Mech - User contributions [en]

Rotary Encoder

2006-11-29T06:45:47Z

58.18.170.86:

[[Category:Sensors]]
__NOTOC__

[[image:encoder disk.jpg|200px]]

A digital optical encoder is a device that converts
motion into a sequence of digital pulses. By counting a single
bit or by decoding a set of bits, the pulses can be converted to
relative or absolute position measurements. Encoders have both
linear and rotary configurations, but the most common type is
rotary. Rotary encoders are manufactured in two basic forms: 1) the
absolute encoder where a unique digital word corresponds to each
rotational position of the shaft, and 2) the incremental encoder,
which produces digital pulses as the shaft rotates, allowing
measurement of relative position of shaft. Most rotary encoders
are composed of a glass or plastic slotted disk. As
radial lines in each track interrupt the beam between a
photoemitter-detector pair (or [[Optointerrupter]]), digital pulses are produced.

 

[[image:encoder diagram.png|center]]

<h4>Absolute encoder</h4>

The optical disk of the absolute encoder is designed to
produce a digital word that distinguishes N distinct positions of
the shaft. For example, if there are 8 tracks, the encoder is
capable of producing 256 distinct positions or an angular
resolution of 1.406 (360/256) degrees. The most common types of
numerical encoding used in the absolute encoder are gray and
binary codes. To illustrate the acion of an absolute encoder, the
gray code and natural binary code dsisk track patterns for a
simple 4-track (4-bit) encoder are illustrated in Fig 2 and 3.
The linear patterns and associated timing diagrams are what the
photodetectors sense as the code disk circular tracks rotate with
the shaft. The output bit codes for both coding schemes are
listed in Table 1.

{|
|+
|[[image:gray.jpg|300px]]||[[image:binary.jpg|300px]]
|}

<table border="1" width="37%">
<tr>
<td width="19%">
Decimal code
</td>
<td width="37%">
Rotation range (deg.)
</td>
<td width="22%">
Binary code
</td>
<td width="22%">
Gray code
</td>
</tr>
<tr>
<td width="19%">
0
</td>
<td width="37%">
0-22.5
</td>
<td width="22%">
0000
</td>
<td width="22%">
0000
</td>
</tr>
<tr>
<td width="19%">
1
</td>
<td width="37%">
22.5-45
</td>
<td width="22%">
0001
</td>
<td width="22%">
0001
</td>
</tr>
<tr>
<td width="19%">
2
</td>
<td width="37%">
45-67.5
</td>
<td width="22%">
0010
</td>
<td width="22%">
0011
</td>
</tr>
<tr>
<td width="19%">
3
</td>
<td width="37%">
67.5-90
</td>
<td width="22%">
0011
</td>
<td width="22%">
0010
</td>
</tr>
<tr>
<td width="19%">
4
</td>
<td width="37%">
90-112.5
</td>
<td width="22%">
0100
</td>
<td width="22%">
0110
</td>
</tr>
<tr>
<td width="19%">
5
</td>
<td width="37%">
112.5-135
</td>
<td width="22%">
0101
</td>
<td width="22%">
0111
</td>
</tr>
<tr>
<td width="19%">
6
</td>
<td width="37%">
135-157.5
</td>
<td width="22%">
0110
</td>
<td width="22%">
0101
</td>
</tr>
<tr>
<td width="19%">
7
</td>
<td width="37%">
15.75-180
</td>
<td width="22%">
0111
</td>
<td width="22%">
0100
</td>
</tr>
<tr>
<td width="19%">
8
</td>
<td width="37%">
180-202.5
</td>
<td width="22%">
1000
</td>
<td width="22%">
1100
</td>
</tr>
<tr>
<td width="19%">
9
</td>
<td width="37%">
202.5-225
</td>
<td width="22%">
1001
</td>
<td width="22%">
1101
</td>
</tr>
<tr>
<td width="19%">
10
</td>
<td width="37%">
225-247.5
</td>
<td width="22%">
1010
</td>
<td width="22%">
1111
</td>
</tr>
<tr>
<td width="19%">
11
</td>
<td width="37%">
247.5-270
</td>
<td width="22%">
1011
</td>
<td width="22%">
1110
</td>
</tr>
<tr>
<td width="19%">
12
</td>
<td width="37%">
270-292.5
</td>
<td width="22%">
1100
</td>
<td width="22%">
1010
</td>
</tr>
<tr>
<td width="19%">
13
</td>
<td width="37%">
292.5-315
</td>
<td width="22%">
1101
</td>
<td width="22%">
1011
</td>
</tr>
<tr>
<td width="19%">
14
</td>
<td width="37%">
315-337.5
</td>
<td width="22%">
1110
</td>
<td width="22%">
1001
</td>
</tr>
<tr>
<td width="19%">
15
</td>
<td width="37%">
337.5-360
</td>
<td width="22%">
1111
</td>
<td width="22%">
1000
</td>
</tr>
</table> 
Table 1. 4-Bit gray and natural binary codes
 

[[image:conv.jpg|395px|right|frame|Fig 4. Gray code to binary code conversion]]

The gray code is designed so that only one track (one bit)
will change state for each count transition, unlike the binary
code where multiple tracks (bits) change at certain count
transitions. This effect can be seen clearly in Table 1. For the
gray code, the uncertainty during a transition is only one count,
unlike with the binary code, where the uncertainty could be
multiple counts.

