Support > Knowledge Base > Length Variance > Encoder Tracking I
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This document is the first in a two-part series on variance problems related to encoder tracking in roll forming applications. With so much information on encoder issues, it made sense to break the article into two pieces. The complete article can be downloaded here in its entirety.
Accurate and consistent part lengths are crucial to quality control in roll forming processes. Even if parts are produced in tolerance, a batch of parts that are inconsistent in length will be more rigorously scrutinized by the end-user than a stack where the ends of all the parts line up.
Encoder and material tracking problems account for most product length variations on computer controlled roll forming lines. From the perspective of a length control system, all information regarding the material - direction, speed, distance traveled - comes from a single point, where the encoder measuring wheel contacts the material. Proper tracking of the encoder measuring wheel is therefore critical to machine performance.
Variance can come from sources other than the measuring system. However, these sources are less likely, and tend to be specific to the machine application type. They are more appropriately addressed by machine application in a separate article, rather as a comprehensive article on length control. Also, since encoders are present on most computer controlled roll forming machines, and because the problems associated with encoder tracking and mounting are similar in each application, the encoder system should always be examined first when troubleshooting length variance.
For technicians, maintenance and engineering, locating the source of a length variance can be a daunting task. This article will help educate the troubleshooter and describe methodologies for correctly implementing the encoder system for a roll former, as well as ‘best practices’ for encoder systems on computer controlled roll forming lines.
This portion of the article deals with the core basics of rotary encoders and measuring wheels. This section covers:
- Basic Encoder Function
- Quadrature
- Circumference and Resolution
- Measuring Wheels
Basic Encoder Function
A rotary encoder is a device that transmits digital pulses as a shaft is turned. A wheel is coupled to the end of the shaft, and that wheel rides on the surface to be measured. As material flows under the wheel, the wheel turns the shaft, and the electronics inside the encoder transmit pulses to a control system. A typical encoder and its associated measuring wheel are depicted in figure 1-1.
Figure 1-1
Inside the encoder there is a small disc typically made of plastic or glass. This disc might have hundreds, or even thousands, of slits cut in the outside diameter (see figure 1-2). As the measuring wheel turns, it spins the shaft of the encoder. The disc is housed so the outside edge moves through an infra-red beam. As the slits in the disc move through the beam, the light is chopped into pulses that are picked up by a sensor located behind the disc.
Figure 1-2
The light pulses are converted electronically into digital pulses and sent to the length control system (figure 1-3).
Figure 1-3
The control system must be programmed with the distance that is passing for each pulse. This is the linear distance measured by the wheel per pulse from the encoder, or resolution.
Quadrature
Quadrature is a method of looking at a bi-directional encoder signal and counting 4 pulses for every 1 pulse received, allowing higher resolution and greater accuracy from a lower resolution encoder. It is the directionality of the encoder signals that allows quadrature counting to be implemented.
With quadrature, there are two signal channels, normally called A and B. The encoder’s disc and read head are arranged so the square wave signal from the A and B channels are 90 degrees out of phase. In figure 1-4 time flows from left to right. The A signal (blue) turns on before the B signal (green), indicating forward motion.
Figure 1-4
When B leads A (figure 1-5), reverse direction is indicated. These signals can be seen using an oscilloscope.
Figure 1-5
Circumference and Resolution
Resolution is calculated by dividing the circumference of the measuring wheel by the number of pulses per revolution generated from the encoder. Circumference is the total linear distance traveled for one revolution of the wheel, and is based on wheel diameter. It’s calculated by multiplying the wheel diameter by pi, a mathematical constant approximately equal to 3.14. Once the circumference of the wheel is known, resolution can be calculated by dividing the circumference by the number of pulses per revolution from the encoder.
Example 1:
An encoder and measuring wheel are installed on a roll former. The wheel is 3.815” in diameter, and the encoder is rated for 8000 pulses per revolution. The circumference equation follows:
C = π · D
C = Circumference
π = 3.14
D = Diameter
π · 3.815 = 11.985”
Resolution is calculated by dividing the circumference of the measuring wheel by the number of pulses per revolution from the encoder. The resolution equation follows:
R = C / PPR
R = Resolution
C = Circumference
PPR = Pulses Per Revolution from Encoder
11.985” / 8000 = 0.001498” per pulse
Encoder Measuring Wheels
In example 1, the measuring wheel is assumed to be concentric (round). Even slightly out-of-round wheels can generate inconsistent results, usually in the form of long-short pieces in cases where part lengths are exact multiples of the measuring wheel circumference. For example, to guarantee the error due to non-concentricity to be less than 0.001” (0.0254 mm), the radius of the measuring wheel must be within 0.00016” (0.004064 mm).
For best results, encoder shafts should always be directly coupled to the measuring wheel, and the wheel should always ride directly on the material. Often, maintenance or engineering will construct a bracket that uses a flex (“zero backlash”), dual-spring, or spline couplers to isolate the encoder from the measuring wheel. This is usually done to protect the encoder from material crashes. Encoders are relatively cheap, and if the material has a tendency to crash in a certain area of the machine, this should be a flag that further engineering (or maintenance) of that machine element is required.
All too often, these couplers create long delays in troubleshooting problems with the encoder system. Flex couplers can break and allow backlash, but not so much that the damaged portion of the coupler is easy to see. The inner spring of a dual-spring coupler will break allowing significant backlash, but the problem can’t be seen until the coupler is physically removed. Spline couplers are notorious for inducing backlash over time, as the spider insert begins to wear.
Machine builders will sometimes couple the encoder shaft to a roll tool or pinch roll. This always results in poor encoder tracking. Unless the rolls are designed to ensure perfect tracking between the roll and material movement (i.e. embossing wheels), it is far better to use a separate, low-mass measuring wheel.
Measuring wheels should suit the application. Painted materials like those used for products that will be visible to the consumer in their finished form usually require a measuring wheel that will not mark the material. Metal wheels are generally not acceptable for such processes. There are several types of polymer wheels that are slick enough to avoid scratching the formed part, yet offer enough friction to track well. In these situations, an encoder should be chosen that offers a very low-friction bearing assembly, so there is less friction in the bearing than between the wheel and the material. Encoder shaft bearings that produce too much drag will cause the wheel to slip.
Hardened steel makes an excellent measuring wheel. It can be knurled so that it “bites” the material. This type of wheel will generally mark the material, but if this is aesthetically acceptable for the finished product, less tension is required to track the material. The marks left on material by a knurled wheel can be beneficial as a quick check for tracking quality. Often, a finished part can be held under the glare of shop lights to make the “tic” marks left on the steel visible to the naked eye. The quality of the encoder tracking can be easily checked based on the type of tracking patterns left on the steel.
Magnetic wheels are often used for steel forming, but the wheels tend to collect debris that can scratch paint, or minimally make the encoder wheel out-of-round. While rubber wheels might be commonly used by machinery builders, they should be avoided in applications requiring accuracy and repeatability. Rubber does not consistently compress, and is sensitive to temperature. On hotter days, rubber will expand and compress more. On colder days, rubber contracts and compresses less. These variances cause a change in the effective resolution that will result in part length variances.
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