sparkfun Servo Trigger Technisches Dokument
Servo Trigger Hookup Guide
Introduction
The Servo Trigger is a small board that helps you deploy hobby RC servo
motors. When an external switch or logic signal changes state, the Servo
Trigger tells an attached servo motor to move from position A to position B.
The Servo Trigger in action.
To use the Servo Trigger, you simply connect a hobby servo and a switch,
then use the onboard potentiometers to adjust the start/stop positions and
transition time. You can use a hobby servos in your projects without having
to do any programming!
In This Tutorial
This hookup guide starts with some background information about hobby
servo motors. From there, it jumps into getting the Servo Trigger working
with a small servo, then examines some of the inner workings. Finally, for
the adventurous, it explains how to customize the Servo Trigger by
reprogramming it.
Suggested Reading
• An Introduction to Motors.
• More information about hobby servo motors.
• Some background on Pulse Width Modulation.
Servo Motor Background
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In the most generic sense, a “servomechanism” (servo for short) is a device
that uses feedback to achieve the desired result. Feedback control is used
in many different disciplines, controlling parameters such as speed,
position, and temperature.
In the context we are discussing here, we are talking about hobby or radio-
control servo motors. These are small motors primarily used for steering
radio-controlled vehicles. Because the position is easily controllable, they
are also useful for robotics and animatronics. However, they shouldn’t be
confused with other types of servo motors, such as the large ones used in
industrial machinery.
An Assortment of Hobby Servos
RC servos are reasonably standardized - they are all a similar shape, with
mounting flanges at each end, available in graduated sizes. Servos often
come with several wheels or levers, known as “horns”, than can be
attached to the shaft, to fit the device they are operating.
Example Servo Horns
Electrical Connection
Most hobby servos use a standard type of 3-pin plug, with the same control
signaling, which makes RC servos reasonably interchangeable.
The connector is a 3-pin, 0.1" pitch header. One thing that can be confusing
is that the wiring color code isn’t always consistent – there are several color
codes at play. The good news is that the pins are usually in the same order,
just that the colors on them are different.
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The table below summarizes common color schemes.
Pin
Number
Signal
Name
Color
Scheme
1
(Futaba)
Color
Scheme
2
(JR)
Color
Scheme
3
(Hitec)
1 Ground Black Brown Black
2Power
Supply Red Red Red or
Brown
3Control
Signal White Orange Yellow or
White
Servo connection Color Coding
Heads up! If you’re in doubt about your color scheme, check the
documentation – don’t plug it in backwards!
Powering Servos
In RC vehicles, 5.5V is the nominal battery voltage. It will be somewhat
higher after a charge, and it will droop as the batteries discharge. As the
voltage drops, the available torque also drops – if you’ve driven RC
vehicles, you’re no doubt familiar with the loss of control that occurs as the
batteries get weaker. It starts to feel sluggish just before it dies.
If you’re not using batteries, the 5VDC available from a garden variety
power supply is a good option. If you’re using the Servo Trigger to control
your motor, the absolute maximum supply voltage that should be applied
is 5.5 VDC.
Regardless of how you’re powering them, it’s worth noting that the current
consumed by the motor increases as the mechanical loading increases. A
small servo with nothing attached to the shaft might draw 10 mA, while a
large one turning a heavy lever might draw an Ampere or more!
Control signal
Servos are controlled with a specific type of pulse train signal. The pulses
occur at a 20 mSec (50 Hz) interval, and vary between 1 and 2 mSec in
width. The Pulse Width Modulation hardware available on a microcontroller
is a great way to generate servo control signals.
Common servos rotate over a range of 90° as the pulses vary between 1
and 2 mSec – they should be at the center of their mechanical range when
the pulse is 1.5 mSec.
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Internally, the mechanism of a servo motor uses a potentiometer attached
to the rotating shaft to sense the position. It measures the width of the
incoming pulse, and applies current to the motor to turn the shaft
correspondingly.
Here are the insides of a servo that’s been dissected. You can see the DC
motor, position potentiometer, and a small PCB. The PCB has a chip on
one side, possibly a small microcontroller.
Inside an RC servo
The other side of the PCB has some discrete transistors, probably in an
H-bridge configuration, which allow the controller to steer current through
the motor in either direction, for both clockwise and counterclockwise
rotation.
