Month: December 2014

December 23, 2014

Options for Linear Position Sensor Technology

There are a number of technologies to choose from when considering the implementation of a linear position sensor.  As CPI’s own sensors focus on the needs of designers of hydraulic cylinders and hydraulic accumulators, it is instructive to review the various technologies being used in this market and the relative tradeoff’s of each, as compared to CPI’s LVDT based Linear Position Sensors.

LVDT based Linear Position Sensors (Traditional)

LVDT’s (Linear Variable Differential Transformers) use magnetic induction to determine position and thus are a completely non-contacting sensor solution. They contain coils wound around a coil form through which a high permeability Nickel-Iron core passes, which is attached to the moving member. Piston displacements generate both a phase angle and differential voltage which are used to infer the direction and displacement of the piston. Signal conditioning and support electronics convert this output to DC voltage or current for ease of integration with control systems. LVDT’s can be engineered with special sealed packaging for industrial and aerospace applications allowing for extreme reliability, high precision and extended temperature range. Disadvantages can include feasibility for measuring large displacements, and cost has historically been higher due to the complexity of production and the support electronics. Modern layer winding techniques and low cost ASIC’s have in recent years however allowed cost
reductions. These solutions can cost effectively displace potentiometer and traditional magnetostrictive solutions in many applications.

Potentiometer Based Position Sensing

Use of a variable resistor attached mechanically to a slider on the piston is one of the cheapest and most conceptually simple detection mechanism’s there is. A large variable resistor is used with a “wiper” attached to the moving piston.  This is a contact based solution requiring real friction between the wiper and the variable resistive element.

Potentiometer based position sensing’s primary issue is its reliance on mechanical contact to provide feedback that causes output signal degradation over time especially in vibration environments. As MTBF are short in a rugged environment, their applications are limited to moisture free, moderate temperature environments where there are smaller stroke lengths and low cost is a primary design consideration.

Magnetostrictive Linear Position Sensors

These position sensors represent the dominant solution for cylinder position sensing in use today. Magnetostriction is a property of a ferromagnetic waveguide, which twists in the presence of a magnetic field, affecting the time for a pulse to travel from one end of the waveguide to the magnet. LPS’s using this technology take advantage of this property to create a non-contacting solution to the problem of linear position detection. The solution is known for being highly accurate and suitable for relatively long measurement ranges (6-120 in).  Inherent in this solution is the use of a waveguide (the “rod”) which must run the length of the cylinder and requires core drilling of the cylinder for insertion.

With magnetostrictive sensors, sagging can be an issue for longer strokes.  Additionally the rods are susceptible to vibration which can cause bending and misalignment which requires them to be replaced. And finally the technology has “dead zones” at the cylinder limits which can be minimized, but not eliminated.

Hybrid Solutions, CPI Linear Position Sensor

As one of the newest hydraulic cylinder / piston accumulator position sensor technologies, CPI’s SL series sensors fit in the class of draw wire sensors but replace the traditional core sensor with a patented sealed LVDT implementation for reliable position measurement in harsh environments. Using a unique linear to rotary to linear mechanism the LVDT can detect displacement at the spool on either the oil or the gas side of the piston.  Using an LVDT that is completely vented, this combination yields a solution that is immune to high pressures, as well as shock, vibration, magnetic field interference and has an extended operating temperature range up to 125C.

While being easy to field retrofit the overall system has high MTBF and accuracy. CPI’s SL series LVDT based Linear Position Sensors are used in accumulators and long stroke hydraulic cylinders where harsh operating environments must be tolerated.

Another significant advantage of the CPI sensor is that a single part number can replace sensors in cylinders of widely varying stroke lengths. No more needing to stock a replacement rod sensor in every single length, now one or two part numbers cover them all.

Draw Wire Sensors

Using a highly flexible steel cable, a spring loaded spool, and a sensor mechanism of some type, Draw-wire displacement sensors measure linear movements. The cable drum is attached to the sensor element which provides a proportional output signal.  Without the use of centered linear alignment, draw wire sensors can measure linear position with high accuracy. They can be configured for wet, dirty or high vibration outdoor environments and applications, and provide installation flexibility. Weaknesses relate more to the sensor mechanism employed, typically a potentiometer (hence the term “string pot) or an optical encoder.