Since the gray code provides data with the least uncertainty
but the natural binary code is the preferred choice for direct
interface to computers and other digital devices, a circuit to
convert from gray to binary code is desirable. Figure 4 shows a
simple circuit that utilizes exclusive OR gates (XOR) to perform
this function.For a gray code to binary code conversion of any
number of bits N, the most signficant bits (MSB) of the binary
and gray code are always identical, and for each other bit, the
binary bit is the exlcusive OR (XOR) combination of adjacent gray
code bits.
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<h4>Incremental encoder</h4>

[[image:increment.jpg|352px|right]]

The incremental encoder, sometimes called a relative
encoder, is simpler in design than the absolute encoder. It
consists of two tracks and two sensors whose outputs are called
channels A and B. As the shaft rotates, pulse trains occur on
these channels at a frequency proportional to the shaft speed,
and the phase relationship between the signals yields the
direction of rotation. The code disk pattern and output signals A
and B are illustrated in Figure 5. By counting the number of
pulses and knowing the resolution of the disk, the angular motion
can be measured. The A and B channels are used to determine the
direction of rotation by assessing which channels "leads" the
other. The signals from the two channels are a 1/4 cycle out of
phase with each other and are known as quadrature signals. Often
a third output channel, called INDEX, yields one pulse per
revolution, which is useful in counting full revolutions. It is
also useful as a reference to define a home base or zero
position.

Figure 5 illustrates two separate tracks for the A and B
channels, but a more common configuration uses a single track
with the A and B sensors offset a 1/4 cycle on the track to yield
the same signal pattern. A single-track code disk is simpler and
cheaper to manufacture.

The quadrature signals A and B can be decoded to yield the
direction of rotation as hown in Figure 6. Decoding transitions
of A and B by using sequential logic circuits in different ways
can provide three different resolutions of the output pulses: 1X,
2X, 4X. 1X resolution only provides a single pulse for each cycle
in one of the signals A or B, 4X resolution provides a pulse at
every edge transition in the two signals A and B providing four
times the 1X resolution. The direction of rotation(clockwise or
counter-clockwise) is determined by the level of one signal
during an edge transition of the second signal. For example, in
the 1X mode, A=<math>\downarrow</math> with B =1 implies a clockwise pulse, and B=<math>\downarrow</math> with A=1 implies a
counter-clockwise pulse. If we only had a single output channel A
or B, it would be impossible to determine the direction of
rotation. Furthermore, shaft jitter around an edge transition in
the single signal woudl result in erroneous pulses..

[[image:quadrature.jpg|500px]]

[[image:encoder parts.png|center]]

<h4>Connecting an Encoder to the PC/104 Stack</h4>

To connect an encoder to the PC/104 stack, you will have to create a suitable ribbon cable connector. The connector that came with the encoder will most likely not match the correct pinout to attach. Consult the [[PC104 I/O]] page to see how to connect the encoder.

==Datasheets==
Datasheet for the Maxon Tacho 103935 encoder: [[Media:maxon_tacho_103935_encoder.pdf]]

==References==
Introduction to Mechatronics and Measurement Systems, Histand & Alciatore, 1999 McGraw Hill

PC104 Power

2006-11-29T06:44:23Z

58.18.170.86: /* Power and battery life */

===Power and battery life===

[[image:Img0067b.jpg|right|300px]]

The whole stack, including the BoB, DAQ, and CPU boards, needs
about 1.3 amps at 5VDC. If you are running the WET54G ethernet
bridge as well, the total will be just under 2A. Any current your
project draws from BoB's 5VDC supply (or its +12 and -12 VDC
supplies) will add to that total.

BoB generates its 5VDC by using a DCDC converter described
earlier. This DCDC converter is quite flexible about its input
voltage; it's happy with anything in the range of 9-18 VDC.
*[[media:DCDC_SC15A.pdf|SC15A11 DCDC converter]]

 

<hr size="1">

[[image:Img0163.jpg|right|300px]]

There are a number of ways to provide 9-18 volts to the DCDC
converter. Most of the time you can use a plug-in adapter. The
one shown provides a regulated 12VDC at up to 1.5A

In practice, the stack (with the wifi bridge) needs 2A at
5VDC, which is 10 watts. The plug-in adapter will be providing
only about 1A at 12VDC, which is 12 watts, to feed the DCDC
converter. The DCDC converter is evidently about about 86%
efficient (10W/12W=86%).

