Behind every great day out on the rides at Blackpool Pleasure Beach is some highly sophisticated equipment. And there’s some that’s not quite so sophisticated, as Victor Harris, Business Unit Manager for ERIKS, explained at the latest ERIKS and Festo Machine Safety Event.
The mercury arc rectifier was invented in 1902, using a cold cathode gas-filled tube and a pool of liquefied mercury as a self-restoring cathode. The result was a rugged, long-lasting drive solution, which caught the eye of the engineers at Blackpool Pleasure Beach.
Just four years later, the builders of the latest attraction at the Beach installed a mercury arc rectifier in the drive system of the Sir Hiram Maxim Flying Machines ride.
Now the oldest ride still operating at the Beach, the Flying Machines recently celebrated their 110th anniversary. To mark the occasion, the owners decided to upgrade the original drive system to more modern drive technology.
ERIKS not only has a long customer relationship with Blackpool Pleasure Beach, but was also prepared to take on the challenge of upgrading the drive system within the tight parameters necessary, when the OEMs either couldn’t or wouldn’t.
13 million people visit the Pleasure Beach every year to be – safely – thrilled, so safety is of paramount importance. The upgrade included a safety system incorporating an emergency stop and over-speed, with both the Emergency Stop and Over-Speed Targets initially being Performance Level (PL)e.
Achieving the Emergency Stop Target was relatively easy.
Meeting BS EN 13849, Stop Category 0 requires immediate removal of power to the actuators. This is sometimes considered as an uncontrolled stop, because in some circumstances motion can take some time to cease. This is because the motor may be free to coast to a halt.
However, achieving the Over-Speed Target proved not to be so simple.
The old drive technology and the tacho generator feedback meant that there was a risk the system could run away if tacho feedback was lost. This meant that the over-speed safety solution was an essential requirement.
The plan was that the specification of the modern DC drive would ensure a limited maximum speed for the system, and would also react to encoder feedback loss. Unfortunately, when it came to the verification process, things were not quite as they are in the technical manual.
In fact, when the encoder was disconnected the system surged for around 3 seconds, then tripped on encoder loss.
The reason for this behaviour was that the current control loop on the series field DC motors has a large time constant. Clearly the solution was to fit over-speed detection on the motors. But sometimes with every solution comes another problem...
The Flying Machines ride is built not from metal, but wood. And wood has a tendency to move and vibrate far more than metal. In this case the vibration was enough to cause the sensors to trip at least twice a day, and the over-speed relay to report sensor faults as the sensors went in and out of range.
Some engineers might have been tempted to give up at this point, but for the ERIKS engineers it was simply time for some lateral thinking.
ERIKS consulted extensively with the customer’s insurance company on the one hand, and with numerous over-speed safety relay manufacturers on the other.
The conclusion was to:
- reduce the Performance Level to Level (d)
- install the sensors on the main drive
- reduce the sensor to a single channel, with feedback via a PLC.
This time, the solution worked, the customer was satisfied, and the Sir Hiram Maxim Flying Machines were cleared for take-off.
To watch Victor Harris’s presentation from the Machine Safety Event, click below.