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ZodiusInfuser 7608e3f293 Encoder driver finalising 2022-04-25 12:28:42 +01:00
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README.md

Motors and Motor 2040

The Motor library lets you drive DC motors from a Raspberry Pi Pico or any other RP2040-based board, such as the Pimoroni Motor 2040.

This library offers two motor implementations:

  • a Motor class that uses hardware PWM to drive a single motor, with support for up to 8 motors.
  • a MotorCluster class that uses Programmable IO (PIO) hardware to drive up to 15 servos.

Table of Content

Motor 2040

Motor 2040 is a standalone motor controller for driving DC motors with encoder feedback. It has JST-SH connectors for plugging in up to 4 motors, with additional bells and whistles like sensor headers, current monitoring, an RGB LED, and a user button that make it ideal for many robotics projects!

For more information on this board, check out the store page: pimoroni.com/motor2040.

Reading the User Button

As is the case for many of Pimoroni's RP2040-based boards, the boot button on Motor 2040 that is used for programming also acts as a user button once a program is running. To simplify the use of this and other buttons, the pimoroni module contains a Button class that handles button debounce and auto-repeat.

Button(button, invert=True, repeat_time=200, hold_time=1000)

To set up the user button, first import the Button class from the pimoroni module and the pin constant for the button from motor:

from pimoroni import Button
from motor import motor2040

Then create an instance of Button for the user button:

user_sw = Button(motor2040.USER_SW)

To get the button state, call .read(). If the button is held down, then this will return True at the interval specified by repeat_time until hold_time is reached, at which point it will return True every repeat_time / 3 milliseconds. This is useful for rapidly increasing/decreasing values:

state = user_sw.read()

It is also possible to read the raw button state without the repeat feature, if you prefer:

state = user_sw.raw()

Reading the Sensors

On the right-hand edge of Motor 2040 are two analog inputs, with neighbouring 3.3V and GND. These let you connect up sensors to enable your mechanical creations to sense how they are interacting with the world. For example, a pair of analog proximity sensors could be hooked up for wall avoidance or line following, or they could have microswitches wired to report when a motor driven mechanism has reached an end-stop.

Motor 2040 also has six internal sensors:

  • A voltage sensor, letting you measure the supply voltage to the motors.
  • Four current sensors, letting you measure how much current each motor is drawing.
  • A fault sensor, letting you know if there is an issue with one of more of your motors These could be used just for monitoring, or as the trigger to turn off motors safely when voltage gets too low or current draw gets too high.

To allow for all of these inputs, Motor 2040 has an onboard analog multiplexer that expands a single analog pin into eight, by use of three digital address pins that select a single input at a time. As such, the setup for these sensors is more involved than it would be to just read eight analog pins directly.

To begin reading these inputs, first import the Analog and AnalogMux classes from pimoroni and the pin, address, and gain constants from servo:

from pimoroni import Analog
from motor import motor2040

Then set up three instances of Analog for the sensor and fault, voltage, and current sensing:

sen_fault_adc = Analog(motor2040.SHARED_ADC)
vol_adc = Analog(motor2040.SHARED_ADC, motor2040.VOLTAGE_GAIN)
cur_adc = Analog(motor2040.SHARED_ADC, motor2040.CURRENT_GAIN,
                 motor2040.SHUNT_RESISTOR, motor2040.CURRENT_OFFSET)

You may notice, all three of these use the same SHARED_ADC pin. This is intentional as it is just a single pin that is being used for all three different functions, only the gains differ.

The next step is to set up the analog multiplexer, by providing it with the three address pins:

mux = AnalogMux(motor2040.ADC_ADDR_0, motor2040.ADC_ADDR_1, motor2040.ADC_ADDR_2)

Note that the AnalogMux does not know about any of the Analog classes that were created before.

With the multiplexer now configured, reading each sensor is simply a case of 'selecting' the sensor on the multiplexer then reading the value from one of the three Analog classes created at the start.