Back of the PCB
One Other Useful Servo
Ordinary RC servos turn over a 90° range – it’s useful for turning a steering
linkage, or adjusting the control surfaces on an airplane, but not so useful
as a drive mechanism. That’s where full or continuous rotation servos
come in.
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Rather than controlling position, the continuous rotation servo translates the
same pulse-train signal into the rotational speed and direction of the shaft.
Otherwise, they’re very similar to regular RC servos – they use the same
power supply, control signals, 3-pin connector, and are available in the
same sizes as RC servos.
The overall speed is relatively low – around 60 RPM is a common
maximum rate – if you need higher rotation speed, servos aren’t the best fit
– DC gearmotors or brushless DC motors are more likely candidates, but
they aren’t directly compatible with servo control signals.
With the Servo Trigger
The Servo Trigger is capable of controlling both regular and continuous
rotation servos. We’ll explore some more specific use cases in the following
sections.
Getting Started Quickly
Let’s jump in and build a circuit to show how the Servo Trigger works!
Materials and Tools
You’ll need to following materials to build this example circuit.
• The Servo Trigger module.
• A hobby servo motor - we’ll be using our micro size metal gear servo.
• A switch - any momentary-contact switch is suitable, so we’ll use a
12mm tactile pushbutton.
• A 5V power supply.
• A barrel jack adapter to make conecting the power supply easier.
• A 3-pin section of a snappable header, either straight or right-angle.
• A small screwdriver to adjust the trimpots.
• Finally, not shown, we’ll need some hookup wire and soldering tools.
Beginning Steps
To start, solder some wires to the tactile switch. If you solder to legs on on
opposite corners (top-right and lower-left, for instance), you can be
confident that you’ll get a contact closure when you press the button.
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Switch Assembly
Then prepare the power plug pigtail. Take a pair of wires, and strip the
ends, then screw them to the power jack adapter – if you look closely at the
adaptor, you’ll notice that there are a small +and -embossed in the
plastic. We used a red wire for VCC on the +terminal, and a black wire for
ground on the -terminal.
Power Jack Closeup
Next, solder the 3-pin header to the 3 pads on the end the board, and plug
the servo into the the header. Be careful to get the plug oriented correctly –
you can check the color code table in the previous section, or consult the
servo manufacturer’s datasheet.
Then solder the switch wires to the IN and GND pads on the Servo
Trigger, and the power pigtail to the VCC and GND pads on the edge of the
board. These are mirrored on opposite edges of the board – they’re wired in
parallel, so you can use either set of pads. The red wire should connect to
the VCC pad, and the black to GND .
Before we power up, take a moment to double-check your work against the
photo below (click on the picture for a larger version). In particular, make
sure that the power and servo connections are oriented correctly.
Adjust the trimpots on the back of the board. Set Afully counterclockwise,
Bfully clockwise, and set Tto the middle.
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Finally, apply power. The servo will probably jump to a new position as you
do this.
Then, press and hold the switch. The servo will rotate, taking a couple of
seconds to reach its new position. Release the switch, and it will go back to
the starting point.
Now you can adjust the trimpots to configure the servo.
•Asets the position the servo sits in while the switch is open.
•Bsets the position the servo moves to when the switch is closed
•Tsets the time it takes to get from A to B and back.
Turning the position pots clockwise will make the motor turn further
clockwise. If A is higher than B, then the servo will turn counterclockwise
when the switch is actuated. The timing range is adjustable between 50
milliseconds and 3 seconds. The transit time is constant – when set to 2
seconds, the servo will take 2 seconds to move between A and B,
regardless of how close the position settings are.
In the next section, we’ll explore some of the finer details of the Servo
Trigger.
More Details
On The Board
Let’s look at the components on the board and examine how it works.
The heart of the Servo Trigger is an Atmel ATTiny84 microcontroller,
running a small program that implements the servo control features we are
discussing here. Just because the Servo Trigger saves you from needing to
write code doesn’t mean that there’s no programming involved!
The servo control signal is generated using a 16-bit hardware timer. It runs
off a 1 MHz clock, counting to 20000 to generate the 20 mSec (50 Hz)
period and configured to generate pulses that range from 1000 to 2000
µSec (1 to 2 milliseconds).
