Technology delivers reliable performance in harsh environments

By Greg Bova, Motion Business Development Manager and Nick Toleos, Motion Control Engineer, Baumer Ltd.

Rotary incremental and absolute encoders can be used in a variety of industrial applications, from speed monitoring on conveyor systems to position control on automated handling machines. Industrial rated encoders function effectively in a range of rotational monitoring applications including motors, drives, automated process machines, robots, and elevators.

Traditional industrial encoders, whether incremental or absolute, can meet the needs of many general industrial motion control applications. However, these devices are more likely to fail when placed in operating environments that subject them to aggressive contaminants, impact, high shock and vibration, long-term submersion in liquids, intensive cleaning procedures or EMI noise.

Many industrial manufacturers like to use standard off the shelf encoders without realizing the total impact of the operating environment on the machine. When standard encoders fail, downtime costs related to encoder failure can quickly grow to several times the cost of the encoder itself.
 
Objective: This article will explain the technology behind heavy-duty encoder designs and illustrate how this technology ensures their long-term survival under extremely challenging conditions. 

This article will also explain the benefits of heavy-duty encoders and will discuss the engineering principals behind their operation. Examples of real world applications will help illustrate uses and benefits.

Challenges Of Harsh End Use Environments

Encoders are commonly used in a range of harsh or challenging end use applications including food/beverage processing, heavy equipment, cranes, specialty vehicles, energy generation equipment; chemical petroleum processing, medical devices, wastewater treatment, shipbuilding and marine vehicles, printing equipment and other indoor and outdoor uses. 

There are three common causes of encoder failure in such harsh environments:

1. Solid particulate or liquid contamination.
2. Mechanical bearing overload.
3. Signal output failure.

As a result of these problems, the encoder will cease to operate or the system will operate erratically.

Ambient temperature variations can accelerate encoder failure rates. During encoder cool down, pressure differences between the outside environment and the inside of the housing can cause the encoder to “breath,” drawing air into the housing.

As the temperature of an encoder’s housing drops, any contained humidity will condense inside of the housing, resulting in the collection of dew on printed circuit boards, wiring and code disk. This liquid ingress can quickly lead to encoder failure.

There are many applications where liquid ingress naturally occurs when encoders come into direct external contact with water, coolants, lubricants, and cleaning agents. Often, these applications are found outdoors or in environments subject to high-pressure wash down such as food and beverage processing facilities.

For example, when yogurt is placed into containers during production, an encoder is often used to monitor the rotation of the rotary table that controls container filling. At the end of a day’s production run, the packaging machine is washed down for sanitation purposes. This process involves cleaning the equipment by spraying high-pressure hot liquid onto the machinery.

Liquids are not the only contaminants that encoders must endure. In harsh environments, encoders are often exposed to particulates such as sand, salt, small wood chips, or dust particles. These particulates will enter the encoder, blocking optical processes and resulting in failure of the device. Paper plants and wood processing are prime examples of environments in which such failure is almost guaranteed with standard rated encoders.

In paper processing, encoders monitor the speed or position of the rotary processes that control the flow of pulp into the machine and the feed of paper onto spools. Paper production is a notoriously dirty process, and the environment is filled with small liquid drops containing pulp.

In its liquid form, pulp collects on nearly everything in the plant, resulting in the formation of a fur on all components, including the encoders. Even standard Industrial encoders with high IP ratings will not withstand such contamination for long. Encoder failure quickly occurs as particulates enter the encoder housings and block the internal optical equipment.

Traditional Encoders & How They Work

There are six main components of every optical encoder.

There are six main components of every optical encoder:

1. The shaft and bearing assembly.
2. The pulse disc.
3. The light source.
4. The grid diaphragm.
5. The photodiode/decoding circuitry.
6. The connector. 

All optical encoders operate in the same basic way.  The light source directs rays through a plane convex lens that focuses the light into a parallel beam.  The light beam passes through the grid diaphragm, which splits it to produce a second beam of light 90° out of phase. 

