For accurate linear measurement, an encoder plus a measuring wheel is a winning combination. The wheel provides increments obtained directly from the moving surface, providing accurate feedback. Here are a few points to consider when specifying an encoder/measuring wheel solution.
1. Mounting: When applying an encoder / measuring wheel solution, start with determining how you plan to keep the wheel in continuous contact with the surface to be measured. Take note of any locating or access constraints, potential mechanical interference points, debris paths, and cabling runs.
Typically a mounting bracket, pivot arm and spring-load assembly is used to achieve continuous wheel contact. In some cases, the weight of the encoder itself may provide sufficient loading. In most applications, however, we recommend using a spring tension method for uniform performance. In the "old days" these components had to be custom-machined or sourced separately. Fortunately, EPC has taken the hassle out of this step by developing several pre-fabricated options.
EPC offers a variety of mounting brackets and pivot arm assemblies for our shaft encoders. In addition, the TruTrac family of products offers a general duty and a heavy duty integrated measuring wheel/pivot arm/encoder solutions-
The internal adjustable spring in the TruTracTM family of products allows you to locate the encoder in any orientation: vertically, horizontal, even inverted.
Interested? Check out these videos detailing the features of the TR1 and TR3.
2. Wheel Load: Loading of the measuring wheel should be sufficient enough to maintain constant contact with the moving surface so as to avoid slippage due to: surface contaminants (dust, dirt, liquid, etc.), vibration, surface irregularities (seams, texture, etc.) or changes in speed, angle or direction. Bouncing or slippage can result in missed counts and inaccurate feedback. At the same time, loading should not be so extreme as to damage the material surface or cause unnecessary wear on the measuring wheel or encoder bearings. A typical wheel pressure is 3 to 5 pounds, but this can vary from application to application. We recommend testing your measuring wheel assembly under various operating conditions to determine the optimal setting. Both the Accu-Coder TR1 and TR3 have easily adjustable internal springs.
3. Single or Double: A single measuring wheel may be a suitable option for your application, provided you have the ability to maintain alignment of the wheel and moving surface. Single measuring wheels must be aligned perpendicular to the material to avoid error induced by uneven wear and a change in the wheel's effective turning diameter. Also, narrow materials, such as wire, tape or pipe, cannot accommodate a dual wheel system. EPC's AccuCoder Cube, Model 702, Model 725, TR1 and TR3 can be configured to provide a single measuring wheel.
Double measuring wheels result in twice the traction without increased bearing load, reducing the potential for wheel slippage and wheel surface wear. When coupled with a double pivot mount that allows the encoder to rotate freely in multiple axes, the measuring wheels will align with the measured material and maintain equal pressure on both wheels. This is an effective safeguard against mis-alignment, excessive movement or vibration of the measured material.
Download our Application Note and learn how the TR3 solved a linear motion feedback challenge for a customer.
4. Circumference: Measuring wheel circumference should be selected to give the best accuracy within the constraints of the system: mounting, speed, controller or counter frequency, and encoder resolution.
Begin by determining your accuracy requirements. When determining accuracy, keep in mind the impact of other system components, particularly those prone to wear over time and usage, such as wheel surface, bearings, pivot points, etc.
For this example, we are specifying a wheel for a cut-to-length operation with an accuracy of .01" .
We can arrive at the desired circumference and encoder resolution with this simple formula:
Accuracy (A) = Circumference (C)/ Resolution (R).
So for our example:
.01 (A) = 12 (C) / 1200 (R)
A 12" wheel and 1200CPR resolution encoder will achieve the desired results. So will a 6" wheel with 600 CPR and other combinations. However, It's most cost-efficient to settle on a combination of variables that leaves you with a standard encoder resolution and wheel size. For example, if you end up calculating something like a 1923 CPR resolution with a 8.76" circumference wheel, the disk, encoder sensor and wheel would need to be custom items-- not economically feasible for most scenarios.
Many printing applications require calculations based on the Lines-Per-Inch (LPI) or Dots-Per-Inch (DPI) resolution of the printer. To calculate the desired wheel and encoder resolution, multiply the DPI or LPI by the wheel circumference and select a CPR that yields the desired number of lines of resolution per inch of linear displacement.
For example, we want to provide feedback for a 150 DPI printer. We'll consider using a 12" circumference wheel.
150 LPI x 12" circumference = 1800 lines of resolution per rotation, or 1800 CPR. We'll want to confirm that resolution is available with the encoder. EPC shaft encoders and Tru-Trac encoders have a wide range of disc resolutions.
EPC also supplies many different measuring wheel sizes, including but not limited to 6", 12", 1/3 meter, 200 mm sizes. In fact, EPC manufactures a greater variety of measuring wheels than any other encoder company.
Here's EPC's full list of measuring wheels.
Finally, some input scaling is usually possible via your counter or controller if needed. Consult your receiving device's documentation to determine if this is a viable solution.
