... from www.positek.com
Conditioning monitoring on high voltage switchgear for the
supply of mains electricity is an important function for preventative
maintenance to ensure continuity of supply to consumers. The
high voltage switchgear at major power distribution facilities
operates in a very harsh environment with very high voltages
and very fast movements.
The condition of the switchgear and mechanism is assessed by
checking the profile of the movement. Because the switch closes
so fast, within a few milli seconds, the sensors measuring the
movement has to have very good frequency response. The P500
rotary sensor has a frequency response of over 10KHz which is
sufficiently fast to give an analogue output with sufficiently
small phase lag. The sensor also provides a 4-20mA output signal
which is more convenient for connection in facilities which
are a long way from the control centre.
The P500 sensor has been tested for a long period and has proven
to be very reliable in the difficult application. The customer
is specifying the P500 sensor across the whole range of switchgear
produced in Europe.
The first support vessel in the USA to be powered by waterjets
is using a Rotary Inductive Position Transducer (P500) from
British company Positek. New Zealand manufacturer Hamilton Jet
supplied the quadruple HM571 waterjet systems for the 140ft
vessel, which can reach speeds of up to 28 knots. The many benefits
of waterjet propulsion, including outstanding manoeuvring capabilities
- necessary for crew and equipment transfer - place high performance
demands on the sensors within the steering and reverse control
system.
The Hamilton waterjets provide manoeuvring by using a reverse
duct that can deflect the jet stream anywhere between maximum
forward and full reverse thrust. In addition a steering deflector
controls the lateral direction of the jet stream. By combining
reverse duct and steering positions, full 360 degree thrust
vectoring is obtained.
Two P500 sensors are used in each hydraulic system to sense
the position of the reverse duct and steering cylinders. Making
very small adjustments of the reverse duct and steering deflector
allows very precise manoeuvring and the ability to 'hover' the
vessel in a given position.
Initially, Hamilton Jet incorporated two potentiometers into
the system but soon found them to be completely inadequate,
as Dick Borrett - Chief Engineer, Control Systems at Hamilton
Jet explains: "In this installation, the potentiometers
were failing after just two weeks in operation as they weren't
robust enough to stand up to the vibration effects. This was
causing mechanical wear on the conductive plastic tracks."
This led him to search for a non-contact position sensor, and
Borrett discovered the Positek P500 product, which offers increased
operational durability compared with potentiometers and better
performance and at lower cost than other inductive sensors.
The P500 is supplied with an internal micro electronic interface
incorporating Positek's unique single chip integrated circuit
design.
"We needed to move quickly and soon bought four of the
devices. Subsequent testing proved them to be extremely satisfactory
in terms of performance and durability. We decided to move over
to the P500 as standard and will continue to do so for future
projects." The accuracy levels required were within 1mm
in terms of actual hydraulic cylinder position which was well
within the capabilities of the P500.
The other problem with potentiometers was that we had to construct
a housing that was oil and water proof to protect them , in
what can be a fairly dirty environment" added Borrett.
"Positek provides a complete 'ready to go' unit. The P500
is totally sealed and enclosed which makes our manufacturing
process a lot easier."
Hamilton Jet was one of the originators of the waterjet system
and has now developed the more powerful HM range of waterjets
for larger sized vessels.
The success of the 140ft support vessel is set to become a
significant landmark for the company, and result in further
business for UK based Positek which has already supplied 650
P500 products to Hamilton.
"We are going through a phase where work is increasing
dramatically, with orders for similar waterjet propulsion systems
coming in from the USA and many other countries around the world."
concludes Borrett.
When valve actuator control supplier Domgas International,
now Limitorque, was looking to utilise an unusual system of
flow control. As Domgas Special Projects Manager, Mike Thurlow,
explained, the actual valve is a "little bit unique"
- a ball valve is being used for flow control and Domgas was
appointed to supply 60 hydraulic systems, complete with micro-processor
for networking, health monitoring and control.
"Conventionally a hydraulic servo-system would be used,
which is a relatively expensive system. We have designed and
engineered a system which provides the accuracy of a typical
servo for half the cost."
For this project, Domgas had developed a system whereby a highly
accurate sensor was required to feed information into the plc
programme - which Limitorque had revised to match the hydraulic
systems to the electric actuator systems.
"Basically the sensor looks at the position of the valve.
The program compares this with the incoming signal - which tells
it where the valve should be. This signal is transmitted to
drive the hydraulic actuator and the valve into the correct
position," says Mike Thurlow.
"The accuracy levels required were highly demanding, 0.4%
for over 90 degrees movement, which means that less than half
of one degree was the level of accuracy demanded." Having
discounted several other options, Mike Thurlow found the answer
to his requirement in a highly accurate position sensor - the
Positek Rotary Inductive Position Sensor.
Launched by Positek in 1994, RIPS® sensor is a non-contacting
sensor which has been developed with an "internal"
electronic interface, utilising unique micro-electronic technology.
It has proved particularly suited to control valve applications,
providing operational durability along with excellent performance
in terms of both accuracy and stability compared with the other
inductive sensors.
"We had looked at straightforward potentiometers and also
at shaft encoders, but neither were suitable as effective solutions
for the accuracy required," says Mike Thurlow.
"Potentiometers are just not accurate enough, particularly
when we were faced with big ambient temperature changes. Shaft
encoders are very expensive and also require additional electronics
- and cheaper versions did not supply the levels of accuracy
we needed.
I first came across Positek at an exhibition symposium and
began discussing the possibilities of utilising the RIPS®
sensor within this application," he adds.
"It provided the accuracy we required, and also had significant
advantages in its price - which seemed very cost-effective.
It is also very compact and well protected. We initially ordered
fifty of the sensors from Positek for the first exercise and
have recently ordered a further twenty."
Mike Thurlow is enthusiastic about the future possibilities
for this system, with the Positek RIPS® sensor an important
component. "We were not previously aware of position sensors
such as RIPS® sensor and believe there is considerable potential
for this application."
