Design, fabrication and test of an embedded ...

International Journal of Intelligent Unmanned Systems Design, fabrication and test of an embedded lightweight kinematic autopilot (ELKA) Luca Petricca Vikram Hrishikeshavan Per Ohlckers Inderjit Chopra

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IJIUS 2,2

Design, fabrication and test of an embedded lightweight kinematic autopilot (ELKA)


140 Received 24 October 2013 Revised 24 October 2013 Accepted 1 April 2014

Luca Petricca Vestfold University College, IMST, Borre, Norway

Vikram Hrishikeshavan Department of Aerospace Engineering, University of Maryland, College Park, Maryland, USA

Per Ohlckers Vestfold University College, IMST, Borre, Norway, and

Inderjit Chopra University of Maryland, College Park, Maryland, USA Abstract Purpose – Unmanned vehicles flight is controlled by embedded circuits in the aircraft, under the remote control of a pilot on the ground. This circuit, called autopilot, represents one of the key elements inside the vehicles. The authors developed one of the smallest autopilot, specifically designed for low-weight low-power applications. The paper aims to discuss these issues. Design/methodology/approach – The system is based on STM32 ARM Cortex M3 microcontroller. It includes an onboard 9 DOF IMU (MPU9150) and a 2.4 GHz wireless transceiver (nRF24L01 þ ). Findings – The embedded lightweight kinematic autopilot (ELKA) can pilot up to eight servomotors, and can be used to monitor more than 100 sensors. The final assembled board is 28 21 mm2 and weighs around 1.2 grams (battery excluded), and has successfully passed initial functionality tests. Originality/value – The authors presented the design, fabrication and initial tests of a lightweight kinematic autopilot (ELKA board version 1.0). The system has been designed in order to upgrade the state-of-art capability in sensing and processing over a previous autopilot (GINA), which is of similar weight and size. The small size (28 21 mm2) and the lightweight (around 1.2 grams) make ELKA one of the smallest autopilot in the world. Keywords UAV, Autopilot, ELKA, Embedded system, NAV/MAV, Unmanned vehicles Paper type Research paper

International Journal of Intelligent Unmanned Systems Vol. 2 No. 2, 2014 pp. 140-150 r Emerald Group Publishing Limited 2049-6427 DOI 10.1108/IJIUS-10-2013-0022

I. Introduction Autonomous vehicles are nowadays in high demand for many applications, spanning from military operations to surveillance and rescue operations. Therefore it is a very hot research topic, with many universities and organizations actively working with it (Petricca et al., 2011). Some commercial players already have products available in the market such as ProxDynamics (ProxDynamics A.S., 2013; BBC, 2013) while others are still in the research and development phase. Among the autonomous vehicles (ground, aerial, etc.) nano-air vehicles (NAVs) have more challenging requirements regarding weight and power budget. As the dimension shrinks down, these vehicles suffer from multiple scaling disadvantages such as poor aerodynamic efficiency, and faster body dynamics that poses challenges in sensing, control and maintaining stability in presence of disturbances (Petricca et al., 2011). Furthermore, shrinking of a complete flying system to less than 15 grams start to be very challenging since at this scale everything must be optimized, from the airframe to the sensor placement, and


the battery. Each gram saved in any of the vehicle section may greatly improve the performance of the vehicles, increasing flight endurance and reducing acoustic noise (stealth). On this already limited weight budget, electronics can use only a small percentage of it (around 10-15 percent of the total weight for a NAV), while keeping most of the functions required for any autonomous vehicles regardless of size and weight, such as ability to wireless communicate with a ground station, receiving and actuating commands, collecting data from sensors, flight stabilization, etc. In the NAV range, the choice of autopilot circuitry that weighs less than two grams and with a low-power capability is very limited. The most interesting autopilot currently available is the GINA board (Pister and Mehta, 2010), developed at University of Berkeley. It includes on board inertial sensing and wireless transceiver. The pitch, roll and angular rates are determined by a three-axis ITG3200 gyroscope from Invensense. The linear accelerations are measured by a three-axis KXSD9 accelerometer from Kionix. The Core consists of on MSP430 16-bit microcontroller, which is part of MSP430F2xx family from Texas Instruments that supports up to 16 MHz operations. While this board is relative small 25 mm 25 mm, and lightweight (1.7 grams), it has been more than three years since latest developments and thus many components and features such as sensor and microcontroller capabilities are outdated. The 16-bit microcontroller may be a limitation for some advanced computations. Therefore, the aim of the present work was to develop an improved lightweight autopilot, with a faster core and better Inertial Motion Units. In order to comply with low-power and low-weight specifications, we designed the system as simple as possible while leaving the possibility of accessing various peripherals through external connectors. Furthermore, we also wanted flexibility and hardware independency, in order to use this board not only on a specific class of vehicles, but one that can be easily adapted from ground vehicles to aerial vehicles; for example we have the possibility to pilot up to eight servo motors, which may be a requirement for more exotic class of flying vehicles such as the opto-copter. Communication was also an integral part of the circuit design. We wanted the system to communicate at normal 2.4 GHz without adding any extra element on-board; so we included a chip antenna on-board, making embedded lightweight kinematic autopilot (ELKA) board ready to be used without any additional components while care was taken to maintain acceptable line-of-sight wireless range (B50-70 m). Table I reports a comparison between ELKA v1.0 and GINA 2.2.

