RPM Revolutions per minute. UAV Unmanned aerial vehicle. I. Introduction. Electric unmanned aerial vehicles (UAVs) are increasingly being operated in a ...
Experimental Testing of Electronic Speed Controllers for UAVs Andrew Gong∗ and Dries Verstraete† The University of Sydney, Sydney, NSW, 2006, Australia
Electronic speed controllers are a vital component in the powertrain of electric propulsion systems in unmanned aerial vehicles (UAVs). However, performance data of speed controllers is scarce. This paper presents the experimental set-up and testing of a range of commercial-off-the-shelf ESCs. Test results are presented and empirical models are derived to predict the performance and efficiency of these ESCs in a variety of operating conditions.
Nomenclature AC DC ESC PWM RPM UAV
Alternating current Direct current Electronic speed controller Pulse width modulation Revolutions per minute Unmanned aerial vehicle
I.
Introduction
Electric unmanned aerial vehicles (UAVs) are increasingly being operated in a range of civilian and military applications.1–5 Within the powertrain of all these UAVs is an electronic speed controller (ESC) that provides a link between the power source and the motor. The ESC is responsible for controlling the power output and rotational speed of the motor in response to the operator’s throttle commands. In most applications, the ESC converts the direct current (DC) electricity from a battery to 3-phase AC to drive a brushless DC motor. This is achieved via high frequency electronic switching, which could yield considerable efficiency losses. However, despite the importance of the ESC, limited data is available on the performance of ESCs and the impact on electric UAV propulsion systems. Performance of ESCs is especially important for advanced fuel-cell propulsion systems where power is limited and the voltage can change significantly under load.6–9 Lundstrom et al.10 tested a series of low current ESCs and extracted the efficiency as a function of RPM and duty cycle for one ESC/motor/propeller combination. Harrington and Kroninger11 presented the efficiency of a single Castle Creations Phoenix 6 speed controller with an AP03-7000 motor as a function of motor torque. Green12 designed a test bed and evaluated four Hobbywing brand ESCs, although at voltages below the recommended voltage. Roessler13 developed simplified linear approximations for ESC efficiency at full and part throttle. Overall, there is a general lack of ESC performance data, ESC modelling and analysis of the effect it plays in the electric propulsion system. This paper presents the hardware, experimental set-up and some test results of selected commercialoff-the-shelf hobby ESCs typically used in small electric UAVs. The performance and efficiency of a series of ESCs is presented for a range of input voltages, output currents and throttle settings. A physics based ∗ Doctoral Candidate, School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, NSW, Australia, 2006, AIAA Student Member † Senior Lecturer, School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, NSW, Australia, 2006, AIAA Senior Member
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model is presented for the ESC and empirical fit constants are determined to allow for estimation of ESC performance under specific operating conditions.
II.
Hardware and Experimental Set-Up
The experimental test bench consists of a DC power supply, the ESC under test, the three-phase power analyser and an electric motor to absorb the power. The test bench is instrumented with high precision voltage and current sensors to measure the DC power supplied to the ESC. The DC power is fed into the ESC where it is converted to 3 phase AC to drive the brushless motor. This 3 phase AC is passed through a Magtrol 6530 3-phase power analyser before it is used to drive the brushless DC motor. The Magtrol 6530 is able to measure 3-phase power to 1 mW resolution with 0.6 % accuracy to 10 kHz. The 3 phase power analyser measures voltage and current across all three phases in addition to the frequency. A figure showing the set up of the test bench with the various voltage and current sensors is shown in Figure 1, with the Magtrol 3-phase power analyser and motor/propeller combination shown in Figure 2. 3-Phase Power Analyzer
A1
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Power Supply
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Figure 1: Experimental Set-up
(a) Magtrol 6530
(b) Motor and Propeller
Figure 2: Test Equipment The brushless motor used in these tests is a Turnigy Propdrive 3530 1100 kV motor. For the reported tests the motor is connected to a Turnigy 10 x 4.5” propeller mounted on a static stand. Four different ESCs were tested. A table of the tested ESCs and their relevant performance specifications can be found in Table
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1. An image showing the ESCs is also given in Figure 3. The efficiency of the speed controller was determined by calculating the 3-phase output power supplied by the ESC and dividing it by the input power supplied to the ESC by the DC power supply. To investigate the impact of the input voltage on the ESC efficiency, a range of input voltages from 7 V to 12 V were tested and the efficiency calculated. Table 1: ESC Specifications ESC Turnigy Turnigy Turnigy Turnigy
Super Brain 40 Aerostar 30 Aerostar 50 Aerostar 80
Voltage Range (V) 6-13 6-17 6-26 6-26
Maximum Current (A) 40 30 50 80
Figure 3: Commercial ESCs tested
III.