Optical Encoders

Optical encoders read optical gratings that are decoded to offer either relative or absolute position. High end devices can offer up to 18 bits of resolution whereas low cost versions provide resolutions of 10-12 bits. These devices can determine speed, position and direction with extreme accuracy. In order to operate however, these sensors require more complex support electronics and are generally suitable for labs and indoor applications where their relative fragility and high cost can be tolerated.

About CPI Linear Position Sensors

CPI has been producing state of the art linear position sensors since 1997. Our current models represent the most evolved solution to position measurement around.  They are deployed throughout the world in Marine, Ocean Drilling, Mobile hydraulics, and mining applications where only the toughest solutions will do.

Visit our website to learn more at: http://www.cpi-nj.com/

December 22, 2014

High Intensity Safety Switch Applications

There is huge potential in the Oil and Mining industry for calamitous explosions due to equipment failure. While not every incident mirrors the Deepwater Horizon oil spill disaster in the Gulf of Mexico, the close proximity of drilling operations to highly flammable oil or gas makes an environment prime for catastrophe.

CPI waterproof limit switches and thermal switches are one of the few safety limit switches in the world relied on to operate in these high intensity environments. Our waterproof limit switches, and thermal switch products are explicitly designed to withstand extreme heat, wind, sand, dust, moisture, EMI, and vibration. Their toughness makes them highly reliable safety controls used in a wide variety of Oilfield and mining equipment.

There are two major causes of unwanted combustion in offshore drilling sites:

  1. Gas and Diesel Engines
  2. The flammability of airborne chemicals.

Controlling Flammability with Safety Limit Switches

The use of Gas and Diesel engines in the same vicinity of flammable airborne chemicals can be risky business. For one, there is the danger of sparking caused by an improper ignition timing of the engine or the flammable mix of fuel and air. Also in certain conditions, the temperature of the engine, can potentially “autoignite” the encompassing atmosphere.

The main cause of engine overheating is the possibility of uncontrolled overspeed that may reach over 2000 rps. Preventing autoignition entails keeping engine surface temperatures below the autoignition temperatures of the flammable chemicals floating in the air.

Avoiding Engine Overspeed: CPI J4 Series Limit Switches

Detecting and preventing overspeed is handled by ASO valve switches which are used to sense the position of the valve. A closed valve means the engine is in a runaway condition and must quickly shut itself off to avoid conditions similar to what is suspected to be the cause of the Deepwater Horizon explosion of 2010.  Engines used on large generators (2 mega watt range) are required to have ASO valves.

CPI’s J4 type switch can handle the extreme temperatures (configurable for operation up to 400F) making them the best fit for this application. CPI’s J4 type switch can also be designed for application in atmospheric pressure up to 50 PSIG, which is typical for these systems.

CPI’s J4 series of waterproof limit switches are constructed entirely with stainless steel. This is the most appropriate material to stand up to the salt fog atmospheres found offshore where incorporation of an Air Shutoff Valve (and associated switch) is constituted. Additionally CPI’s J4 Switch can easily endure the high vibration environment where ASO valves are used on diesel engines.

About CPI Limit Switches

CPI has been designing and manufacturing extreme use switches and sensors for over half a century. Our J4 series of waterproof limit switches endure the toughest environment applications in the world. They succeed where other switches regularly fail.

Whatever switch you need in your application, our engineers can design, manufacture, and qualify it. We are experts in providing form, fit, and function replacements for other switches that fail in these intense, mission critical environments.

Why wait? CALL CPI ENGINEERING to discuss your oilfield safety requirements today. Or visit us at http:/www.cpi-nj.com

 

December 18, 2014

Limit Switches & Their Unlimited Use

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For over 5 decades, CPI has been manufacturing custom designed waterproof limit switches that have been used for a plethora of applications. Few of their uses, like the panel mounted kill switch for a motorboat engine or the dishwasher door closed detector, are easy to understand. But other more complex applications include high tech industrial equipment and military grade uses. Some of these are top secret, so even we aren’t exactly sure what their uses are.

