Industrial Bus Health Test using Fluke 125 ScopeMeter

1. Industrial Bus Health Test using Fluke 125 ScopeMeter Application

2. 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

3. 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?

4. 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?

5. System description (at top of elevator shaft) This particular elevator was of a newer design: - it had the motor and a variable speed drive 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’) motor Variable speed motor drive (in the technical room, at ground floor)

6. So, what next….? • A Fluke 125 ScopeMeter was then used to verify the quality of the signals on this communication link • 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. • 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. ScopeMeter 125 used in Bus Health test and Eyepattern mode reveals that bussignals are submerged in noise

7. 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

8. 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)

9. 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. The manufacturer’s instructions to use shielded signal cable had not been recognized, and consequently had not been implemented. High levels of noise were induced on the communication link, which occaisionally resulted in an incorrect message to stop the elevator. A clear violation of the basic Profibus assembly rules !

10. 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!