Light passes from the original and the second channel through a tempered glass, polycarbonate, or metal pulse disc onto the photovoltaic or photodiode array.  The pulse disc turns, creating a light/dark pattern through the clear and opaque segments of the disc.

The light/dark pattern is read and processed by the photodiode array and decoding circuitry.  Light beams A and B are each received by a separate diode and converted into two squarewave signals, 90° out of phase, commonly known as a quadrature output.  The quadrature output is then fed into a controlling device that can process the signal to determine the number of pulses, direction, speed, and other information.

To operate flawlessly, optical encoders require a clean path from their emitter diodes to their receiver array through the pulse disk, which is naturally located directly between the two. The encoder shaft – and therefore the connected pulse disk – rotates, forming a critical interface between the encoder electronics shaft and bearing assembly and the outside environment.

It is not possible to manufacture a perfectly-sealed optical encoder as bearing to shaft assembly will never be perfectly tight.  There must be a clearance that allows the bearing to slide over the shaft during assembly of the encoder.  This clearance creates openings or paths that contaminants can wick through.

Even high IP rated bearings with rubber or plastic lip seals cannot cover all rotational speeds, encoder designs, and mounting positions. All seals are subject to wear, aging and the effects of UV radiation.

Bearings allow the shaft to turn while permitting the housing to remain still.  As they are not designed to support high loads under normal circumstances, mechanical bearings can become overloaded in harsh environments.

Common causes of bearing failure are shock, vibration, and excessive radial and axial loads.  These loads can cause an excessive strain on encoder bearings, forcing them to become noisy and to fail.

On a factory floor, standard industrial encoders can deliver erratic signal output resulting from damage to the sensitive ICs that transmit quadrature output to the controlling device. Such damage can be caused by EMI or radiated noise from factory equipment surrounding the encoder. In large factories, long distances between the encoder to the controlling electronic can impair the quadature output, resulting in intermittent failures.

Heavy Duty Encoders

Firgure 4: With the potential to flow through the earthed encoder housing to the ground, these shaft currents are very dangerous as spark erosion can cause lasting damage to the balls and the bearing surfaces.

Running at speeds up to 30,000 rpm with nearly no wear, heavy duty encoders monitor speed and position in a wide variety of harsh end use applications including wind energy, steel processing, heavy industrial equipment, heavy duty vehicles; oil and gas processing, printing equipment, metal stamping and die casting, and motor and drive control.

Incorporating creative design and process technologies, heavy duty encoders can be used in various applications and environments that would normally cause standard encoders to fail.

The robust design of heavy duty encoders is vastly different from that of standard encoders and allows them to withstand solid or particulate contamination, mechanical bearing failures and signal output failures.

To withstand liquid and solid particulate contamination, heavy duty encoders feature solid die-cast housings and robust designs that permit their reliable operation in open-air applications where normal encoders would fail.

As their first line of defense, heavy duty encoders incorporate labyrinth seals with reverse-lead spiral grooves that prevent the ingress of liquids and particulates into the housing. These seals allow heavy duty encoders to be exposed to moisture, temperature extremes, salt spray, chemicals and vibration above the limits of traditional encoders.

Heavy duty encoders feature larger ball bearings fitted on opposite sides of the solid die-cast housing surrounding sensor electronics. Although these encoders typically take up more space, they feature a highly durable shaft preload condition that can withstand much greater forces in both radial and axial directions.

This robust construction allows harsh-duty encoders to survive shocks up to 500g and operate reliably across a wide range of operating temperatures from -40 to +100° C.

Patented ceramic ball bearings with special isolation resistance between the housing and shaft of the encoder are used to prevent the buildup of shaft currents from large AC motors and generators. With the potential to flow through the earthed encoder housing to the ground, these shaft currents are very dangerous as spark erosion can cause lasting damage to the balls and the bearing surfaces (Figure 4).