What brings limit switch customers to our door:

• Custom requirements – i.e. changing an unreliable switch to one that is for a form-fit-and-function replacement

• A high failure rate on a limit switch that alleged to be waterproof

• Operation in a hot, dirty, dusty and wet environment. Our our limit switches succeed in even the most challenging environments

• Safety Applications – When machine or human safety is a concern, then cutting costs by looking for cheap “water resistant” limit switches is a huge risk

Uses of CPI Waterproof Limit Switches

Vermeer Brush Chippers

In this application our client had become frustrated by the lack of water resistance of the previous switch along with other common field failures. CPI was asked to replace a roller actuator style limit switch used as a direction indicator in a brush chipper from Vermeer. In other words, a form-fit-and-function replacement.

“Essentially they just sent us the Cams and said to make it work,” says CPI’s chief application engineer, Pete Gagliardi.

To set a truly waterproof solution, a custom angle mount was developed to fit the existing mounting provisions. Our basic switch was drawn from the E1 series with a sealed roller actuator design. In the CPI solution, the switch was completely sealed in santoprene insuring security from the intense wood chipper operating environment. Also a “right and left” version of the actuator were created. Our field testing allowed our E1243, and E1320 to be deployed as a permanent production solution.

Military Vehicle “Cab Down” Detection – Oshkosh Truck LTAS vehicle

2In this LTAS compliant vehicle developed for the US Army, you must raise the entire cab to access the engine. Furthermore the operating environment of a hot enclosed engine bay in a vehicle design for active military duty… well it does not get any rougher than that.

Due to high failure in dust, dirt and mud, Oshkosh was once again choosing to replace another manufacturers’ switch. Oshkosh chose our E1 series plunger switch for this critical application and we were able to fully qualify it demonstrating improved electromechanical endurance and an extreme resistance to temperature and operating environment particulates. Having a 100% USA designed, tested and manufactured product doesn’t hurt either.

Other applications for Limit switches:

• Safety detection – Safety interlock for I-Beam drill press mounting.

• Safety detection – Pin present/not present detection for a lawnmower deck (J4 series)

• Safety detection – Top of travel detection for crane cable
• Panel Switch – City bus fuel conservation system.

• Safety detection – Torpedo hatch door open/closed.

Limit Switches by CPI

Our line of limit switches can be customized to your requirements and fully tested and qualified in your application, no matter what or where it is. If you have an application tough enough for our limit switches, give us a call today or visit http://www.cpi-nj.com/

December 8, 2014

Design Factors for Hydraulic Linear Actuators

A Linear Actuator generally refers to apparatuses that adapt electro-rotational motion into linear motion.  Hydraulic linear actuators are devices where an unbalanced fluid compression of the piston can generate a force great enough to move an external object.  The size and noise of gas compressors and inclination to leaks at high pressure marks fluid mediums as the best implementation for large force application.  Since hydraulic fluid oils are nearly incompressible, a hydraulic cylinder can provide controlled precise linear displacement of the piston for many kinds of applications.

Hydraulic Linear Actuator Position Sensing

Heavy duty machinery requires linear actuators to provide accurate position sensing while functioning in some of the most corrosive environmental conditions.  Extremely high oil pressure coupled with heat, harsh salt and sand, sea water, EMI, and constant multi-frequency vibration mean that these hydraulic actuator assemblies must have sealing and mechanical designs that minimize the number and complexity of moving parts while providing consistent position sensing.

The CPI SL series of linear position sensors is based on an enhancement to the decades old draw wire sensor technology. Our LVDT technology converts the linear motion of the piston into the rotary movement of the spool which then converts into linear displacement signals accurate to 1mm of position.  Our system can provide absolute position measurement removing the need for calibration.  CPI sensors are designed to omit core drilling of the piston for insertion.

Success with CPI Sensors

Specialized military and industrial markets have relied on Control Products Inc. switch and sensor designs for half a century. All our products are made and manufactured in the USA. If you are in need of a hydraulic linear actuator that requires reliability and longevity in extreme conditions, then look no further then CPI.

Discuss your Linear Actuator design with our engineers today. Visit us at http://www.cpi-nj.com/.