To read the two sensor headers:

for addr in range(motor2040.NUM_SENSORS):
    mux.select(addr + motor2040.SENSOR_1_ADDR)
    print("Sensor", addr + 1, "=", sen_fault_adc.read_voltage())

To read the voltage sense:

mux.select(motor2040.VOLTAGE_SENSE_ADDR)
print("Voltage =", vol_adc.read_voltage(), "V")

To read the current draw in amps (A):

for addr in range(motor2040.NUM_MOTORS):
    mux.select(addr + motor2040.CURRENT_SENSE_A_ADDR)
    print("Current", addr + 1, "=", cur_adc.read_current(), "A")

Configuring Pulls

For some sensors, as well as the internal fault sensor, you may need to have the input be pulled high or low before taking a reading. To support this there is an optional muxed_pin parameter that can be passed into the AnalogMux when creating it, which gives the multiplexer access to the pin to control the pulls.

mux = AnalogMux(motor2040.ADC_ADDR_0, motor2040.ADC_ADDR_1, motor2040.ADC_ADDR_2,
                muxed_pin=Pin(servo2040.SHARED_ADC))

From there the pull state of each of the multiplexer's addresses can be configured independently by calling .configure_pull(), with the address and the pull state (either Pin.PULL_UP, Pin.PULL_DOWN, or None).

The below example shows how to set both sensor addresses to have pull-downs:

sensor_addrs = list(range(motor2040.SENSOR_1_ADDR, motor2040.SENSOR_2_ADDR + 1))
for addr in sensor_addrs:
    mux.configure_pull(addr, Pin.PULL_DOWN)

Fault Sensing

The drivers on Motor 2040 can detect when there is a fault with their connected motors, such as if a short occurs, and shut themselves off for safety. As part of this they also signal that a fault has occurred, which can be read. The way they signal this is by pulling the line to ground. This means that the line needs to be high, and so the 'AnalogMux' needs to be configured accordingly.

adc_as_io = Pin(motor2040.SHARED_ADC)
mux = AnalogMux(motor2040.ADC_ADDR_0, motor2040.ADC_ADDR_1, motor2040.ADC_ADDR_2,
                muxed_pin=adc_as_io)
mux.configure_pull(motor2040.FAULT_SENSE_ADDR, Pin.PULL_UP)

Then in your main code:

mux.select(motor2040.FAULT_SENSE_ADDR)
if not adc_as_io.value():
    print("Fault!")
else:
    print("No Fault")

//TODO make this neater

Controlling the LED

Between Motor 2040's for motor connectors is a single addressable RGB LEDs. This works using the same chainable 1-wire signalling as WS2812 LED's, commonly known as Neopixels. As such, it can be controlled using the same Plasma Library used by the Pimoroni Plasma 2040.

To set up the LED, first import the WS2812 class from the plasma module and the pin constants for the LED from motor:

from plasma import WS2812
from motor import motor2040

Then construct a new WS2812 instance, specifying the number of LEDs, PIO, PIO state-machine and GPIO pin.

led = WS2812(motor2040.NUM_LEDS, 1, 0, motor2040.LED_DATA)

Finally, start the LED by calling start:

led.start()

For more information on how to control the LED, please refer to the Plasma Library.

Pin Constants

The motor module contains a motor2040 sub module with constants for the motor, encoder, LED, sensor and button pins.

Motor Pin Tuples

  • MOTOR_A = (4, 5)
  • MOTOR_B = (6, 7)
  • MOTOR_C = (8, 9)
  • MOTOR_D = (10, 11)

Encoder Pin Tuples

  • ENCODER_A = (0, 1)
  • ENCODER_B = (2, 3)
  • ENCODER_C = (12, 13)
  • ENCODER_D = (14, 15)

UART/ULTRASOUND Pins

  • TX_TRIG = 16
  • RX_ECHO = 17

LED Pin

  • LED_DATA = 18

I2C Pins

  • INT = 19
  • SDA = 20
  • SCL = 21

Button Pin

  • USER_SW = 23

Address Pins

  • ADC_ADDR_0 = 22
  • ADC_ADDR_1 = 24
  • ADC_ADDR_2 = 25

ADC Pins

  • ADC0 = 26
  • ADC1 = 27
  • ADC2 = 28
  • SHARED_ADC = 29

Other Constants

The motor2040 sub module also contains other constants to help with the control of Motor 2040's various features:

