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SF11
Product manual
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SF11
Laser altimeter
The SF11 is a compact, lightweight
laser altimeter for above-groundlevel measurement from small fixed
wing or multi-rotor craft.
Features:
The SF11 laser altimeter is ideal for
automated landings, precision
hovering and terrain following.
• Fast update rate of 16 readings per second.
serial, I2C, USB and analog interfaces
• Includes
with programmable capabilities.
The SF11 is directly compatible with
the Pixhawk and other flight
controllers.
to configure using the built-in menu and
• Easy
LightWare Terminal software.
The SF11 laser altimeter makes
accurate distance measurements to
solid surfaces up to an altitude of 120
meters and water up to 40 meters.
• Very compact and lightweight - 35 grams.
AGL altitude measurements on ground,
• Accurate
foliage and water.
• Fully calibrated and ready to run.
with Pixhawk, APM compatible and
• Compatible
other flight controllers.
reliable altitude measurements in
• Accurate,
sunlight or dark conditions.
affected by: speed; wind; changes in
• Not
barometric pressure; noise; ambient light; terrain
or air temperature.
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SF11
Laser altimeter
Product manual
1. Overview ................................................................................................................................................... 3
2. Quick start guide .........................................................................................................................................4
3. Making connections to the SF11 ........................................................................................................................5
4. Menu options .............................................................................................................................................. 7
5. Instructions for safe use ............................................................................................................................... 11
Appendix A :: Specifications .............................................................................................................................. 12
Appendix B :: Dimensions ................................................................................................................................. 12
Appendix C :: Main cable type 1, 35 cm ................................................................................................................ 13
Appendix D :: Connecting to Pixhawk Autopilot using “serial 4” .................................................................................... 14
Appendix E :: Electromagnetic interference (EMI) graphs ............................................................................................14
Revision history ............................................................................................................................................. 16
Table of figures
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Figure 1 :: The main features of the SF11 ................................................................................................................ 3
Figure 2 :: Power from the USB port ..................................................................................................................... 5
Figure 3 :: Regulated +5 V DC power supply connections .............................................................................................. 5
Figure 4 :: USB communications ........................................................................................................................... 5
Figure 5 :: Analog voltage connections ................................................................................................................... 6
Figure 6 :: Serial interface connections .................................................................................................................. 6
Figure 7 :: I2C interface connections ..................................................................................................................... 6
Figure 8 :: LightWare Terminal showing menu options expanded .................................................................................... 7
Figure 9 :: Altitude represented by distance (Serial / I2C) and analog voltage .................................................................... 8
Figure 10 :: Median Filter Response graph .............................................................................................................10
Figure 11 :: Adaptive Filter Response graph ...........................................................................................................10
Figure 12 :: Labelling on the SF11........................................................................................................................ 11
Figure 13 :: Dimension drawings of the SF11 ...........................................................................................................12
Disclaimer
Information found in this document is used entirely at the reader’s own risk and whilst every effort has been made to
ensure its validity neither LightWare Optoelectronics (Pty) Ltd nor its representatives make any warranties with respect the
accuracy of the information contained herein.
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SF11
Table of contents
SF11
Laser altimeter
Product manual
The light-weight, SF11 laser altimeter is an essential addition to any unmanned aircraft that needs fast, accurate and reliable AGL
altitude measurements over solid ground and water.
Operating from a regulated 5 V DC supply, the SF11 includes both analog and digital interfaces that can be easily connected to a
Pixhawk flight controller or other standard processing platform. Each interface on the SF11 can be configured using a simple
software menu that is accessible through the built-in, micro USB port.
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The SF11 works by measuring the time it takes for a very short flash of laser light to travel to the ground and back again. The
accuracy of the measurement is not affected by the colour or texture of the ground nor the angle of incidence of the laser beam.
The SF11 is virtually immune to background light, wind and noise making it the ideal AGL altimeter for all kinds of terrain.
