Vernier Motion Encoder System - Manual (Page 2)

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Calibration of the Vernier Motion Encoder System
This sensor is equipped with circuitry that supports auto-ID. When used with
LabQuest, LabQuest 2, or LabQuest Mini, the data-collection software identifies the
sensor and configures an experiment appropriately.
No calibration of the Vernier Motion Encoder System is necessary. The printed bars
on the track determine the scale, and the cart encoder counts the passage of the bars.
Available units are meters and feet, selectable in software.
In contrast, it is possible and desirable to zero the encoder. Unlike an ultrasonic
motion detector, there is no way for the system to have an unchanging reference
position; it can only count bars from the point at which the cart is placed on the
track. As a result, you may want to move the cart to the receiver end of the track and
zero the reading in software.
The positive direction can be reversed so that readings increase as the cart moves
toward the receiver. A reversed coordinate system is helpful when using two Vernier
Motion Encoder Systems to monitor the motion of two encoder carts, so that the
positive direction is the same in both cases.
Because the track strip must be continuous, the Vernier Motion Encoder System
cannot be used with the Track-to-Track Coupler.
Power
The Motion Encoder Cart requires two AAA batteries. Either NiMH rechargeable
batteries or alkaline disposable batteries can be used. Turn on the cart by pressing the
clear power button on the cart endcap. It will glow blue when power is on. Press
again to turn off. The cart will turn itself off after 20 minutes of inactivity. Any
motion on the track will cause the timer to be reset. The receiver is powered by the
interface.
Battery life depends on use and the range setting. Low battery level may cause
erratic detection of the cart motion, including incorrect velocity signs. Replace the
batteries if this is seen.
Range Setting of the Motion Encoder Cart
The IR transmitter on the cart has two power levels available. The default 1 m
setting conserves battery power. If the cart is used on a 2.2 m track, set the cart to the
higher 2 m power level. If this setting is not used, the receiver will not reliably sense
the position of the cart at the far end of the track. The switch is located inside the
battery compartment.
Use of Two Vernier Motion Encoder Systems on the Same Track
Some experiments require measuring the motion of two carts. This can be done by
purchasing the Motion Encoder Cart and Receiver (order code: MEC-VDS) to add a
second encoder cart, receiver, and strip to your Vernier Motion Encoder System. A
Motion Encoder Receiver is placed at either end of the track, and two Motion
Encoder Carts are used on the track, each with its transmitter facing the unobstructed
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receiver. A second encoder strip must be applied to the track, one on either side of
the center slot.
Consider reversing the direction of one receiver so that the same direction is positive
for each system. Put the carts together, and zero both systems. This will put the carts
on the same coordinate system; if they move together in contact, their position
readings will be the same.
Use of Multiple Vernier Motion Encoder Systems in the Same
Room
Because of the narrow IR beam used for signaling between the cart and receiver,
interference should be rare. However, if one apparatus is apparently interfering with
another, the problem can be resolved by repositioning one of the tracks.
All Motion Encoder Carts are interchangeable; that is, there is no matching of cart to
receiver.
Data-Collection Notes for the Motion Encoder System
 The optical motion encoder can only make relative position measurements, so that
the zero point is initially determined by where on the track the cart is first placed
when the power is on. If you want zero to be near the receiver, initially place the
cart next to the receiver. This behavior is very different from the ultrasonic
Motion Detector, which by default uses a fixed origin near the detector.
 The motion encoder is nearly immune to interference, but it cannot work if the IR
beam between the cart and receiver is blocked. Keep your hand away from this
region.
 Since the zero position (origin) of the encoder depends on where the cart is placed
initially, it is often useful to zero the encoder in software. Place the cart in the
position you want to declare as zero. On LabQuest, tap the meter display to access
the zero command. In Logger Pro, use the toolbar button.
 It can also be useful to reverse the direction of the coordinate system, so that
values increase as the cart moves toward the receiver. Do this from the meter
screen on LabQuest, or by using the sensor popup menu in the Set Up Sensors
dialog for your interface on Logger Pro.
 High data-collection rates are not useful for the motion encoder. Rates above
30 Hz will produce noisy velocity and acceleration graphs because of few counts
during each time period.
 Just like the ultrasonic Motion Detector, it can be useful to adjust the number of
points used to calculate derivatives for velocity and acceleration graphs. Higher
values create quieter graphs, while lower values result in more temporal detail.
Adjust this value in LabQuest preferences or in the Settings for… File menu item
in Logger Pro.