The DCDC converter can provide up to 3A of current at
5VDC.

*[[media:ACDC_Jameco312864.pdf|Wall-plug switching supply 12VDC 1.5A]]

 

<hr size="1">

[[image:Img0165.jpg|right|300px]]

The wall-plug switching power supply has a barrel-type
connector; male on the supply and female on BoB (there is some
ambiguity about the anatomy.)

There are a lot of barrel type connectors. The ones we will be
using for 9-18VDC are 5.5/2.1mm, which are the OD and ID of the
plug respectively.

5.5/2.5mm plugs and jacks are quite common too; both are shown
in the photo. They are hard to distinguish visually.

Note that BoB has a typo; it says 5.5/2.5 but actually uses
5.5/2.1.

We will use the more common center-positive convention, which
is what the symbol in the inset shows.

 

<hr size="1">

[[image:Img0166.jpg|right|300px]]

For mobile projects, you may want to use a rechargeable
battery pack. You can use anything in the 9-18VDC range.

The one shown is rated 9.6VDC, 2200mAH. It consists of 8 1.2
volt NiCd cells in series. That's 21 watt-hours; I found that it
ran the stack (which was consuming 10 watts) for 90 minutes.

[[media:NiCd_2200MaH.pdf|Discharge curves for 1.2V 2200mAH NiCd cell

Warning. If you short a rechargeable battery
pack, very large currents will flow. It can get hot very quickly,
and may catch fire or burst, spattering you with hot caustic
toxic chemicals. Treat it like a car battery.
Having a barrel connector on it helps a lot. Loose wires or clips
are a hazard.

If you are putting a barrel connector on a battery pack, use a
prefabricated molded connector with wires (as shown in this
photo); don't use a do-it-yourself connector (as shown in the
previous photo) which is prone to shorts. Strip one wire from the
battery pack and solder it to the molded cord and insulate it,
before you strip the other wire. This way the battery's wires
can't touch accidentally while you work on them. Test for
center-positive with a voltmeter.

 

<hr size="1">

[[image:Img0162.jpg|right|300px]]

You can charge a variety of battery packs with this smart
charger. I've added a molded inline jack so that it connects
easily to the battery pack. There's an inline fuse, too.

Do not use clips.

*[[media:SmartChargerInstructions.pdf|Charger instructions]]

 

<hr size="1">

[[image:Img0180.jpg|right|300px]]

You are also welcome to make up your own battery pack out of
rechargeable AA cells, which are now easy to get up to 2500mAH.
The smart charger above will handle up to 10 of them in series, a
nominal 12V pack.

Or you can use off-the-shelf chargers like the one shown
here.

 

<hr size="1">

[[image:Img0182.jpg|right|300px]]

12 rechargeable AA cells, 2500mAH each, gives a whopping 36
WH, which ought to run the stack and wifi bridge (at 2A, 5V) for
about 2.5 hours.

 

<hr size="1">
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===Power for your project===

[[image:Img0168.jpg|right|300px]]

You can draw power for the electronics of your project from
BoB's DCDC converters.

Up to 1A at 5VDC and 100mA at +12 and -12 are available. If
you exceed the capabilities of the DCDC converters, they will
"hiccup", shutting down and trying to restart over and over. You
will see the +5 and +/-12 indicator LEDs go on and off if this
happens.

You can use the terminal block to make connections, or a
ribbon cable. The pinout for the ribbon cable is shown above it.
Pin 1 is on the upper right, and is the brown wire in the photo.
It's labeled "0" for ground.

 

<hr size="1">

[[image:Img0170.jpg|right|300px]]

Do not use BoB's supplies for running motors and other high
power things. In fact ideally you should use a separate battery
pack for these, not share the one that powers the stack.

To power the WET54G ethernet bridge, use a molded 5.5/2.5mm
barrel connector and cable, and connect it (center-positive) to
the +5VDC and ground connections of BoB.

Don't connect your 12VDC chargers or wall-plug supplies or
battery packs to the ethernet bridge, which expects 5VDC. We'll
try to stick to this convention:

*5.5/2.1mm barrel connectors ... 12VDC
*5.5/2.5mm barrel connectors ... 5VDC

Suppliers for all parts are listed on the parts list

[[media:BoB_partslist_526_2005-12-28.xls|Parts list]]

 

[[Category:PC104]]