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Figure 1 :: The main features of the SF11
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SF11
1. Overview
SF11
Laser altimeter
Product manual
SF11
2. Quick start guide
1. CAUTION - The SF11 laser altimeter contains a laser and should never be
aimed at a person or an animal. Do not look at the beam directly with
optical instruments.
2. Download LightWare Terminal software from www.lightware.co.za >
Library > Documents > Software onto your PC. Open the installer package
and follow the installation instructions. Everything needed for
communicating with the SF11 will automatically be installed.
3. Plug the “micro-B to type A” USB cable provided into the SF11’s micro
USB connector and connect the other end to your PC. This provides both
power and communication to the unit.
4. Start the LightWare Terminal software and click the “Connect” icon to
open a communications port.
5. If the connection isn’t made automatically, click the “Laser” icon and
select the correct USB port from the list shown.
6. Press the <SPACE> key to display the main menu. This menu includes a
list of all the settings that can be changed in the SF11. A summary of the
settings is given below:
Setting
Range of values
a: Zero datum
-10.00 m … +10.00 m
Description
Adjusts the zero point from which measurements are taken.
b: Hide / Show communication menu
> c: Serial port baud rate
9600 … 115200
> d: I2C bus address
0 … 7F
Sets the baud rate for the serial port.
Sets the I2C address.
e: Hide / Show analog output menu
> f: Analog distance range
1.00 m … 120.00 m
Sets the distance at which the voltage output will show either
2.56 V or 3.30 V.
> g: Analog voltage range
0.00 V … 2.56 V or
0.00 V … 3.30 V
Selects the maximum output voltage of the analog output: either
2.56 V or 3.30 V.
0.00 m = 0.00 V | 25.60 m = 2.56 V Selects the polarity of the analog output so that the maximum
or
voltage selected using menu item <g> occurs at the maximum
25.60 m = 0.00 V | 0.00 m = 2.56 V distance set using menu item <f> or at 0.00 m as required.
i: Hide / Show filter menu
> j: Output on lost signal
10 m ... 130.00 m
>> k: Confirm lost signal
0.0 ... 120.0 seconds
Sets how long the signal may be lost before the out of range
distance is output.
1 ... 120 results
Sets how many valid readings must be obtained before a signal is
confirmed as usable.
> l: Confirm recovered signal
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> h: Analog polarity
> m: Median filter
>> n: Median filter size
> p: Adaptive filter
ON / OFF
3 … 32 results
ON / OFF
Sets the distance output on all ports when out of range.
Enables the Median smoothing filter.
Sets the number of results included in the median filter.
Enables the Adaptive filter.
7. Once you have confirmed your settings, press the <SPACE> key to start taking distance measurements and the results
will be displayed in the Terminal window.
8. Press the “Disconnect” icon before unplugging the USB cable.
9. There are several interface options available on the main connector. These connections are used to integrate the SF11
into your system and details of all the options are explained later in this document.
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SF11
Laser altimeter
Product manual
The SF11 gets power from either a regulated +5 V DC supply on the main connector or via the USB port when it is connected to a PC.
There are a number of digital and analog interfaces on the main connector and either one or a combination of interfaces may be
connected to a host controller. The built-in micro USB port can be used to input settings and to test the performance of the SF11.
Power supply option 1: USB
The SF11 can be powered directly from the USB port of a PC or laptop. This is particularly useful for testing the SF11 before it is
installed in your system and also for changing the settings in readiness for the final application.
Figure 2 :: Power from the USB port
Power supply option 2: Regulated +5 V DC
The second power supply option is to connect a regulated voltage of 5 V ± 10% DC to the main connector. If the power wires are more
than 30 cm long, we recommend using a decoupling capacitor of 100 µF, or other noise suppression components to reduce the chance
interference being picked up on the power wires. It is important that this voltage is stable and well regulated.
