Parallax 28044 Manuel utilisateur

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Parallax Laser Range Finder (#28044)

Designed in conjunction with Grand Idea Studio (www.grandideastudio.com), the Parallax Laser Range

Finder (LRF) Module is a distance-measuring instrument that uses laser technology to calculate the

distance to a targeted object. The design uses a Propeller processor, CMOS camera, and laser diode to

create a low-cost laser range finder. Distance to a targeted object is calculated by optical triangulation

using simple trigonometry between the centroid of laser light, camera, and object.

Features

Optimal measurement range of 6–48

inches (15–122 cm) with an accuracy

error <5%, average 3%

Maximum object detection distance of

~8 feet (2.4 meters)

Compact module with integrated CMOS

camera and laser system

Single row, 4-pin, 0.1” header for easy

connection to a host system

All engineering materials released as

open source under a Creative Commons

license; see page 11

Key Specifications

Power Requirements: 5 VDC @ 150 mA

Communication: Asynchronous serial

300–115,200 baud with automatic baud

rate detection

Operating temperature: 32 to 122 °F

(0 to 50 °C)

Dimensions: 3.95" W x 1.55" H x 0.67" D

(10.05 W x 3.95 H x 1.7 D cm)

Application Ideas

Distance or liquid level measurements

Object detection and/or avoidance

Item counting

Table of Contents

Connections ..................................................2

Usage ...........................................................2

Command Set................................................3

Range and Accuracy.......................................8

Error Modes...................................................9

Electrical Characteristics ...............................10

Hardware Design .........................................10

Open Source Files ........................................11

Additional Resources.................................... 11

LRF Image Viewer ....................................... 12

Theory of Operation..................................... 14

Camera Interface......................................... 17

Care and Handling ....................................... 20

Safety......................................................... 20

Example Code ............................................. 21

Connections

The LRF Module interfaces to any host microcontroller or computer system using only four connections

(GND, VCC, SOUT, SIN).

Pin Name Type Function

1 GND G System ground. Connect to power supply’s ground (GND) terminal.

2 VCC P System power, 5 VDC input.

3 SOUT O Serial output to host. 5 V TTL-level interface, non-inverted, 8 data bits, no

parity, 1 stop bit, baud rate matched to host.

Note: The LRF Module hardware design provides bidirectional communication

capability for the SOUT pin. This allows the use of a standard three-pin servo

extension cable (Parallax #805-00001) for connecting the LRF Module to the

host. However, bidirectional communication requires changes to the LRF

Module firmware, not currently supported by Parallax.

4 SIN I Serial input from host. 3.3 V to 5 V TTL-level interface, non-inverted, 8 data

bits, no parity, 1 stop bit, baud rate matched to host.

Type: I = Input, O = Output, P = Power, G = Ground

Use the following example circuit for connecting the Parallax Laser Range Finder Module:

Usage

The LRF Module is controlled by the host via a serial communications interface. To use, simply align the

LRF Module towards the target object and send the desired command.

The serial interface is configured for 8 data bits, no parity, 1 stop bit (8N1). Automatic baud rate

detection occurs on initial power-up of the LRF Module. The Module waits for a “U” character to be sent

by the host and will set its baud rate to match that of the host. Supported baud rates include 300, 600,

1200, 2400, 4800, 9600, 19200, 38400, 57600, and 115200.

When the LRF is ready to receive commands, it will send a “:” to the host. The LRF waits in an idle state

until it receives a valid command, at which time it performs the command and returns command-specific

data (if any). The LRF will return a “?” upon receiving an invalid command.

The camera system used in the LRF Module has automatic white balance, automatic exposure, and

automatic gain control enabled by default, and will automatically adjust its image to account for sudden

changes in lighting conditions. However, the LRF works best in a controlled environment, such as indoors

with minimal changes in brightness across the frame. The LRF is also less reliable when the laser is

shining onto a bright object (for example, a white piece of paper), since the background subtraction done

during image processing could potentially “subtract” the bright laser from the already bright frame. Giving

the camera time for its automatic white balance and automatic exposure to settle helps a bit to make the

laser spot stand out. Using a red filter over the camera will also help the red laser spot become more

visible to the camera in certain situations. See the “E” command in the Command Set section below, and

Range and Accuracy, page 8, for more details.

Status Indicator

A visual indication of the LRF Module’s operating state is given with the on-board LED (Light-Emitting

Diode). The LED is located on the back side of the LRF near the center of the board. The LED denotes

four states of the Module:

1. Green: Idle state. Waiting for a valid command to be sent by the host.

2. Red: Active state. For example, performing a range calculation or capturing an image with the

camera.

3. Orange (Solid): Baud rate detection state. The LRF is waiting for a “U” character to be sent by

the host in order to automatically set the communications baud rate. Occurs on LRF power-up

only. See, Usage, page 2, for more details.

