Nubotics Support - Frequently Asked Questions

Sections:

General Questions

WW-01 Questions

Designed for
servo motors

WW-02 Questions

Designed for
Solarbotics
GM-2/3/8/9 motors

WheelWatcher^TM Encoder Frequently Asked Questions

General Questions

Where can I buy it?
Click here!
What happens if the codewheel sticker is not centered on the wheel?
This will result in a wobbling pattern to the signals that repeats for each wheel rotation. It would be hard to correct for this in firmware, so go slowly and take care when mounting the sticker. This won't affect you much if you are only using the WheelWatcher for odometry (distance measurement), but will affect you if you are using it for velocity control.
What do the LEDs mean?
The RED LED (DL1) is connected to the ChA signal, so it pulses on and off 32 times per wheel rotation to show that the WheelWatcher is working.
The GREEN LED (DL2) is connected to the DIR signal, so it turns on when the wheel rotates clockwise (CW) and off when it rotates counter clock wise (CCW).
How can I clean the codewheel stickers?
We recommend only gentle cleaning with a tissue or Q-Tip moistened with clean warm water. Do not use isopropyl (rubbing) alcohol or other cleaners, as they can cloud the silver areas and render them less reflective. They could also damage the adhesive.
Can I run the WheelWatcher from my +6v (or +9v, ...) battery pack?
No. The ICs on the board require Vcc to be between +4.5v and +5.5v. We recommend the use of a regulated +5v supply.
How can I tell how far my robot has travelled?
This is called odometry. Simply count up ChA and/or ChB rising, falling or both edges, or count CLK pulses, as the robot moves. See the next question for clues as to how to convert these counts to real world distance units. Note that wheel slippage or uneven terrain will result in inaccurate distance measurements, no matter what encoder system you use; this is the bane of dead reckoning..

How can I measure the velocity of the wheel?
Two methods are commonly employed. The easiest is to count the number of clock pulses N (or changes to ChA and/or ChB) over a certain interval of time T. Velocity V = N / T in terms of counts per unit time. If T is too small you sometimes won't get any counts, even if the wheel is still turning.

The standard injection molded wheel used with the WheelWatcher is 2.75" in diameter (for O-ring wheels). Pi * D gives a circumference of 8.64". If you are using the CLK signal, the counts per rotation C = 128. That gives C / (PI * D) = 128/8.64 = 14.8 clocks per inch of linear travel.

So, if you measured N using CLK and T in seconds, then V = (N / T) * pi * D / C in terms of inches per second.

The limitation with the first method is that you can only update your servo control pulse width or PWM value every T seconds. For a normal servo, the highest rotation speed is usually around 60 RPM, which is one rotation per second, or 128 CLK pulses per second; at slower speeds, you will get fewer pulses per second. Your update rate (or control loop bandwidth, in control theory language) will be slow.

Another method is to measure the time, using your microcontroller's hardware timers, between each CLK pulse (or ChA or ChB pulse), then take the inverse (1/T) to get counts per unit time. At 60RPM, you could update your servo control pulse value or PWM value every 8 ms instead of every second, and have much better resolution too.

However, regardless of how well aligned the board is and well centered the sticker is, manufacturing error and alignment error will still result in some time variation from clock to clock, so we recommend that your firmware calculates a running average of the time between the 4 most recent pulses when using this method.
My microcontroller is too slow to measure the CLK signals. What do I do?
For slow chips like the Basic Stamp, use the raw quadrature signals instead, such as ChA, ignoring ChB and CLK. If you need your program to know the direction of wheel rotation, you can easily read the DIR signals. While you will get less precision in positioning your robot, you will still get approximately 1/4" resolution.
How can I control the velocity of the wheel?
This is an area of engineering known as control systems theory. There are many methods for using feedback to control velocity of a motor.

In the simplest method, one measures the actual velocity of the wheel, calculates an error signal by subtracting the actual velocity from the commanded or goal velocity, multiplies that by a constant, then feeds that value to the motor. Thus, if the wheel is spinning too slowly, the motor is told to speed up, and vice versa. What I just described is known as a P loop -- proportional control loop -- since the only error term is proportional to the difference in velocity.

One common technique that works better for varying terrain is called a PID loop, which stands for Proportional Integral Differential. Much has been written about this technique, and Google is your friend.

See the Example Code area for examples of P and PI loops using the WheelWatcher for various robot platforms.

