US Digital

Does US Digital provide encoders with differential line driver outputs?

Yes. US Digital has the E5D, E5MD, E6D, E6MD, E7PD, H15D, H5D, H6D, H6MD, HD25, PED, S5D and S6D encoders, which are US Digital encoders with a built-in line driver board. This board uses an industry standard 26LS31 line driver IC. The line driver outputs are RS-422 compatible and provide differential outputs. Both two and three channel encoders are available with the line driver option. We also have our PC4/PC5 external mini-boards that connect to the output of single-ended encoders to provide line driver outputs.

I am having some problems with noise affecting my encoder signals. What should I do?

There are several ways to improve noise immunity. Separate motor driver wires from encoder wires, and be sure to separate encoder channels from each other (i.e., do not twist CH A with CH B or CH I). Shielded twisted pair cable is recommended for encoder lines. Also, be sure to separate the ground lines of the motor/amplifier and the encoder since high current switching in the motor can cause noise in the encoder output lines. Pull-up resistors help somewhat with noise rejection. Additional circuitry, such as line drivers/receivers and optocouplers, is much more effective in rejecting noise. In really noisy environments, additional circuitry may be the only way to totally eliminate a noise problem.

I am trying to drive a fairly long length of cable. What kind of additional circuitry should I consider?

US Digital makes a variety of cable drivers. For maximum noise immunity and minimum cross talk, a differential cable driver and receiver is recommended for cables longer than the 6 to 10 foot range. Our line drivers use the industry standard 26C31, which sources and sinks 20 milliamps at TTL levels. Each channel pair should drive a twisted pair in a multi-pair cable. US Digital receivers terminate each pair with a 110 Ω resistor to eliminate ringing.

Typically, how much current do the LED and photodetector IC draw?

In the two channel modules and encoders, the LED typically draws about 15 mA and the IC draws about 2 mA. In the three channel modules and encoders, the LED draws about 45 mA and the IC draws about 10 to 12 mA.

What kind of output drives do US Digital encoders have?

The HEDS transmissive optical encoder module does a good job of sinking current (3.2 mA or 3.86 mA to ground), but they dont pull up to +5V as well (40 μA or 200 μA to +5V). The encoder module outputs are open collector with a 65 kΩ internal pull-up resistor. When driving a cable, they will provide good high-to-low transition times. But low-to-high times will stretch out in proportion to the cable length and capacitance. If a cable driver is not used, you can add a pull-up resistor (2.7 kΩ to 3.3 kΩ) to +5V on each output to improve these rise times.

The EM1 transmissive optical encoder module will source and sink 8 mA at TTL levels.

What is an encoder?

An encoder is a sensor of mechanical motion. It translates motion (such as speed, direction, and shaft angle) into electrical signals.

What are the encoder disks made of?

Encoder disks are made of Mylar (.007" thick). The disks are durable, can be handled without damage, can be easily cleaned, and are virtually unbreakable. They can be bent almost in half and will spring back as long as they have not been creased. They are stable over time and temperature and support high-resolution patterns.

What is the encoder module lens made of?

The lens is polycarbonate.

What is the material of the encoder module housing?

For the EM1 & HEDS transmissive optical encoder modules, the material is Ryton R-4, a polyphenylene sulfide which contains 40% glass fiber.

Does US Digital provide optical encoder rotary disks and/or linear strips?

Yes, please see our Optical Encoder Disks page and our Optical Encoder Linear Strip page.

How thick are US Digital encoder disks and/or strips?

Encoder disks and linear strips are 0.007 inches (0.18 mm) in thickness.

On the HEDS-9000/9100/9200 encoder modules, there are two metal straps on either side of the five pinouts. What are they for? Can they be cut or removed?

These are the Ground and VCC lines which carry current to the LED light source. The pinouts, where the user connects Ground and VCC, are on the detector side of the module. The "straps" carry current to the emitter side of the module. If these lines are cut, the LED will not light up and the module will obviously not function.

Can adhesives be used instead of screws?

Screws provide the most secure method for mounting our encoders. If threaded mounting holes are not available, US Digital optical kit encoders can be ordered with a transfer adhesive pre-attached to the mounting surface (T-option). The adhesive sticks immediately on contact. Strength increases with time (up to 24 hours) and with heat (good for 100 C). A centering tool is highly recommended when using the T-option.

Another common adhesive used is RTV, made by GE or Dow Corning (GE 162, Dow Corning 3145) among others. Epoxy can also be used.

How will the gap between the encoder disk and the detector side of the encoder module affect the output?

