By Jose Alcantar, E&C Engineer

Introduction:

The GPS information received through the Arxterra control panel can be sent through three different data types. The first data type is a 32-bit floating point, the second is a 64-bit floating point or “double” and lastly a 32-bit integer or “long”. Depending on how the settings are configured through the “Robot Capabilities Configuration” menu in the app, the user can choose between any of these three data types. For further information on the data types see the blogpost by Jeff Gomes,

https://www.arxterra.com/arxterra-now-supports-waypoint-navigation/

For our purpose, the 32-bit integer was selected as the data type due to some formatting issues when using the 32-bit floating or 64-bit floating types.

To begin with formatting the coordinates, it must first be understood what values are being sent through the Arxterra app. From the link above, Gomes explains that for the 32-bit integer the units are set to decimicrodegrees. This means that for example, coordinates 33.794904, -117.913712 would be encoded as 33794904, -117913712. The app sends the coordinate values through a 16-byte packet, in which the first 4 bytes are the latitude and the last 4 bytes are the longitude coordinates, bytes being represented in hexadecimal.

There are different commands available related to GPS navigation, further described in this link,

https://www.arxterra.com/pathfinder-arxterra-communication-commands/

Example:

The following firmware code is based on the WAYPOINT _COORD command, which is used to receive the waypoint coordinate packet data and to format the coordinates into usable values.

After a waypoint is created, for example, 33.794904, -117.124567, the app converts these values into decimicrodegrees and sends the values to the MCU as a 32-bit long. The data being sent will be as follows

{A5, 10 (packetlength),14 (command id), 02 03 AB 58 F9 04 D6 80, (checksum)};

To extract the data through the Arduino some variables must first be declared:

long waypointLat4;

long waypointLat3;

long waypointLat2;

long waypointLat1;

unsigned long waypointLat;

long waypointLon4;

long waypointLon3;

long waypointLon2;

long waypointLon1;

unsigned long waypointLon;

The byte vales are extracted from the packet and the bits are shifted to be stored into a 32-bit unsigned long.

void waypoint_coordHandler (uint8_t cmd, uint8_t param[], uint8_t n)

{

waypointLat4 = param[0];

waypointLat3 = param[1];

waypointLat2 = param[2];

waypointLat1 = param[3];

waypointLat = (waypointLat4 << 24 | waypointLat3 << 16| waypointLat2 << 8| waypointLat1);

waypointLon4 = param[4];

waypointLon3 = param[5];

waypointLon2 = param[6];

waypointLon1 = param[7];

waypointLon = (waypointLon4 << 24 | waypointLon3 << 16| waypointLon2 <<                  8| waypointLon1);

waypointLon = ~waypointLon;

waypointLon = (waypointLon + 1) * -1;

}

The longitude coordinates require a secondary step to get the right value. Because longitude is a negative value, a 2’s complement operation must be done on the final value to get the right coordinate. Using this formatting, successfully converts the packet data being sent into usable coordinates needed for GPS navigation.

Conclusion:

Following this method, the GPS data sent through the Arxterra control panel can be converted into usable values, which will help with autonomous navigation.

Resources:

https://www.arduino.cc/en/Reference/Bitshift

https://www.arduino.cc/en/Reference/UnsignedLong

https://www.arduino.cc/en/Reference/Long

https://en.wikipedia.org/wiki/Two’s_complement

By Jose Alcantar, Electronics and Controls Engineer

Introduction:

To drive the six motors independently as per requirement, 3 dual motor drivers are being utilized. Three VNH5019 motor drivers will be used to control each motor along with a PCA96805 I/O expander and a hex inverter.

VNH5019:

The VNH5019 dual motor driver requires three pins to control each of the two H-Bridges integrated onto the board. The two-motor direction pins (M1INA and M1INB) for the first H-Bridge use a digital output high and low to set the direction of the motor. The following truth table summarizes the different settings for the pins:

For example, to drive the motor clockwise M1INA is set high, while M1INB is set low. To reverse the direction M1INA is set low and M1INB is set high.

The third pin (M1PWM) is used to control the speed of the motor via a PWM signal.

The pathfinder will utilize three VNH5019 dual motor driver boards, with each board requiring 6 pins to control each motor. This gives us a total of 18 pins needed to control the motors. Due to the limitation of pins on the Arduino Leonardo, a PCA9685 16-channel 12-bit I/O expander along with a hex inverter will be used to control each of these pins.

PCA9685 & 74HC04 Hex Inverter:

The PCA9685 I/O expander allows the user to expand the capabilities of the Arduino by providing an additional 16 channels through I2C interface. Six of these channels will be dedicated for controlling the PWM input of all six H-Bridges. The PCA9685 has a dedicated library available that simplifies the use of the board. The following sample code shows how to set the output of one of these channels.

pwm.setPWM(0, 0, 4095);   //Sets the output to 100% duty cycle

In the above sample code the first input is the channel output pin, followed by the off time for the duty cycle and finally, the last value is the on-time for the duty cycle. Because the board has 12-bit resolution, the values for the duty cycle range between (0 – 4095), giving 4096 steps.

To control the motor direction, the use of a Hex inverter along with the I/O expander will be used to reduce the number of pins being utilized. Using an inverter, the motor direction can be controlled clockwise and counterclockwise with the use of a single pin. The following diagram shows the basic idea of how the direction will be determined.

In this case, setting the channel output HIGH or 100% duty cycle on the I/O expander, will set the direction pin M1INA high and M1INB low, driving the motor in the clockwise direction. Setting the channel output LOW, or 0% duty cycle, will do the opposite.

Conclusion:

With this configuration, the 18 pins needed to drive the motors is reduced to only 12 pins from the I/O expander. This leaves four more pins on the expander to drive servos or other actuators.

Current Sensing Experiment:

Purpose:

Measuring motor draw current on current sensing pins via the VNH5019 to identify no load conditions and determine no-load threshold to shut off motors.

The VNH5019 provides two current pins to the user, which can be used to measure the instantaneous current absorption by each H-Bridge.

Procedure:

While running a motor through the VNH5019, the current was measured using the Arduino Leonardo via the current sensing pins M1CS and M2CS. Through software, a running average smoothing filter was used to remove inconsistencies in the data. After graphing the current data drawn by the averagre, a threshold was determined. An ammeter was attached to the motor to directly compare the current measured with the Arduino data.

Results:

After reading the current measured by the Arduino, a current range was determined at which the motor ran under no load conditions. From the data, it was determined that under no load conditions, the current draw was between 450 mA and 600 mA. Using the Ammeter as a reference, it was found that the actual current draw of the motor was about 620 mA. The threshold for the current under no load conditions was chosen to be 620 mA.