4. Orange (Blinking): Error state. The LRF has malfunctioned. A message identifying the failed

operation will be transmitted on the SOUT (Serial Out) pin. See Error Modes, page 9, for

more details.

If the LED is OFF, the LRF may not be receiving power.

Command Set

All commands are single-byte, ASCII printable values and are not case-sensitive (upper case and lower

case will both work). The basic and advanced command sets are described below.

Basic Commands

R Single range measurement (returns a 4-digit decimal value in millimeters)

B Single range measurement (returns a 4-byte binary value in millimeters)

L Repeated range measurement (any subsequent byte will stop the loop)

E Adjust camera for current lighting conditions

S Reset camera to initial settings

V Print version information

H Print list of available commands

Advanced Commands

O Display coordinate, mass, and centroid information for all detected blobs

X Calibrate camera system for range finding

G Capture & send single frame (8 bits/pixel grayscale @ 160x128)

C Capture & send single frame (16 bits/pixel YUV422 color @ 640x16) w/ laser enabled

P Capture & send image processed frame (16 bits/pixel YUV422 color @ 640x16) w/

background subtraction

Command Details

R: Single range measurement (decimal)

Takes a single range finding measurement and returns the distance to the target object as a printable

ASCII string (“D = “) with a

4-digit decimal

value in millimeters.

The LRF Module is most accurate within its optimal measurement range of 6–48 inches (15–122 cm). In

ideal conditions, a distance measurement of up to ~8 feet (2.4 meters) may be possible. While the

accuracy will be hindered at distances outside of the optimal range, the LRF could still be used for gross

distance measurements or simple object detection. See Range and Accuracy, page 8, for more details.

Example:

D = 0288 mm

B: Single range measurement (binary)

Takes a single range finding measurement and returns the distance to the target object as a

4-byte

binary

value in millimeters. Data is sent MSB first.

The LRF Module is most accurate within its optimal measurement range of 6–48 inches (15–122 cm). In

ideal conditions, a distance measurement of up to ~8 feet (2.4 meters) may be possible. While the

accuracy will be hindered at distances outside of the optimal range, the LRF could still be used for gross

distance measurements or simple object detection. See Range and Accuracy, page 8, for more details.

Example:

L: Repeated range measurement

Continuously calls the Single range measurement (decimal) command (“R”). Once the loop has started,

any byte sent to the LRF Module will stop the command.

Example:

D = 0288 mm

D = 0289 mm

E: Adjust camera for current lighting conditions

Calibrates the LRF Module’s camera for the current lighting conditions. This may aid the camera in

successfully detecting the laser spot within the frame, which increases accuracy of the range finding

functionality.

The command first ensures that automatic white balance (AWB), automatic exposure control (AEC), and

automatic gain control (AGC) are enabled. Then, after a 10 second delay for the camera image to settle,

AWB, AEC, and AGC are disabled. The EV/exposure level is also reduced to a minimum value. This will

cause the entire camera frame, including any previously bright areas, to appear dark. A bright laser spot

may now be more easily identifiable within the frame.

Example:

The camera’s settings can be reset using the “S” command.

S: Reset camera to initial settings

Resets and initializes the LRF Module’s camera to its default, power-up configuration, including enabling

automatic white balance (AWB), automatic exposure control (AEC), and automatic gain control (AGC). It

may take up to 10 seconds for the camera image to settle after reset.

This command is particularly helpful when the “E” command has been used to adjust the camera for

current lighting conditions and the user wishes to reset the camera settings without power cycling the

LRF Module.

Example:

V: Print version information

Lists version and calibration information for the LRF Module. This data is useful for troubleshooting and

debugging.

FW: Firmware revision (major.minor)

MFG and PID: Manufacturer ID and Product ID of the LRF’s on-board camera

SLOPE, INTERCEPT, PFC_MIN: Device-specific values calculated during the calibration process

(“X”) and used for range finding (if the LRF Module is uncalibrated, the values will all be

0xFFFFFFFF)

Example:

Parallax Laser Range Finder

Manufactured and distributed by Parallax [[email protected]]

FW = 1.0

MFG = 7FA2

PID = 7691

SLOPE = +0.001413373 (3AB940EC)

INT = -0.01235497 (BC4A6C80)

PFC_MIN = 30

H: Print list of available commands

Lists all available commands that the LRF Module supports.

Example:

Basic Commands:

R Single range measurement

B Single range measurement (binary response, 4 bytes)

< more commands listed, but not shown in this manual >

O: Display coordinate, mass, and centroid information for all detected blobs

Displays coordinate, mass, and centroid (center of mass) information for up to 6 detected blobs within

the camera’s field-of-view. This information can be used for custom image processing or object detection

outside of the standard LRF Module functionality. The blob detection begins on the left side of the frame

and “scans” to the right. See Image Processing and Blob Detection, page 18, for details.