In general, US Digital encoders are not that sensitive to the encoder/disk gap. This is because the lens collimates the LED light into a parallel beam and keeps the light/dark pattern fairly uniform. However, as the encoder disk gets closer to the detector, the shadows cast by the encoder disk get cleaner and sharper. This results in better light/dark contrast and subsequently somewhat better performance.

What is meant by "artwork side of encoder disk?"

The thick side of the optical encoder module is the emitter (the light source) and the thin side is the detector. The encoder disk is a piece of .007" thick Mylar photographic film with the emulsion pattern developed on one side. The detector (the thin side of the module) sees a slightly cleaner, sharper image when the emulsion side is closest to it. The US Digital EM1 module uses a nonsymmetrical index code. The index channel will only work when the disk is not upside down. A code wheel consists of a disk and a hub. The code wheel cannot normally be installed upside down because it only fits into the encoder housing one way. However, when bare disks (disks without a hub) are purchased, instructions are included that specify which side is up.

What is TIR?

TIR stands for "Total Indicated Runout" and is the total movement that the encoder disk window/bar pattern will have in a radial direction. Thus it is the total eccentricity of the encoder disk. The factors which contribute to TIR are:

• Shaft eccentricity.
• Shaft undersize relative to hub.
• Off-center placement of hub relative to encoder disk.
• Shaft radial play due to bearing tolerance and uneven loading TIR contributes directly to the cycle error and position error of the encoder.

How well do US Digital encoders work in a dusty environment?

US Digital encoders are fairly robust to dusty environments. The encoders look at multiple sets of window and bar pairs. Thus a speck of dust at any one window will be averaged out by the other sets of windows and bars. Because of this averaging effect, a number of windows can be covered completely and the module will still not lose a count. The phased array differential modules that read the disks have plenty of signal margin. An impressive demonstration of this is to color the disk completely black with a Sharpie pen. The encoder will continue to work without errors.

What solvents can be used to clean in the vicinity of the encoder modules?

US Digital has found that almost all solvents (besides water) will attack some part of the encoder module. This includes alcohols and freon-based cleaners. US Digital has not qualified any solvent to be compatible with the encoder modules.

Will US Digital encoders work in a fluid?

US Digital offers a housing seal on many of our encoders (HS-option) and some are available with a shaft seal for harsh environments. Care must be taken to seal the encoder so fluids cannot enter and damage it. After mounting the encoder, you may need to seal it with silicone or a similar sealant.

Will US Digital encoders work in a radioactive environment?

Our encoders have not been tested in a radioactive environment. We believe the encoders will perform just fine in this environment. Regardless, this should be verified with your own tests.

Will US Digital encoders work in a vacuum?

Our encoders have not been tested in a vacuum. We believe the encoders will perform just fine in this environment. Regardless, this should be verified with your own tests.

How can I get a rotary encoder resolution that is not one of the standard resolutions?

US Digital has developed a device that allows you to generate a wide variety of custom resolutions from a standard fixed resolution encoder. We call it the EDIVIDE. When placed in-line with the encoder cable, it can divide the encoder resolution by any integer number by setting the DIP switches.

US Digital routinely makes custom disks for very reasonable costs. There is no minimum order for specials, and lead time is typically around one week. We can laser-cut virtually any ID, OD and hole patterns in the .007" Mylar material. Simply talk with us. We will take care of the details to make your job easier.

If your desired resolution is within 3% of one of our standard resolutions, only a custom disk is required; no mechanical changes are necessary. All of our assemblies will work.

If your desired resolution is greater than 3% away of our standard resolutions, we can stretch or shrink the disk and the optical radius to obtain virtually any resolution. We will select one of our standard encoder modules to read that special disk and give you the mounting specifications. You would need to drill and tap two holes in your mounting surface and use two #4-40 screws to mount the module. We will specify the mounting hole locations since our standard packages will not fit the special disk diameter. You can provide a dust cover if desired.

How closely does the line density of the encoder disk or linear strip have to match the line density of the module?

For modules with less than 200 LPI resolution, a general rule of thumb is that the line densities must match within 5 lines/inch for the module to function (0.2 lines/mm). Encoding performance degrades with mismatch and thus US Digital cannot guarantee encoding specifications when significant mismatch occurs.

What are the minimum and maximum resolutions available with US Digital encoder modules?

Resolution is measured in cycles per revolution. Each cycle can provide 1, 2 or 4 counts depending on how many signal edges the quadrature encoder counts. For example, 1000 CPR can provide 1000, 2000 or 4000 counts or pulses per revolution.

For 1" diameter disks, we can provide 32 CPR to 1250 CPR using either the EM1 or HEDS modules.

For 2" diameter disks, we can provide 64 CPR to 2500 CPR using the EM1 or HEDS modules.

For linear strips we can provide 120 CPI to 500 CPI using the EM1 or HEDS modules.