Counts

  • NUM_MOTORS = 4
  • NUM_ENCODERS = 4
  • NUM_SENSORS = 2
  • NUM_LEDS = 1

Addresses

  • CURRENT_SENSE_A_ADDR = 0b000
  • CURRENT_SENSE_B_ADDR = 0b001
  • CURRENT_SENSE_C_ADDR = 0b010
  • CURRENT_SENSE_D_ADDR = 0b011
  • VOLTAGE_SENSE_ADDR = 0b100
  • FAULT_SENSE_ADDR = 0b101
  • SENSOR_1_ADDR = 0b110
  • SENSOR_2_ADDR = 0b111

Sensing

  • VOLTAGE_GAIN = 0.28058
  • SHUNT_RESISTOR = 0.47
  • CURRENT_GAIN = 5
  • CURRENT_OFFSET = -0.02

Motor

Getting Started

To start using motors with your Motor 2040, you will need to first import the Motor class.

from motor import Motor, motor2040

If you are using another RP2040 based board, then motor2040 can be omitted from the above line

To create your motor, choose which GPIO pins it will be connected to, and pass that into Motor. For this example we will use one of the handy constants of the motor2040.

m = Motor(motor2040.MOTOR_A)

You now have a Motor class called m that will control the physical motor connected to MOTOR_A. To start using this motor, simply enable it using:

m.enable()

This activates the motor and sets it to its last known speed. Since this is the first time enabling the motor, there is no last known speed, so instead it will be zero.

Once you have finished with the motor, it can be disabled by calling:

m.disable()

From here the motor can be controlled in several ways. These are covered in more detail in the following sections.

Control by Speed

The most intuitive way of controlling a motor is by speed. Speed can be any number that has a real-world meaning for that type of motor, for example revolutions per minute, or the linear or angular speed of the mechanism it is driving. By default the speed is a value ranging from -1.0 to 1.0 but this can be changed by setting a new speed_scale. See Configuration for more details.

The speed of a motor can be set by calling .speed(speed), which takes a float as its speed input. If the motor is disabled, this will enable it. The resulting duty cycle will also be stored.

To read back the current speed of the motor, call .speed() without any input. If the motor is disabled, this will be the last speed that was provided when enabled.

At this time the speed of a motor is the same as m.duty() * m.speed_scale(), though there could be the option in the future to add a curve to a motor's speed that make the relationship between speed and duty cycle become non-linear.

Common Speeds

To simplify certain motion patterns, a motor can be commanded to three common speeds. These are, full negative, full positive, and stopped. These are performed by calling .full_negative(), .full_positive(), and .stop(), respectively. If the motor is disabled, these will enable it.

The full negative and full positive speed can be read back using .speed_scale(). This can be useful as an input to equations that provide numbers directly to .speed(speed), for example.

Control by Percent

Sometimes there are projects where a motor needs to drive based on the reading from a sensor or another device, but the numbers given out are not easy to convert to speeds the motor accepts. To overcome this the library lets you drive the motor at a percent between its negative and positive speeds, or two speeds provided, based on that input.

With an input between -1.0 and 1.0, a motor can be driven at a percent between its negative and positive speeds using .at_percent(in).

With an input between a provided min and max, a motor can be driven at a percent between its negative and positive speeds using .at_percent(in, in_min, in_max).

With an input between a provided min and max, a motor can be driven at a percent between two provided speeds using .at_percent(in, in_min, value_min, value_max).

If the motor is disabled, these will enable it.

Control by Duty Cycle

At a hardware level DC motors operate by receiving a voltage across their two terminals, with positive causing a motion in one direction and negative causing a motion in the other. To avoid needing a negative voltage supply, motor drivers employ a H-Bridge arrangement of transistors or mosfets to flip which side of the motor is connected to ground and which is connected to power. By rapidly turing these transistors or mosfets on and off both the speed and direction of the motor can be varied. The common way this is achieved is by using a pair of pulse width modulated signals, where the duty cycle of the active signal controls the speed, and which signal is active controls the direction. Braking can also be controlled (see //TODO)

The duty cycle of a motor can be set by calling .duty(duty), which takes a float from -1.0 to 1.0 as its duty input. If the motor is disabled this will enable it. This function will also recalculate the related speed.

To read back the current duty cycle of the motor, call .duty() without any input. If the motor is disabled, this will be the last duty that was provided when enabled.

At this time the duty cycle of a motor is the same as m.speed() / m.speed_scale(), though there could be the option in the future to add a curve to a motor's speed that make the relationship between speed and duty cycle become non-linear.