L: X coordinate of the beginning (left side) of the detected blob

R: X coordinate of the end (right side) of the detected blob

M: Mass of blob (sum of all valid pixels within the blob)

C: Centroid (center of mass) of blob

In some cases, only a single blob is detected within the frame and is typically the laser spot. In other

cases, there may be reflections of the laser light or other spots that are not related to the laser.

Generally, the blob with the largest mass within the frame can be considered the actual laser spot.

Example:

0: L = 81 R = 88 M = 38 C = 84

1: L = 137 R = 232 M = 917 C = 181

2: L = 235 R = 254 M = 170 C = 244

X: Calibrate camera system for range finding

To account for manufacturing and assembly variances, particularly related to the camera and laser diode

alignments, each LRF Module must be calibrated. This occurs during production, but the LRF can be

recalibrated by the user at a later date if desired.

The calibration routine requires the user to place the LRF Module at 6 fixed distances (from 20 cm to 70

cm at 10 cm intervals). The LRF takes measurements at each distance and calculates the SLOPE,

INTERCEPT, and PFC_MIN values (the routine is based lightly on the one found at

www.eng.umd.edu/~nsw/ench250/slope.htm). The values are then stored in an unused portion of the

non-volatile boot Serial EEPROM. They will remain intact even if the LRF firmware is re-programmed onto

the Module.

The SLOPE and INTERCEPT are used to convert the pixel offset to angle using a best-fit slope-intercept

linear equation. The PFC_MIN value is used to set the maximum allowable distance of the LRF Module,

which is represented by a minimum pixels from center value. See Optical Triangulation, page 15, for

more details.

A video demonstrating the calibration process can be found on YouTube:

www.youtube.com/watch?v=1gk_tRbJO84

Example:

Are you sure you want to calibrate (Y/N)?Y

Set LRF to D = 20 cm and press spacebar (any other key to abort)

pfc: 246 angle: 0.3718561

pfc: 245 angle: 0.3718561

pfc: 244 angle: 0.3718561

< more steps listed, but not shown in this manual >

Set LRF to D = 70 cm and press spacebar (any other key to abort)

pfc: 66 angle: 0.1109708

pfc: 65 angle: 0.1109708

pfc: 67 angle: 0.1109708

SLOPE = +0.001450448 (3ABE1CF8)

INT = +0.01748975 (3C8F46A8)

PFC_MIN = 9

Write new values (Y/N)?Y

G: Capture & send single frame (8 bits/pixel grayscale @ 160x128)

Capture a 160x128 resolution grayscale image with the LRF Module’s camera and return the data in a

binary format.

The data is sent MSB first, one byte per pixel, starting at the upper-left pixel location (0,0), moving left to

right, top to bottom across the frame, and ending at the lower-right pixel location (160,128). Each byte

corresponds to the brightness value of a single pixel where 0x00 is black and 0xFF is white.

A total of 20,480 bytes of binary data is sent. An ASCII footer (“END”) is then attached to the end of the

binary data stream to assist in identifying the end of frame.

This command can be used from within the LRF Image Viewer tool to easily view the image.

Example:

<binary data>END

C: Capture & send single frame (16 bits/pixel YUV422 color @ 640x16)

Capture a 640x16 resolution image with the LRF Module’s camera and return the data in a binary format.

The laser diode is enabled during the frame grab for testing and alignment purposes.

The data is sent MSB first, two bytes per pixel, starting at the upper-left pixel location (0,0), moving left

to right, top to bottom across the frame, and ending at the lower-right pixel location (640,16). Every two

bytes corresponds to a single pixel. The pixel format is YUY2 (www.fourcc.org/yuv.php), a subset of

YUV422 formatting in which each 16-bit pixel is given an 8-bit Y component (luma/brightness) and

alternating 8-bit U or 8-bit V component (chroma). See Camera Interface, page 17, for more details.

A total of 20,480 bytes of binary data is sent. An ASCII footer (“END”) is then attached to the end of the

binary data stream to assist in identifying the end of frame.

This command can be used from within the LRF Image Viewer tool to easily view the image. YUVTools

(www.sunrayimage.com) can also be used to convert the raw binary data into a YUV422-decoded color

image.

Example:

<binary data>END

P: Capture & send image processed frame (16 bits/pixel YUV422 color @ 640x16)

Capture a 640x16 resolution “processed” image with the LRF Module’s camera and return the data in a

binary format. This mode is specific for range finding functionality and consists of a background

subtracted image where one frame is taken with the laser diode off, one frame taken with laser diode on,

and the data subtracted to help isolate the laser spot from the rest of the image frame. See Image

Processing and Blob Detection, page 18, for more details.