The three potentiometers are connected as voltage dividers between VCC
and ground. They are read using analog inputs ADC0, ADC3, and ADC7.
The switch input is read using PortA, input pin 1. It is debounced in
software and can be configured to watch for a switch closure, or a logic
level pulse.
If you’re interested, you can download the schematic, PCB layout and
firmware files from the Servo Trigger GitHub Repository. The board also
includes the common 6-pin in-system-programming header, which we’ll
discuss in the Expert Exercises section. But we’re getting a bit ahead of
ourselves – there are configuration options you can use without
programming.
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Configuratio
n
The Servo Trigger has a couple of configuration options. If you look at the
back of the PCB, you’ll notice two solder jumpers that can be used to
change Servo Trigger’s response.
Configuration Jumpers, SJ1 and SJ2.
When it first powers up, the servo trigger reads these jumpers, and
configures itself accordingly.
Modes
The Servo Trigger has two different servo control modes, selected with
solder jumper 1 (SJ1). They can be used to tailor the response of the board
for different applications.
The default mode implements bistable control – the servo will sit at position
A or position B, depending on the input actuation. While the switch stays in
a state, the servo stays in the corresponding position – it is stable in two
different states.
Jumper Cleared - Bistable Control
This behavior can be changed by flowing solder between the pads of the
jumper.
With the solder jumper closed, the mode changes to one-shot or
monostable. When the input is actuated, the servo will move from A to B,
then back to A – the servo is stable in the A position, and only passes
through the B position momentarily. Regardless of when the input is
cleared, the servo will make a complete transit.
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Jumper Soldered - One-Shot Control
Input Polarity
The Servo Trigger input sensitivity can also be changed, using solder
jumper 2 (SJ2).
The default configuration, with no solder applied, configures the Servo
Trigger for use with a normally-open switch, with the internal pull-up resistor
on the microcontroller enabled. This configuration is also suitable for use
with an active-low logic input.
With SJ2 closed, the internal pull-up is disabled, and the input is set as an
active-high logic input.
If SJ2 is closed, be careful about powering up the Servo Trigger when the
input is not connected to anything. When the input is floating, it can
randomly toggle between active and inactive and may cause the motor to
behave unpredictably.
A note about nomenclature here: since the input polarity can be swapped, it
can be hard to talk about – the voltage might be high, but when the sense is
inverted, it indicates that the input isn’t being actuated. To help navigate
this, the polarity-neutral terms active or asserted are used to describe
when the input is being used, and inactive or deasserted to describe the
default state.
More components
The servo trigger can be used with a wider variety of external components
than used in the example above. We used a mid-sized servo, though we
have many other candidates, in a wide variety of sizes & torque ratings.
You can also use different switches, such as micro switches and foot pedal
switches.
Power Notes
Compared to a servo motor, the Servo Trigger board draws very little
current – roughly 5 mA.
The motors draw significantly more – a quick bench test using a small
servo, with only a lightwieght horn attached, shows the motor draws 10 mA
sitting idle, and about 70 mA while moving. Grabbing the horn and twisting
causes the controller to apply current to the motor, counteracting the twist.
It drew 700 mA during this test – a larger servo could draw even more!
These currents can get surprisingly high as you add more motors to the
system – you’ll need to select a power supply with adequate capacity.
The Servo Trigger is designed to make it easy to daisy chain boards – you
can simply connect the VCC and GND pads on adjacent boards.
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In applications where the motors are moving non-trivial loads, it’s a better
bet to use heavier gauge wires and give each Servo Trigger a direct
connection to the power supply. The configuration is commonly known as
“star power.”
When in doubt, grab a multimeter, measure the current consumed, and
check whether VCC at the board input is falling below the rated voltage
when the servos are turning.
Troubleshooting
If there’s no motion when you actuate the input, first check that A and B are
not set the same, otherwise there’s no position change!
If you’re feeding the input with a logic signal from an external device, be
sure to drive the signal for more than 50 milliseconds. The PWM signal is
updated every 50 mSec, and events shorter than that may missed.
It’s also possible to set T shorter than the time it take the servo motor to
physically rotate. In this case, the motor may not reach B before returning
to A. Try turning up T, to see if a longer transition time allows the motor to
turn.
Expert Exercises
Customizing the Servo Trigger
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