Frequency Control

Motors can be driven at a variety of frequencies, with a common values being above the range of human hearing. As such this library uses 25KHz as its default, but this can easily be changed.

The frequency (in Hz) of a motor can be set by calling .frequency(freq), which takes a float as its freq input. //TODO The supported range of this input is 10 to 350 Hz.

To read back the current frequency (in Hz) of the motor, call .frequency() without any input.

Note that changing the frequency does not change the duty cycle sent to the motors, only how frequently pulses are sent.

Configuration

There are several options for configuring a motor. As mentioned in earlier sections, the first is to adjust their speed scale. There is also the option to change their direction, change their decay mode, and add in a dead-zone. This is a percentage of the duty cycle below which the motor will be stopped. This is included to avoid annoying situations where the duty cycle is too low for a motor to move, but enough for it to make an audable sound.

Decay mode is //TODO .brake() .coast()

Function Reference

Here is the complete list of functions available on the Motor class:

//TODO
Motor(pins, calibration=ANGULAR, freq=50)        # calibration can either be an integer or a Calibration class
pins()
enable()
disable()
is_enabled()
duty()
duty(duty)
speed()
speed(speed)
frequency()
frequency(freq)
stop()
coast()
brake()
full_negative()
full_positive()
at_percent(in)
at_percent(in, in_min, in_max)
at_percent(in, in_min, in_max, speed_min, speed_max)
direction()
direction(direction)
speed_scale()
speed_scale(speed_scale)
deadzone()
deadzone(deadzone)
decay_mode()
decay_mode(mode)

PWM Limitations

Although the RP2040 is capable of outputting up to 16 PWM signals, there are limitations of which pins can be used together:

  • The PWMs output from pins 0 to 15 are repeated for pins 16 to 29, meaning that those pins share the same signals if PWM is enabled on both. For example if you used pin 3 for PWM and then tried to use pin 19, they would both output the same signal and it would not be possible to control them independently.
  • The 16 PWM channels are grouped into 8 PWM slices, each containing A and B sub channels that are synchronised with each other. This means that parameters such as frequency are shared, which can cause issues if you want one motor to operate at a different frequency to it's pin neighbour or to drive an LED with PWM at a high frequency.

The official RP2040 datasheet, includes the handy Table 525 that details the pwm channel for each GPIO pin. This is shown below for convenience:

GPIO 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
PWM Channel 0A 0B 1A 1B 2A 2B 3A 3B 4A 4B 5A 5B 6A 6B 7A 7B
GPIO 16 17 18 19 20 21 22 23 24 25 26 27 28 29
PWM Channel 0A 0B 1A 1B 2A 2B 3A 3B 4A 4B 5A 5B 6A 6B

MotorCluster

Getting Started

An alternative way of controlling motors with your Motor 2040 is to use the MotorCluster class. This can be imported as follows:

from motor import MotorCluster, motor2040

(If you are using another RP2040 based board, then motor2040 can be omitted from the above line)

The next step is to choose which GPIO pins the cluster will be connected to and store them in a list. For example, using the handy constants of the motor2040, the below line creates the list [ (4, 5), (6, 7), (8, 9), (10, 11) ]

pins = [ motor2040.MOTOR_A, motor2040.MOTOR_B, motor2040.MOTOR_C, motor2040.MOTOR_D ]

To create your motor cluster, specify the PIO, PIO state-machine and GPIO pins you chose a moment ago, and pass those into MotorCluster.

cluster = MotorCluster(0, 0, pins)

You now have a MotorCluster class called cluster that will control the physical motors connected to MOTOR_A, MOTOR_B, MOTOR_C, and MOTOR_D. To start using these motors, simply enable them using:

cluster.enable_all()

or

cluster.enable(motor)

where motor is the motor's number within the cluster from 0 to cluster.count() - 1.

These functions activate the motors and drives them at their last known speed. Since this is the first time enabling the motors, there are no last known speeds, so instead they will be set to zero instead.

Once you have finished with the motors, they can be disabled by calling:

cluster.disable_all()

or

cluster.disable(motor)

where motor is the motor's number within the cluster from 0 to cluster.count() - 1.

From here the motors can be controlled in several ways. These are covered in more detail in the following sections.