Figure 3 :: Regulated +5 V DC power supply connections
Communications using the USB port
Once communication has been established, settings can be changed by pressing the <SPACE> key to access the menu and then
selecting the menu item that needs changing. Pressing the <SPACE> key again restarts the measuring process. If no settings are
entered then the SF11 automatically restarts after two minutes. More details of the menu items are discussed in the “Menu options”
section below.
If you want to use a different serial emulation program then the USB serial protocol should be set to 115200 baud with 1 stop bit and
no parity or handshaking. All communications are in standard ASCII format.
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The SF11 has a micro USB port that can be used to communicate with LightWare Terminal software on a PC. This connection also
provides power to the unit thereby presenting a quick way to test and configure the SF11. The LightWare Terminal software will
automatically detect the USB port that is connected to the SF11 and communications can be established by clicking on the
“Connect” icon. If more than one compatible device is present, click the “Laser” icon to select which USB port should be active.
Figure 4 :: USB communications
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SF11
3. Making connections to the SF11
SF11
Laser altimeter
Product manual
The analog interface on the main connector produces a voltage proportional to the measured altitude. The scale can be set to
deliver a maximum voltage of 2.56 V or 3.3 V depending upon the menu selection. The physical altitude in meters that equates to
the high end of the scale can be adjusted through the USB menu system. For example, an altitude setting of 51.20 m would produce
a linear voltage output of 50 mV per meter on the 2.56 V scale.
Figure 5 :: Analog voltage connections
Serial interface
The serial interface on the main connector outputs the measured altitude in meters as an ASCII encoded number. This interface uses
3.3 V logic levels and can be connected directly to any similar, compatible interface. Distances are transmitted whenever the SF11
receives an ASCII ‘d’ character from the host controller. The baud rate for the serial interface is selectable through the USB menu
system. The maximum delay between receiving a character and returning the altitude is 25 ms.
Figure 6 :: Serial interface connections
The I2C interface on the main connector outputs a value that represents the altitude in centimetres. This interface operates in
“slave” mode and uses 3.3 V logic levels. The I2C address can be set through the USB menu system. The host controller acts as the
I2C “master” and sends the address to the SF11 as an 8 bit value (7 address bits plus 1 read bit). The SF11 then returns the altitude
as a 16 bit integer. The maximum delay between receiving the address and returning the altitude is 25 ms.
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I2C interface
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Figure 7 :: I2C interface connections
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SF11
Analog voltage interface
SF11
Laser altimeter
Product manual
The SF11 can be connected through the on-board USB port to a Terminal emulation program running on a PC. The LightWare Terminal
software is available for download from www.lightware.co.za.
Once the USB connection is made, the Terminal window displays the distance reading from the SF11. Pressing the <SPACE> key stops
the measuring process and changes the display to a menu that lists all the available settings and configuration options. Pressing the
<SPACE> key again restarts the measuring process. If no changes are made, the unit will automatically begin to measure again after
two minutes.
Figure 8 :: LightWare Terminal showing menu options expanded
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Menu item <b> is used to expand or hide the communications menu. The settings available in this menu relate to the serial port and
the I2C port.
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The point from which altitude measurements are taken can be adjusted using menu item <a>. The range of values that can be
entered are from -10.00 m to +10.00 m and this value is subtracted from the altitude reading before it is made available on any of
the interfaces. The zero datum offset can be used to compensate for the mounting position of the SF11 in the airframe, where
distance readings may best be interpreted from a suitable point on the landing gear, rather than from the front face of the SF11.
•
a: Zero datum offset (-10.00 m … +10.00 m)
b: Show or hide the communications menu
c: Serial port baud rate (9600 … 115200)
The serial port transmits a serial string of ASCII encoded data from the SF11 to the host controller. The baud rate of transmission is
selected by menu item <c> and toggles through the standard baud rates from 9600 to 115200. There are 8 data bits, 1 stop bit and
no parity or handshaking on this serial port.