In all rotary encoders, sensor electronics and code disk are located inside the housing between the bearings. In heavy duty encoders, code disks are now made of metal instead of glass or plastic. Glass disks are very susceptible to scratching and fracture under shock.

While plastic disks are quite shock resistant, they are more likely to warp and lose their shape at higher temperatures.  Plastic disks are also more likely to break down in chemically aggressive environmental conditions.  Metal disks better withstand shock, heat and chemicals and will not break down in harsh duty applications.

The die cast housing found on heavy duty encoders allows the length of the shaft to be extended between bearings to allow a second encoder device or a mechanical centrifugal switch to be mounted.

The second device triggers an action when a specific speed limit is exceeded and is electronically independent of the first system, providing the redundancy needed in applications where extreme safety is required. 

In all rotary encoders, sensor electronics and code disk are located inside the housing between the bearings. In heavy duty encoders, code disks are now made of metal instead of glass or plastic. Glass disks are very susceptible to scratching and fracture under shock.

For example, if the blades on a wind turbine are turning too fast in a wind storm, the secondary device slows the blades to protect the turbine from damage.

• Heavy duty Shaft Assy (1).
• Insulated Ceramic bearings (2&3).
• Metal Disk instead of Glass or plastic (4).
• Heavy duty Electronic Board (5).
• 300 mA Power Transistors (6).
• Redundant Speed switch (7&8).
• Separated Terminal boxes (9&10).
• Separator plate (11).
• Special seals, labyrinth (12).

To prevent signal output failure, specially-coated large terminal boxes can provide resistance to electro-magnetic compatibility (EMC) fields. Encoders designed with large terminal boxes can be rotated through 180 degrees to position the cable opening to the left or the right of the encoder. This facilitates the encoder’s installation into any system.

Encoder Testing

Heavy duty encoders must be continually tested to ensure a toughness that standard encoders cannot match. Standard testing verifies that heavy duty encoders can withstand trying environments. 

Resonant frequencies are checked during the development stage when the assembled printed circuits are tested and optimized for the resonant frequencies of the components.  This testing ensures that components do not vibrate excessively to the detriment of the output circuit or break loose using a measurement rig with continuously tunable frequency and amplitude (a sine-wave sweep from 10 to 2000 Hz).  Electromagnetic compatibility (EMC) is tested for burst-voltage capability in a pulse voltage test setup basis.

These encoders then go through vibration, continuous shock, dust protection, and water jet testing. The water jet test subjects the encoder to a strong jet stream at a pressure of 12 bars and a flow rate of 100 liters/minute. The encoder is also immersed in water to a depth of 1 meter to test that sealing complies with IP67 standards.
 
Some encoders are exposed to a sprayed salt mist test for marine applications and a humid heat test to verify their tropical suitability. These tests usually ensure that the corrosive failures that happen on standard encoders in harsh environments will not occur on heavy duty encoders in the same environments.

For marine and specialized food grade and wash-down environments, heavy duty encoders with full stainless steel housings utilize a specialized Simmering at seal for deep immersion, and a hermetic encapsulation process that protects the electronics of the encoder from the outside world.

The hermetic encapsulation allows the encoder to work unimpaired by environmental forces as the only thing that can pass through the encapsulation is the magnetic field of the permanent magnet on the encoder. These encoders reach the highest protection classes of IP 68 and IP 69K. 

These heavy duty encoders can be submerged in liquids for long time periods or cleaned with pressure washers without failing, qualities that make these devices ideal for marine applications.

One of the toughest real world applications for heavy duty encoders involves their use on the latest generation of fire extinguisher boats.  In this application, heavy duty encoders manage the positioning of remote-controlled jet pipes that throw tons of seawater per minute over a distance of more than 100 meters directly onto a fire.

In order to effectively control the jet pipes, the encoders are positioned with the shaft side facing upward.  This positioning would allow seawater to rapidly enter a standard industrial encoder. With heavy duty encoders design that relies on hermetic sealing, sea water cannot enter the encoder housing, preventing associated damage to internal electronics.