Control by Speed

The most intuitive way of controlling a motor is by speed. Speed can be any number that has a real-world meaning for that type of motor, for example revolutions per minute, or the linear or angular speed of the mechanism it is driving. By default the speed is a value ranging from -1.0 to 1.0 but this can be changed by setting a new speed_scale. See Configuration for more details.

The speed of a motor on a cluster can be set calling .speed(motor, speed) or .all_at_speed(speed), which take a float as their speed input. If a motor is disabled, these will enable it. The resulting duty cycle will also be stored.

To read back the current speed of a motor on the cluster, call .speed(motor). If the motor is disabled, this will be the last speed that was provided when enabled.

Common Speeds

To simplify certain motion patterns, motors on a cluster can be commanded to three common speeds. These are, full negative, full positive, and stopped. These are performed by calling .full_negative(motor), .full_positive(motor), and .stop(servo), respectively. If the motor is disabled, these will enable it. There are also .all_full_negative(), .all_full_positive(), and .stop_all() for having all the motors on the cluster drive at once.

The full negative and full positive speed of each motor on a cluster can be read back using .speed_scale(motor). This can be useful as an input to equations that provide numbers directly to .speed(motor, speed), for example.

Control by Percent

Sometimes there are projects where motors need to move based on the readings from sensors or another devices, but the numbers given out are not easy to convert to speeds the motors accept. To overcome this the library lets you drive the motors on a cluster at a percent between their negative and positive speeds, or two speeds provided, based on that input.

With an input between -1.0 and 1.0, a motor on a cluster can be driven at a percent between its negative and positive speeds using .at_percent(motor, in), or all motors using .all_at_percent(in).

With an input between a provided min and max, a motor on a cluster can be driven at a percent between its negative and postive speeds using .at_percent(motor, in, in_min, in_max), or all motors using .all_at_percent(in, in_min, in_max).

With an input between a provided min and max, a motor on a cluster can be driven at a percent between two provided speeds using .at_percent(motor, in, in_min, speed_min, speed_max), or all motors using .all_at_percent(in, in_min, speed_min, speed_max).

If the motor is disabled, these will enable it.

Control by Duty Cycle

At a hardware level DC motors operate by receiving a voltage across their two terminals, with positive causing a motion in one direction and negative causing a motion in the other. To avoid needing a negative voltage supply, motor drivers employ a H-Bridge arrangement of transistors or mosfets to flip which side of the motor is connected to ground and which is connected to power. By rapidly turing these transistors or mosfets on and off both the speed and direction of the motor can be varied. The common way this is achieved is by using a pair of pulse width modulated signals, where the duty cycle of the active signal controls the speed, and which signal is active controls the direction. Braking can also be controlled (see //TODO)

The duty cycle of a motor on a cluster can be set by calling .duty(motor, duty) or .all_at_duty(duty), which take a float as their duty input. If a motor is disabled, these will enable it. These functions will also recalculate the related speed.

To read back the current duty cycle of a motor on a cluster, call .duty(motor). If the motor is disabled, this will be the last duty that was provided when enabled.

Frequency Control

Motors can be driven at a variety of frequencies, with a common values being above the range of human hearing. As such this library uses 25KHz as its default, but this can easily be changed.

The frequency (in Hz) of all the motors on a cluster can be set by calling .frequency(freq), which takes a float as its freq input. //TODO The supported range of this input is 10 to 350 Hz. Due to how MotorCluster works, there is no way to set independent frequencies for each motor.

To read back the current frequency (in Hz) of all the motors on a cluster, call .frequency() without any input.

Note that changing the frequency does not change the duty cycle sent to the motors, only how frequently pulses are sent.

Also, be aware that currently the frequency changes immediately, even if part-way through outputting a pulse. It is therefore recommended to disable all motors first before changing the frequency.

Phase Control

When dealing with many servos, there can often be large current draw spikes caused by them all responding to pulses at the same time. To minimise this, the ServoCluster class allows for the start time of each servo's pulses to be delayed by a percentage of the available time period. This is called their phase.

The phase of a servo on a cluster can be set by calling .phase(servo, phase) or .all_to_phase(phase), which take a float between 0.0 and 1.0 as their phase input.

To read back the current phase of a servo on a cluster, call .phase(servo).

