DRIVE AGAINST TERRORISM
September 2008

With the terrorist threat regularly at heightened levels, security gates are becoming ever more high tech and responsive to emergency demands.

Frontier Pitts, a manufacturer of high performance perimeter security systems has worked with drives supplier Silverteam to develop a heavy duty gate capable of sliding shut in seconds to stop, say, a truck laden with explosives dead in its tracks.

Neil Fairchild, the engineer behind the TerraGate, explains: “The concept is simple; if the gate weighs four or five tonnes and its well supported, its going to stop just about anything on wheels. The problem is shutting something that heavy fast enough, given that you may only have a couple of seconds to react.”

Frontier Pitts offers a number of basic gate designs, but each has to be customised to meet the exact needs of the location. Often, because of space restrictions, a sliding gate is required, and these may have to be 4-5m wide to allow lorries to pass.

There are several options for the opening and closing mechanism, but Fairchild prefers a chain and sprocket drive because of the simplicity and reliability. This has to be motor driven, and in these days of rising energy costs and increasing environmental awareness, he likes to make sure the motor is not oversized yet is powerful enough to cope day in and day out.

“Security gates are generally kept closed, being opened – possibly hundreds of times a day – to let legitimate traffic through,” says Neil. “Given the weather exposure, occasional knock by a careless driver and other factors, robustness has to be a design watchword for us.”

To ensure a long trouble-free working life the gates have to move smoothly, without too much acceleration, at a sensible speed and without crash stops. However, should an emergency arise, all that is out the window - its lockdown in the minimum time!

“All this sounds fairly straightforward, but it took several years from the 9/11 attacks to formalise things like long term reliability, operational health and safety, performance parameters, etc into internationally acceptable standards.”

Once the Standard was agreed (European Has68 and US K12) and Frontier Pitts called in motion experts from Silverteam to begin working on a suitable drive system.

Two operating patterns were needed, regular and emergency. The first uses gentle acceleration and deceleration to avoid unnecessary stress on the drive system, gate and supporting structure. Maximum speed attained in a typical installation would be 500mm/sec, giving a closing time of 10-15 seconds.

For the emergency pattern, the idea is to shut the gate a quickly as possible, accepting shock loads to the system and assuming the structure can withstand the impact stresses. Here the drive is used to generate maximum acceleration and the gate is allowed to slam shut against a ‘faceage’ a rubber strip type shock absorber. Typically the gate will reach a speed of 1000mm/sec or more, meaning that even the widest gate will be secure within seconds.

Silverteam assessed the two patterns and considering the shock loads, the harsh outdoor environment of most applications and the need for long term reliability, suggested a Hitachi SJ 300 variable drive coupled to a Bauer geared motor.

The SJ 300 is specifically designed for highly dynamic applications such as printing presses, textile machinery and machine tools industries; its dynamic performance also make it ideal for lifts and cranes. So its abilities to cope with large loads at high speeds and accelerations made it an ideal choice for the TerraGate.

It uses Using Hitachi's "Advanced Sensorless Vector Control" which offers 200% starting torque and full torque at 0Hz without feedback. For the TerraGate this means maximum acceleration from the very outset, with no over-driving or slipping and no delicate feedback encoder loop.

The integrated Hitachi 32-bit SuperH RISC processor delivers the dynamic response needed for high-end applications. To compensate for possible temperature fluctuations within the motor, which might adversely affect smooth control of the motor, an online/offline motor-autotuning function is included. A newly developed internal control process helps to drastically reduce any rotational fluctuation at low speed, which for Frontier Pitts enhances gate stability and precision stopping during regular operation
In essence the TerraGate is a simple application, requiring brawn rather than brain. However the SJ 300’s host of high tech features means the TerraGate performs its emergency functions with a level of finesse that ensures years of reliable operation.

The SJ300 carries UL and CSA approvals as well as being CE certified for low voltage and EMC Directives. This makes it usable in all countries, a point of great significance to Frontier Pitts, which exports to all corners of the world. It is available from 1.5kW to 132kW, although Frontier Pitts has used only 4kW to 11kW versions so far.