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SF11
4. Menu options
SF11
Laser altimeter
Product manual
SF11
The ASCII string representing the altitude is in floating point format with two decimal places followed by the analog voltage, carriage
return and line feed:
“22.48 m 1.045 V\r\n”
where carriage return and line feed are given by the hexadecimal ASCII characters:
\r = 0x0D
\n = 0x0A
The altitude is sent out of the serial port when the ‘d’ ASCII character is transmitted by the host controller to the SF11.
d: I2C bus address (0x00 … 0x7F)
The I2C bus operates in slave mode and accepts a 7 bit address (7 address bits plus 1 read bit) before responding with a 16 bit,
binary coded integer that is the altitude in centimetres. The address can be set by selecting menu item <d> and is entered as a 7
bit, hexadecimal number.
e: Show or hide the analog output menu
Menu item <e> is used to expand or hide the analog output menu. The settings available in this menu relate to the analog voltage
port.
f: Analog distance range (1.00 m … 120.00 m)
The distance at which the maximum analog output of 2.56 V or 3.3 V occurs can be set by selecting menu item <f>. The output
voltage can be converted back into a distance by using the formula:
d=v/g * f
where:
d = measured distance
v = voltage measured by the ADC of the host
g = 2.56 V or 3.3 V scale
f = maximum distance setting
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The analog voltage output updates 16 times per second and has a 12 bit resolution.
Figure 9 :: Altitude represented by distance (Serial / I2C) and analog voltage
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SF11
Laser altimeter
Product manual
Selects the scale of the analog output voltage - either 2.56 V or 3.30 V - by selecting menu item <g>.
h: Analog polarity (0.00 m = 0.00 V | 25.60 m = 2.56 V or 25.60 m = 0.00 V | 0.00 m = 2.56 V)
Selects the polarity of the analog output so that the maximum voltage selected using menu item <g> occurs at the maximum
distance set using menu item <f> or at 0.00 m as required.
i: Show or hide the filters menu
Menu item <i> is used to expand or hide the numerical filter menu. The settings available in this menu relate to providing smoothed
data to all the output ports and rejecting momentary changes in altitude caused by trees, fences, lost signals and out of range
conditions.
j: Output on lost signal (10 m ... 130.00 m)
Menu item <j> is used to set the altitude that is output on all ports when the signal has gone out of range. This is set to 130 m by
default and is required to ensure that there is a clean transition between out of range results and valid altitude measurements.
k: Confirm lost signal (0.0 ... 120.0 seconds)
Menu item <k> is used to set how long a lost signal condition is permitted before the out of range altitude is output. When operating
close to the measuring limit of of the SF11 or over water, signals may be lost intermittently. Rather than jumping between the
measured altitude and the out of range altitude, this menu item holds the last valid altitude for the entered time. If the out of range
condition persists for longer than this time then the out of range altitude is output.
l: Confirm recovered signal (1 ... 120 results)
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Menu item <l> sets how many times successive good results must occur before transitioning from a lost signal condition to normal
altitude measurements. This setting prevents isolated, transient signals from being accepted as good readings. In practice, this
setting can be used at a value of 1 when measuring difficult surfaces such as water whilst for most other surfaces a value of 3
provides more reliable results.
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SF11
g: Analog voltage range (0.00 V … 2.56 V or 0.00 V … 3.30 V)
SF11
Laser altimeter
Product manual
Menu item <m> is used to enable the median filter that can be used to reject transient changes in altitude caused by rough terrain,
bushes, fences, ditches or other small obstacles. This improves flight stability during automatic terrain following.
Figure 10 :: Median Filter Response graph
n: Median filter size (3 … 32 results)
Menu item <n> sets the number of readings that are used by the median filter. A higher number provides a smoother output but slows
down the rate of response of the output to changes in altitude.
p: Enable or disable the adaptive filter (ON / OFF)
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Menu item <p> is used to enable the adaptive filter that can be used to smooth the altitude readings when operating above flat
surfaces such as indoors. This filter adaptively smooths fast changes in altitude but follows slow changes.