By default all servos on a cluster will start with different phases unless auto_phase=False is provided when creating the ServoCluster.

Calibration

There are different types of servos, with ANGULAR, LINEAR, and CONTINUOUS being common. To support these different types, each ServoCluster class contains calibration objects for each of its servos that store the specific value to pulse mappings needed for their types. A copy of a servo's calibration on a cluster can be accessed using .calibration(servo). It is also possible to provide a servo on a cluster with a new calibration using .calibration(servo, calibration).

Delayed Loading

To match behaviour with the regular Motor class, MotorCluster automatically applies each change to its motor's states immediately. However, sometimes this may not be wanted, and instead you want all motors to receive updated duty cycles at the same time, regardless of how long the code ran that calculated the update.

For this purpose, all the functions that modify a motor state on a cluster include an optional parameter load, which by default is True. To avoid this "loading" include load=False in the relevant function calls. Then either the last call can include load=True, or a specific call to .load() can be made.

Function Reference

Here is the complete list of functions available on the MotorCluster class:

//TODO
MotorCluster(pio, sm, pins, calibration=ANGULAR, freq=50, auto_phase=True)    # calibration can either be an integer or a Calibration class
count()
pins(motor)
enable(motor, load=True)
enable_all(load=True)
disable(motor, load=True)
disable_all(load=True)
is_enabled(motor)
duty(motor)
duty(motor, duty, load=True)
all_at_duty(motor, load=True)
speed(motor)
speed(motor, speed, load=True)
all_at_speed(speed, load=True)
phase(motor)
phase(motor, phase, load=True)
all_to_phase(phase, load=True)
frequency()
frequency(freq)
full_negative(motor, load=True)
all_full_negative(load=True)
full_positive(motor, load=True)
all_full_positive(load=True)
stop(motor, load=True)
stop_all(load=True)
coast(motor, load=True)
coast_all(load=True)
brake(motor, load=True)
brake_all(load=True)
at_percent(motor, in, load=True)
at_percent(motor, in, in_min, in_max, load=True)
at_percent(motor, in, in_min, in_max, value_min, value_max, load=True)
all_at_percent(in, load=True)
all_at_percent(in, in_min, in_max, load=True)
all_at_percent(in, in_min, in_max, value_min, value_max, load=True)
load()

PIO Limitations

The RP2040 features two PIOs with four state machines each. This places a hard limit on how many MotorClusters can be created. As this class is capable of driving all 30 GPIO pins, the only time this limit will be of concern is when motors with different frequencies are wanted, as all the outputs a MotorCluster controls share the same frequency. Relating this to the hardware PWM, think of it as a single PWM slice with up to 30 sub channels, A, B, C, D etc.

When creating a MotorCluster, in most cases you'll use 0 for PIO and 0 for PIO state-machine. You should change these though if you plan on running multiple clusters, or using a cluster alongside something else that uses PIO, such as our Plasma library.

Configuration

After using servos for a while, you may notice that some don't quite go to the positions you would expect. Or perhaps you are giving values to a continuous rotation servo in degrees when it would make more sense to use a speed or rpm. To compensate for these cases, each Servo class or servo within a ServoCluster has an individual Calibration class. This class contains a set of pulse-value pairs that are used by the .value(value) functions (and those similar) to convert real-world numbers into pulses each servo understand.

Common Types

There are three common types of servo's supported:

  • ANGULAR = 0 - A servo with a value that ranges from -90 to +90 degrees.
  • LINEAR = 1 - A servo with a value that ranges from 0 to +1.0.
  • CONTINUOUS = 2 - A servo with a value that ranges from -1.0 to +1.0.

By default all Servo classes or servo within a ServoCluster are ANGULAR. This can be changed by providing one of the other types as a parameter during their creation, as shown below:

angular = Servo(servo2040.SERVO_1)  # ANGULAR is the default so does not need to be specified here
linear = Servo(servo2040.SERVO_2, LINEAR)
continuous = Servo(servo2040.SERVO_3, CONTINUOUS)

Custom Calibration

As well as the common types, a custom calibration can also be provided to one or more servos during creation. Below is an example that creates an angular servo that can only travel from -45 degrees to 45 degrees.

cal = Calibration()
cal.apply_two_pairs(1000, 2000, -45, 45)
s = Servo(servo2040.SERVO_1, cal)

This could be useful for example if the servo turning beyond those values would cause damage to whatever mechanism it is driving, since it would not be possible to go to angles beyond these unless limits were disabled (see Limits). Also it lets the exact pulse widths matching the angles be set (the 1000 and 2000 in the example). Discovering these values can take some trial and error, and will offen be different for each servo.

