ROM Fundamentals

1. ROM Fundamentals

2. Contents • Introduction • Features • Specification • Basic Theory of Operation • Advanced Issues effecting Operation • Do’s and Don'ts

3. Railway Overhead Measurement (ROM) Introduction The Suparule Railway Overhead Measurement (ROM) system is an ultrasonic measuring system specifically designed for measuring the height and stagger of overhead cables in electrified rail networks. The ROM is the safest way to measure height and stagger as the system requires no contact with the live overhead network and therefore reduces the risk of electrocution due to dirty or damp measuring rods.

4. Railway Overhead Measurement (ROM)Features • Safest method for measurement of height & stagger • No contact with overhead cables • Accurate measurements at the push of a button • Each measurement is time & date stamped • Electronics housed in robust stainless steel fixture • Instrument centres itself between the rails • Instrument can be removed rapidly from the track in an emergency • Backlight on the instrument display facilitates nighttime use • Automatically compensates measurements for current air temperature • Allows user to store up to 200 measurements • Software included for downloading of measurements to PC • Software also allows advanced configuring of instrument

5. Specifications

6. Specifications (ctd.)

7. Basic Theory of Operation • Height & Stagger Trigonometry • Temperature Effects on Accuracy • Difference between 1-wire & 2-wire Modes • Measurement Window • DNC’s and OR’s • Echo Lock On Algorithm

8. Basic Theory of Operation • The instrument consists of a stainless steel fixture which is placed onto a railway track below the wires to be measured • The instrument has a total of four ultrasonic transducers – two housed at each end of the fixture • When the measure button is pressed, the transducers emit ultrasonic bursts and listen for echoes from the wires • The instrumentmeasures the time from transmission of the bursts to the receiving of the echoes • The software in the instrument converts this time data into distance data by calculating the speed of sound at the temperature sensed and multiplying this value by the time data. • It then performs trigonometric calculations to calculate the height and stagger of the overhead cables and displays the results.

9. Height & Stagger Trigonometry

10. Height & Stagger Trigonometry L - distance from left transducer to cable, measured R- distance from right transducer to cable, measured S - stagger (or distance from centre of track), calculated H - Height of the cable above the track, calculated G - distance between centres of transducers, constant

11. Height & Stagger Trigonometry From Pythagoras Theorem:  L2= H2 + (G/2 + S)2 and R2= H2 + (G/2 - S)2 Substituting for H2 gives S = (L2 - R2)/2G Therefore, by knowing G and measuring L and R, stagger S can be determined. Substituting for S into the first two equations gives the height.

12. Height & Stagger Trigonometry The distances L and R are calculated as follows: The software measures the time T from transmission of the ultrasonic burst to receiving the echo from the wire The distance D travelled by the signal during time T is D = Speed of Sound x T The distance from the transducer to the wire is D/2

13. Temperature Effects on Accuracy

14. Difference between 1-Wire and 2-Wire Modes In 1-wire mode the firmware assumes that the first echo from each side corresponds to the lowest wire and the second echo from each side corresponds to the second lowest wire i.e. L2 > L1 and R2 > R1

15. Difference between 1-Wire and 2-Wire Modes In a 2-wire setup, the first echo from the left and first echo from the right do not correspond to the same wire. If 1-wire mode is used in this situation it would calculate the wire to be in between the 2 wires and slightly lower in height. 2-wire mode,therefore, performs a test to determine whether the overhead cables are vertically positioned or horizontally positioned.

16. Measurement Window The angle of the transducers at each end of the ROM, together with the limits of the transmission and receiving electronics means there is a defined “window” within which the wires being measured must be. The window is between 3.8 and 7m in height, and +/- 450mm stagger. Wires outside this window will result in Does Not Compute (DNC) or Out of Range (OR) errors.

17. DNC’s and OR’s If first wire's stagger value is found to be >450 but < 650 mm it is deemed to be Out of Range and the display will show OR. If the reading is >650 mm it is deemed to be a 'Does Not Compute' and the display will show DNC. If the stagger is <450mm, the height is then calculated and if it does not fall within 3.8 to 7m, the value OR is displayed

18. DNC’c explained

19. DNC’c explained In this example shown, the right hand transducer cannot ‘see’ the lower wire. As a result, the transducer returns the echo from the second wire as the first. This causes the unit to assume the situation below right, and the unit tries to match two readings from two separate targets, which in turn leads to a stagger value greater than is possible. The windowing detects this and a DNC error is indicated.

20. Echo Lock-On Algorithm To ensure echo received is from a steady target and not a moving obstruction. For each wire from each side the unit takes two readings. If it does not receive the same value twice in a row, it discards the reading and starts again. With this algorithm, readings from a temporary obstruction between the unit and the wire, or a moving wire, are ignored.

21. Do’s and Don’ts • Ensure ROM battery is fully charged before going on site. • Ensure the ROM is levelled firmly on the tracks. • Ensure the ROM is at right angles to the track. • Ensure when ROM is in position the handle is tightened, to enable quick removal in case of emergency. • Avoid positioning the ROM close to obstacles, e.g. poles, etc. • Avoid using the ROM in rain or snow. • Avoid using ROM in location with high background noise. • Avoid standing in a position that obstructs the transducer signals.