Design, fabrication and test of an ELKA 141

II. ELKA design and fabrication ELKA (v1.0) consists of three main blocks (Figure 1): (1)

microcontroller (mC);

No. of chips used (IMU þ RF þ mC) Transmission range (m)a Gyro spectral noise (1/sec) Bits mC Speed (MHz) RAM (kB) ROM/FLASH (kB) Note: aEstimation

GINA

ELKA

6 20 0.03 16 16 8 116

3 80 0.005 32 72 20 128

Table I. GINA vs ELKA comparison

IJIUS 2,2


142

(2)

inertial motion unit (IMU); and

(3)

wireless transceiver (RF).

Furthermore two extra connectors (EXT) are available on-board for accessing extra features. The microcontroller here used is the STM32F103CB from ST Microelectronics (STM), a 32-bit microcontroller based on the ARM CORTEX M3 core. In particular, the F103 is one of the high-performance microcontrollers of the F1 family. The core works at 72 MHz, while the large memory available onboard (flash memory up to 128 kB and SRAM up to 20 kB) allows storing large programs and executing complex algorithms. The STM device also offers two 12-bit ADCs, three general purpose 16-bit timers (four channel each), one PWM timer, as well as advanced communication interfaces such as: two I2Cs (inter-integrated circuit), two SPIs (serial peripheral interface) and three USARTs. The device accepts 3.3 V as supply voltage. A 48-pin package (LQFP-48) was chosen as it was the perfect trade-off for this application between size and number of pins. The instruction set of the F1 family includes Thumb, Thumb-2 and Saturated Math. Regarding the IMU the choice was a single chip which included all the required sensors. In particular we chose the state of the art 9 DOF MPU9150 from Invensense which include a three-axis accelerometer, gyro and magnetometer. Furthermore it also includes a thermal sensor which could be used for compensate the thermal drift of the sensors. Having all the motion sensors in one chip provide several advantages: it requires less footprint on the board, saving weight and space; it helps to save power, since the power consumption of one single chip is in general lower than three discrete chips; and most importantly, eliminate the misalignment error between gyro, accelerometer and magnetometers axis. The gyro noise is 0.005 full-scale range (dps)/Hz and has a user-programmable full-scale range setting for the accelerometer and gyroscope (Invensense, 2013a). For the transmission we chose a single-chip, very low-power RF transceiver with a data rate up to 1 Mbit/s, packed in a QFN24 package (5 5 mm). The chip is the NRF24l01 þ from Nordic Semiconductor. As power supply we decided to use a single cell Li-PO battery with 3.7 V nominal voltage, further stabilized and lowered to 3.3 V by using two low-drop voltage regulators TPS79301 from Texas Instruments. In Figure 2 the complete schematic of the board is shown. The layout was partially generated using EAGLE AutoRoute (CadSoft EAGLE). The signals were split into three classes with three different line widths and VIA size: (1) (2)

standard signals: with a minimum trace width, trace spacing and VIAS of 4 mils; power signals (VCC, GND): with a minimum trace width, trace spacing and VIAS of 8 mils; and

(3)

RF signals: with a preferable trace width of 32 mils and VIAS of 12 mils.

IMU EXT

Figure 1. ELKA block diagram

µC RF

15pF

C2

DGND

1 4 2

3

MOTOR_1 MOTOR_2 MOTOR_3 MOTOR_4

V2 V1 GND1 GND

4.7uF

C6

1 3 5 7 9

15pF

C3

1 3 5 7 9

DGND

DGND

100nF

DGND

C5

100nF

R2

100k

4.7uF

2 4 6 8 10

2 4 6 8 10

PC13 PC14 PC15

U1PORTC

FTE-105-01-G-DV

J3

VCC

2 3 4

DGND

PD1/OSC=> PD0/OSC