Figure 11 :: Adaptive Filter Response graph
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SF11
m: Enable or disable the median filter menu (ON / OFF)
SF11
Laser altimeter
Product manual
The SF11 is a laser based altimeter that emits ionizing laser radiation. The level of the laser emission is Class 1M which indicates that
the laser beam is safe to look at with the unaided eye but must not be viewed using binoculars or other optical devices at a distance
of less than 15 meters. Notwithstanding the safety rating, avoid looking into the beam and switch the unit off when working in the
area.
CAUTION -- The use of optical instruments with this product will increase eye hazard.
The SF11 should not be disassembled or modified in any way. The laser eye safety rating depends on the mechanical integrity of the
optics and electronics so if these are damaged do not continue using the SF11. There are no user serviceable parts and maintenance
or repair must only be carried out by the manufacturer or a qualified service agent.
No regular maintenance is required for the SF11 but if the lenses start to collect dust then they may be wiped with suitable lens
cleaning materials. Make sure that the SF11 is switched OFF before looking into the lenses.
The SF11 should be mounted using the four holes provided in the circuit board. Do not hold or clamp the lens tubes as this may cause
damage and adversely affect the laser safety rating.
Laser radiation information and labels
Specification
Value / AEL
Laser wavelength
905 nm
Pulse width
< 20 ns
Pulse frequency
Peak power
< 36 kHz
< 10 W
Average power
< 0.6 mW
Average energy per pulse
< 300 nj
15 m
50 millimeter aperture at 2 meters
7 millimeter aperture
Distance beyond which binoculars with may be used safely
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NOHD
Notes
Figure 12 :: Labelling on the SF11
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SF11
5. Instructions for safe use
SF11
Laser altimeter
Product manual
SF11/C (120 m)
Range
0.1 … 120 meters (natural targets), 2 ... 40 meters (moving water)
Resolution
1 centimeter
Update rate
16 readings per second
Accuracy
±0.1 meter (70% reflective target @ 20°C)
Power supply voltage
5.0 V ± 0.5 V DC
Power supply current
200 mA (maximum)
Outputs & interfaces
Serial, I2C & analog with maximum latency of 65 ms
Dimensions
30 x 56.5 x 50 millimeters
Weight
35 grams (excluding cables)
Connections
Plug & socket, micro USB
Laser power
20 W (peak), <15 mW (average), Class 1M
Optical aperture
51 millimeters
Beam divergence
0.2°
Operating temp.
0 ... 40°C
Approvals
FDA: 1410968-002 (2016/01)
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Appendix B :: Dimensions
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Figure 13 :: Dimension drawings of the SF11
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SF11
Appendix A :: Specifications
SF11
Laser altimeter
Product manual
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SF11
Appendix C :: Main cable type 1, 35 cm
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SF11
Laser altimeter
Product manual
SF11
Appendix D :: Connecting to Pixhawk Autopilot using “serial 4”
Appendix E :: Electromagnetic interference (EMI) graphs
The SF11 family has been tested for radio frequency interference in accordance with MIL-STD-451C. The results are well within the
required limits so that neither direct radiation nor secondary radiation from wiring should cause interference to on-board systems
such as GPS and optical flow.
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1. 14 kHz to 1 GHz - narrowband
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SF11
Laser altimeter
Product manual
SF11
2. 14 kHz to 1 GHz - broadband
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3. 1 GHz to 10 GHz - narrowband
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SF11
Laser altimeter
Product manual
Version
Date
Authors
Comments
2016/01/29
TLP
Update minimum range on natural targets to “0.1” in “Appendix A ::
Specifications” (page 12). Update FDA accession number “1410968-002 (2016/01)” in
“Appendix A :: Specifications” (page 12).
Rev 0
2015/11/13
JEP
First edition
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Rev 1
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Revision history
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