WW-01 Questions

Designed for servo motors

Do I need to use the alignment tool?
Yes, it is highly recommended. If the board's sensors are not aligned well with the codewheel sticker, the timing of the various signals will be strongly affected, to the point that you may end up not getting 128 clock pulses per rotation. Further, the time between each clock pulse will not be consistent, but will instead vary from pulse to pulse, usually with a pattern that repeats every 4 clocks. Severe misalignment may result in ChA and ChB not always being in phase with each other, resulting in inconsistent counts.
Do I need to use the adhesive on the back of the board? I might want to remove the WheelWatcher later.
The adhesive helps prevent the board from falling into misalignment if the mounting screws come loose. We chose a type of adhesive that sticks well to many types of plastic, but not so well that you cannot remove the board if you need to.
Why am I not getting good counts from the encoders?
Check the behavior of the LEDs while your servos are turning. Do this either by turning the wheels slowly by hand (rough turning can strip the gears in the servos, so be gentle!) with the power on to the WW-01s but off to the servos, or by using your own test program. The green LEDs should be solid on or off, and should only change state when the direction of wheel rotation changes. The red LEDs should blink on and off, once per stripe -- 32 times on and 32 off for each wheel rotation.

If the green LED is flashing when it shouldn't be, or if the red LED is not pulsing 32 times per rotation, make sure your wheels are on tight. If the wheels are too high above the WW-01 PCBs, there won't be enough light reflected to the sensors on the boards. It is best to use the screws provided with your servos to hold the wheels in tight.
If you are still having problems, check the alignment of the PCB with the AL-02 alignment tool with the wheel temporarily removed. Also, check that the codewheel stickers are flat against the wheel, without bubbles under them, and that they are clean.
What mounting hardware is provided?
We provide four 4-40 x 7/8" flat head machine screws, with a 100 degree head angle so that the screw heads will not rub on the wheel. We also provide four 4-40 k-lock nuts, four .25" x .278" #4 custom spacers, and four .25" x .31" #4 spacers. See the product manual for suggested spacers to use with various popular servo models.
My robot uses DC motors and hand made wheels. Can I use the WheelWatcher WW-01 with it?
Maybe. You will need to mount the WW-01 such that it is aligned with your motor's shaft, and place the codewheel sticker on a flat surface (your wheel or something attached to the motor shaft) so that it is parallel to the surface of the WW-01 PCB. The distance between the top of the 2 sensors on the WW-01 and the codewheel sticker must be close to 1.1 mm (0.043") for it to function.

Beyond that, good luck. We can't support nonstandard motors, wheels, and mounting schemes -- the WW-01 is designed specifically for standard injection molded wheels and standard size RC servos. Let us know how it works for you, though.

WW-02 Questions

Designed for Solarbotics GM2, GM3, GM–8, and GM9 motors

Why am I not getting good counts from the encoders?
Check the behavior of the LEDs while your motors are turning. Do this either by turning the wheels slowly by hand (rough turning can strip the gears in the motors, so be gentle!) with the power on to the WW-02s but off to the motors, or by using your own test program. The green LEDs should be solid on or off, and should only change state when the direction of wheel rotation changes. The red LEDs should blink on and off, once per stripe -- 32 times on and 32 off for each wheel rotation.

If the green LED is flashing when it shouldn't be, or if the red LED is not pulsing 32 times per rotation, make sure your wheels are on tight. If the wheels are too high above the WW-02 PCBs, there won't be enough light reflected to the sensors on the boards. It is best to use screws to hold the wheels in tight.

If you are still having problems, check that the codewheel stickers are flat against the wheel, without bubbles under them, and that they are clean.
What mounting hardware is provided?
The WW-02 is provided without mounting hardware at this time, as the appropriate hardware depends on the chassis to which the motor is attached.
My robot uses nonstandard motors and wheels. Can I use the WheelWatcher WW-02 with it?
Maybe. You will need to mount the WW-02 such that it is aligned with your motor's shaft, and place the codewheel sticker on a flat surface (your wheel or something attached to the motor shaft) so that it is parallel to the surface of the WW-02 PCB. The distance between the top of the 2 sensors on the WW-02 and the codewheel sticker must be close to 1.1 mm (0.043") for it to function.

Beyond that, good luck. We can't support nonstandard motors, wheels, and mounting schemes -- the WW-02 is designed specifically for standard injection molded wheels and Solarbotics GM-2, GM-3, GM-8, and GM-9 motors. Let us know how it works for you, though.

WheelWatcherTM Encoder Frequently Asked Questions

General Questions

WW-01 Questions

WW-02 Questions

WheelWatcher^TM Encoder Frequently Asked Questions