- and
maintenance group that performs many
of the maintenance tasks themselves
• Maintenance includes a total of 67
elevators
Application: Bus Health Test at Woonstede
Situation:
• For one particular elevator, reports were received that the
elevator randomly stopped in between floors.
• These complaints were taken serious, as multiple users had
reported such experiences, encountered at different moments in
time
• But during repeated visits
of the service engineer,
the phenomena was
never seen….
• So, how then to find
the root cause?
First findings: a Mains problem
• Electric power was verified using a Fluke 1735 over a 36 hours
timeframe, and a voltage dip was recorded on the supplied mains
This was seen as a potential cause of error for the motor drive unit, that might ‘lock-up’ on it
• So, the power company was informed about
this irregularity and they improved the local
mains system

they installed a new power feed into the building
• No voltage drops were recorded ever since,
but the trouble with the elevator was not
over yet.
So, how to proceed?
System description
This particular elevator was of a newer design:
- it had the motor and a variable speed drive
(at top of elevator shaft)
installed near the top of the shaft
- it had the main control-unit installed in the
‘Technical Room’ near the base
of the elevator shaft
- communications between the
two units was by means of an
RS-485 link, running along
the wall of the elevator shaft
- installed cable was a good
quality, low voltage signal
cable (like ‘telephone wire’)
(in the technical room, at ground floor)
motor
Variable speed
motor drive
So, what next….?
• A Fluke 125 ScopeMeter was then used to verify
the quality of the signals on this communication link
The Fluke 125 is a dual channel oscilloscope and
dual multimeter which has additional capabilities
to verify the performance of industrial buses,
including RS-485 based buses.
• It was quickly seen that –once the motor was running–
excessive noise was present on the signal wires
….. and once the motor stopped, the noise was
gone again
• Further investigations learned that the true source of
this noise was in the Variable Speed motor-drive, sitting
near the top of the elevator shaft.
ScopeMeter 125 used in Bus Health test and Eyepattern
mode reveals that bussignals are submerged in noise
Signal Quality?
The elevator manufacturer had published a recommendation to use
only shielded cable for the link between controller and motor drive
…but during the installation
of this particular elevator
system, such information
had not been recognized….
Cable hanging
down the shaft wall
Second Improvement….
As a test, a new length of cable was installed, using shielded
cable instead
The shielding was properly connected to system ground ,
(chassis of the hardware) on each end of the cable
The noise levels were verified again.
It turned out, the noise level was greatly reduced now, and
the bus signals were within spec.
The cable was then installed for permanent use.
The elevator has not
shown any malfunctioning
since this modification
ScopeMeter 125 measurements
after the new cable was installed
Shielding (brading)
The root cause
The communication link between upper and lower
units of the elevator had been installed using plain,
unshielded and untwisted ‘basic’ signal wire.
Part this cable is really close to the variable speed
motor drive, which –by nature– generates powerful,
high frequency noise.
A clear
violation of the
The manufacturer’s instructions to use shielded signal cable had
not Profibus
basic
been recognized, and consequently had not been implemented. assembly
rules !
High levels of noise were induced on the communication link, which
occaisionally resulted in an incorrect message to stop the elevator.
Conclusions
• Finding the root cause of intermittent errors in a system can be
really difficult
• Continuous measurements over a longer period of time can give
an insight in what is really happening
• Finding an error doens’t necessarily mean you’ve found the root
cause of the problem – there may well be multiple errors
involved!
• Control wires and cables easily pick up noise from the
environment. Therefore, if any potential source of noise is
nearby, better use shielded cable on forehand!
Fluke – Keeping your world up and running!