5. Speed Considerations: When selecting an encoder for use with a measuring wheel, confirm that it is capable of handling both the mechanical and electrical speeds that will be generated in your application. Using our previous example, let's suppose our material is traveling at brisk 1000 ft. per minute, a relatively fast rate for most applications, but not unheard of by us.
Mechanical Speed: At 1000 ft per minute with our 12" wheel example equates to shaft speed of 1000 RPMs, well within the capabilities of EPC shaft encoders, usually 6,000 RPMs and higher.
Maximum Frequency: Based on our first example above we can determine the frequency of the encoder output signal.
1200 pulses per foot x 1000 ft. per minute = 1,200,000 pulses per minute
1,200,000 / 60 = 20,000 pulses per second or 20kHz.
In this case, 20kHz is well within the capabilities of EPC's rotary shaft encoders, but you'll want to confirm your receiving device can accommodate the frequency of the encoder output. For instance, EPC's model TR1 can handle applications with linear speeds up to 3000 feet per minute and electrical frequencies up to 1 MHz, however many controllers or counters have maximum input limits that are lower.
If needed, consider adjusting the encoder CPR or measuring wheel circumference to acheive the desired frequency.
6. Width: Wheel width should be selected in light of factors such as mechanical installation constraints, the traction afforded by the moving surface, material width, softness or hardness of material or surface, and alignment. EPC's integrated solutions, the TR1 and TR3 have wheel widths that have been pre-determined by EPC's engineers to provide optimal alignment and wear for most general applications. EPC's measuring wheel widths range from 0.25" to 1", however, not all width, circumference and material combinations are available.
7. Bore: Be sure to confirm the bore of the measuring wheel matches the shaft diameter of your encoder. Standard bore sizes are 1/4" and 3/8". However, not all wheel widths and materials are available in both bore sizes. EPC shaft encoders suitable for measuring wheels have both 1/4" and 3/8" standard shaft sizes.
8. Wheel Surface: The wheel surface should be chosen to provide optimal traction without unduly compromising wear. Over time, a worn encoder wheel will can introduce count errors.
Material Properties: Give consideration to the properties of the material to be measured: texture, softness, compressibility, cleanliness, etc. You may want to test one or more wheel surfaces to confirm the best choice.
- EPC's rubber wheel surface offers the best traction in most applications, but it can be short lived with some materials.
- The 80 urethane is somewhat harder than the rubber and usually lasts longer.
- The 90 urethane is the hardest of the coated wheels and provides the longest life under the most circumstances at the cost of less traction. Performance may vary depending on your application.
- Knurled aluminum is the most durable but may mar glossy surfaces or slip on some hard materials.
Operating Conditions: Also, review the operating conditions for two factors that can impact the measuring wheel's traction: temperature and surface contaminants. Heat and cold can impact the durability and grip of the measuring wheel surface, as can the presence of moisture, oil, dirt, dust and other contaminants. In some applications involving hot extruded material, adequate cooling of material should be provided prior to contact with a measuring wheel-- to protect the extrusion and/or the wheel. See the chart below regarding operating temperature ranges for wheel surfaces. Tip No. 9 below addresses the subject of debris.
EPC offers two types of measuring wheels in both English and metric circumferences. The first type, Faced Measuring Wheels, has a wide contact surface with a variety of faced coatings, one of which will be suitable for nearly every specialized application.
The second, Rubber Insert Measuring Wheels, uses a replaceable rubber insert, or "spare tire", that is easier to set up and maintain, and fits most general-purpose requirements. In some high-wear, high-accuracy applications, periodic measurement of the measuring wheel is advised to ensure accurate encoder feedback.
The Selection Guide below offers some recommendations for a various materials and surface.
Recommended Use for Measuring Wheels
-40° F to +155° F
-40° F to +155° F
|* 90 urethane is a more durable and performs better for tracking rough or hard fibers than the slightly softer 80 urethane material. The above recommendations are only guidelines. Performance may vary depending on your application. Contact Customer Service for specification assistance.|
9. Debris: Debris collecting on a measuring wheel will increase the effective diameter of the wheel and cause potentially unacceptable error. If there will be significant debris in your application, it is best to install the measuring wheel encoder in a location that is least likely to have the debris collect on the wheel. Rather than mounting the measuring wheel on the top surface of a conveyor belt, mount it upside down and on the interior surface of the belt.
If not possible, then installing a brush on the measured material just ahead of the wheel, or in contact with the wheel itself can reduce or even eliminate this problem.
10. Limit radial loading: When it's time to install the measuring wheel on the encoder, be sure to place the wheel as close to the encoder housing as possible. This will minimize radial loading to the encoder bearings. See our previous blog post on the subject. TR1 and TR3 measuring wheels are applied in the optimal shaft position at the factory.
In the world of industrial automation, selecting and specifying a measuring wheel is not a difficult task compared to some others. However, understanding the application and product variables, features, constraints and potential "gotchas" will ensure the proper measuring wheel is installed. When done correctly, the result is trouble free operation and accurate feedback.