Modifying a Calibration

It is also possible to access and modify the calibration of a Servo or a servo on a ServoCluster after their creation. This is done by first getting a copy of the servo's calibration using .calibration() or .calibration(servo), modifying its pulses or values, then applying the modified calibration back onto to the servo.

Below, an angular servo is modified to increase its reported rotation range from 180 degrees to 270 degrees.

wide_angle = Servo(servo2040.SERVO_1)
cal = wide_angle.calibration()
cal.first_value(-135)
cal.last_value(+135)
wide_angle.calibration(cal)

Movement Limits

As well as providing a mapping between pulses and values, the calibration class also limits a servo from going beyond its minimum and maximum values. In some cases this may not be wanted, so the state of the limits can be modified by calling .limit_to_calibration(lower, upper) where lower and upper are booleans. Additionally, the current state of these limits can be queried by calling .has_lower_limit() and .has_upper_limit(), respectively.

A case where you may want to disable limits is if you want a servo to go to a value (e.g. 90 degrees), but are not physically able to get a pulse measurement for that but can do another value instead (e.g. 60 degrees).

Note, even with limits disables, servos still have hard limits of 400 and 2600 microsecond pulse widths. These are intended to protect servos from receiving pulses that are too far beyond their expected range. These can vary from servo to servo though, with some hitting a physical end-stop before getting to the typical 500 and 2500 associated with -90 and +90 degrees.

Populating a Calibration

To aid in populating a Calibration class, there are five helper functions that fill the class with pulse-value pairs:

  • apply_blank_pairs(size) - Fills the calibration with the specified number of zero pairs
  • apply_two_pairs(min_pulse, max_pulse, min_value, max_value) - Fills the calibration with two pairs using the min and max numbers provided
  • apply_three_pairs(min_pulse, mid_pulse, max_pulse, min_value, mid_value, max_value) - Fills the calibration with three pairs using the min, mid and max numbers provided
  • apply_uniform_pairs(size, min_pulse, max_pulse, min_value, max_value) - Fills the calibration with the specified number of pairs, interpolated from the min to max numbers provided
  • apply_default_pairs(type) - Fills the calibration with the pairs of one of the common types

Once a Calibration class contains pairs (as checked .size() > 0), these can then be accessed by calling .pair(index) and can then be modified by calling .pair(index, pair). The former function returns a list containing the pulse and value of the pair, and the latter accepts a list or tuple containing the pulse and value. For situations when only a single element of each pair is needed, .pulse(index) and .value(index) will return the current numbers, and .pulse(index, pulse) and .value(index, value) will override them.

For further convenience there are functions for accessing and modifying the .first() and .last() pair/pulse/value of the calibration.

Viewing the Mapping

To aid in visualising a calibration's pulse-value mapping, the pulse for any given value can be queried by calling .value_to_pulse(value). Similarly, the value for any given pulse can be queried by calling .pulse_to_value(pulse). These are the same functions used by Servo and ServoCluster when controlling their servos.

Function Reference

Here is the complete list of functions available on the Calibration class:

Calibration()
Calibration(type)
apply_blank_pairs(size)
apply_two_pairs(min_pulse, max_pulse, min_value, max_value)
apply_three_pairs(min_pulse, mid_pulse, max_pulse, min_value, mid_value, max_value)
apply_uniform_pairs(size, min_pulse, max_pulse, min_value, max_value)
apply_default_pairs(type)
size()
pair(index)
pair(index, pair)
pulse(index)
pulse(index, pulse)
value(index)
value(index, value)
first()
first(pair)
first_pulse()
first_pulse(pulse)
first_value()
first_value(value)
last()
last(pair)
last_pulse()
last_pulse(pulse)
last_value()
last_value(value)
has_lower_limit()
has_upper_limit()
limit_to_calibration(lower, upper)
value_to_pulse(value)
pulse_to_value(pulse)