The funicular railway, run by a charity, consists of two coaches linked by a steel cable threaded around a drive wheel at the top of the 250m-long slope. Each coach functions as a counterweight for the other; when one goes up, the other goes down. In this way, the only energy required to make the railway go is to overcome the weight of the passengers.
Originally, the funicular railway operated by water; tanks full of water pumped up from the sea filled a tank fitted to the uppermost carriage, providing the ballast to pull up the ascending carriage.
It was electrified in 1921. The latest generation 110V AC electronic control system’s design dates back to 1978. That consisted of a control panel at the bottom of the hill, a manned wheelhouse at the top of the hill, and a loop of multicore cable linking the two. Operators at the bottom of the hill and in the wheelhouse controlled the train with essentially four buttons: run forward, run backward, jog forward and jog backward. As a safety precaution, operators must keep pressed a deadman-type foot switch to work the controls. Emergency stops were also fitted in the wheelhouse and at the cabinet.
HOW IT WORKS
In normal operations, the train carriages were stopped automatically, based on input from limit switches installed along the track. One set indicated the end of normal travel; a second set indicated a more extreme stopping point. To run the main pulley flywheel in the reverse direction, a set of contactors mounted on the output of the drive reversed the polarity of the current fed from the drive to the 44kW brushless DC motor powering the main pulley flywheel.
At that time, the system would have been cutting-edge, as not only did it feature automatic stopping, but also incorporated a first-generation Mentor 1 variable-speed drive from Control Techniques predecessor KTK, whose claim to fame was being the world’s first VSD to incorporate a microprocessor, according to the supplier.
Forty years on, however, electronic control panel components were becoming increasingly hard to find. No longer available from the original suppliers, sourcing parts required increasingly esoteric searching techniques (including internet auction sites).
A second concern was the functional safety of the electronic system, which did not meet modern-day functional safety criteria recalls Gez Evans, The Motion Control Warehouse’s manufacturing director (see also https://is.gd/miyeso). He worked to renovate the system alongside an HSE inspector with the required electronics engineering expertise.
Evans says: “From our investigatory work we found the original safety circuit to consist on single channel limit switches, emergency stops, and overspeed detection devices all wired in series with a basic latching circuit on a singular e-stop contactor. If a contact were to stick and not open when the device was actuated, there would be no failsafe. Likewise if the main e-stop contactor were to stick, there would be no other form of isolation.”
Discussing the project in late October, Evans is at pains to point out that, although antiquated, the system was not unsafe. It was just that such an arrangement didn’t rule out the potential of a catastrophic fault - for example, were the cable running alongside the track to be cut, or if there was a short-out in the safety system, causing the safety functions in the wheelhouse to stop working. However, even that extreme case would not necessarily allow the carriage to plummet down the track. First, the railway travels relatively slowly- 4mph. Second, a mechanical brake acting on a second cable in the track bed acts a final safety brake.
THE NEW ARCHITECTURE
Now, a Schneider Electric safety relay rated to SIL 3 runs at 24V DC on two channels, to reduce the consequences of a broken connection. Second, every e-stop, overspeed sensor and limit switch was wired into its own input into the relay, in parallel rather than the series configuration used before, further increasing safety through diversity. Third, dual e-stop contactors that feature monitored reset were used, so if one contactor were to fail and stick in the system would not reset.
At the centre of the relay is a new Control Techniques Mentor MP DC drive with four-quadrant control technology, for finer bidirectional motion control that occurs inside the drive. This controls motion in all four potential cases: motor going clockwise and controller driving clockwise (which raises one car and lowers the other); motor going clockwise and controller driving anticlockwise (to slow the ascent of the first car and the descent of the second); motor going anticlockwise and controller driving anticlockwise (to raise the second car and lower the first); motor going anticlockwise and controller driving clockwise (to slow the ascent of the second car and the descent of the first).
Physical limit switches were again used for automatic train motion control, but their role has increased, linking car position to drive quadrants and pre-selected ramps and speeds. The rising car increases to top speed and continues travelling at top speed until, toward the end of the line, it hits the first limit switch. That cues the drive to go into deceleration mode. Once the car reaches the station, it hits another limit to indicate a need to stop, after which the speed ramps down to zero. A third set of limit switches set slightly farther is reserved for emergency use in case the car over-travels, and triggers a response from the safety circuit. A similar deployment of limit switches is mirrored on the down slope. To further increase safety, two different types of limit switches were installed, each with a different mechanism, to reduce the risk of common failure modes.
In the final few seconds before final stop, if the system’s inertia happens to exceed braking force, and the motor becomes a generator, the drive technology can accept that energy, convert it into AC and send it into the mains network. This means that the train stops crisply; it doesn’t coast to a stop.
What with the parallel architecture, new limit switches and extra channels, the new network was significantly more complicated than its predecessor. Rather than running multiple lengths of cable all of the way down to the control panel, which would have been expensive and would require transformers along the track to overcome voltage drop, Motion Control Warehouse wired inputs at the wheelhouse into a local remote input/output unit, and laid a single low-voltage RS485 twisted pair cable back down to the control panel. In fact it wasn’t that simple, as the track was too long to run a single length of RS485 cable. Instead, four repeaters were installed at 50m intervals trackside, fed by 24V DC power supply daisy-chained between them.
Despite installing a network architecture that ensured safe operations to modern standards, the retrofit has kept a low profile aesthetically in the wheelhouse and in the control room – which was a departure from normal industrial projects, muses Evans. “You would expect flashy HMIs in an industrial environment to provide loads of information. But they wanted to keep the older style feel in how it is used.
“They didn’t want all new operation stations. They have small pushbutton stations. We were to keep the existing furniture.”
Evans denies that this was an odd request. “Over the years, we’ve restored theme park rides, such as old carousels. They want to keep things simple to help blend in with older equipment, so from an operator point of view what is seen doesn’t change drastically. The clever stuff is inside the cabinet.”