Test Results
Each of the ESCs was tested in a linear sweep from zero throttle to full throttle. Figure 4 shows the variation of efficiency at 7, 9 and 11 V with input current, corresponding to this throttle sweep. It can be seen that at low throttle settings the efficiency is low but rapidly rises and levels off between 80%-90% efficiency. All ESCs had an efficiency above 80% by 5 A, and the efficiency levels off and does not exceed 90%. There is some variation in efficiency with input voltage. It can be seen that all models are more efficienct at 7V than at 11V. This implies that running at a lower voltage can yield efficiency improvements of 2-3% at low throttle settings. However, a higher input voltage allows the ESC to supply a higher current and hence more power to the motor. Variation also exists between different ESC models. For example, the Aerostar 80 is approximately 2-3% more efficient than the Aerostar 30 for input currents above 5 A. This improvement in efficiency is achieved in the larger high power rated ESC due to the lower resistance components used. This allows for a decrease in the electrical power required to provide the same propulsive force at moderate to high current conditions, at the expense of an increase in cost and weight. When flying aircraft, the propulsive requirement is driven by the thrust required to overcome the drag of the aircraft. Hence, a specific propulsive power is required rather than a specific current value. Plotting the variation of efficiency with ESC output power shows an even greater variation between the different input voltages. By running at a lower voltage, the input current required for the same power is higher and this pushes the ESC into the more efficient range of its operating curve. For example, in Figure 5a, the SuperBrain 40 has an efficiency of 84% at 50 W when operating at 11 V, but 87% when operating at 7 V. This improvement in low power efficiency with lower input voltages is seen in all ESCs tested, and supports the findings of Lundstrom in 2010.10 A higher input voltage does however allow a greater power and thrust 3 of 10 American Institute of Aeronautics and Astronautics
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(c) Aerostar 50A
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Figure 4: ESC Efficiency vs Current
output for the same motor and propeller configuration. A similar result is seen in Figure 6, where for a given motor and propeller RPM a lower input voltage allows the ESC to operate more efficiently but with a reduction in maximum RPM achievable. This has implications for electric UAV flight, as the battery voltage tends to drop under load and is the opposite effect to the ideal where the voltage would increase as increasing power is required. This effect is even more pronounced for advanced electric power systems such as fuel cells as these show even greater changes in voltage with load.
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Figure 5: ESC Efficiency vs Power
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Figure 6: Efficiency vs RPM
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IV.
Modelling of Electronic Speed Controllers
To allow the prediction and simulation of the performance of ESCs, an empirical model is presented here based on some of the physical losses that occur. A simplified diagram of the basic electrical architecture of an ESC is presented in Figure 7. DC electricity from the battery or other power source is passed through a circuit of six electronically controlled MOSFETs. These MOSFETs act as digital switches that convert the DC into pulse width modulated (PWM) 3 phase electricity to power the brushless motors.
Figure 7: ESC Circuit Diagram
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Due to the switching nature of the ESC, the three phase electric drive is not a smooth sine wave like most AC sources. Instead, it has high frequency pulses where the current and voltage increase with increasing pulse width. An example of this can be seen in Figure 8. Figure 8a shows a low throttle setting, which is characterised by short pulses of voltage (and current). With increasing throttle setting, the time in which the switches are conducting increases, and the average power increases (Figure 8b). At full throttle, the switches are conducting for as long as possible and the high frequency switching disappears (Figure 8c).
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Figure 8: ESC Output Waveforms for a Range of Throttle Settings For high frequency electronic switches devices such as the ESCs, there are two dominant forms of losses. The first, conduction loss, is the result of the inherent resistance in each of the components used in the switching circuitry. The conduction loss is proportional to the square of the input current and can be approximated by the formula5 2 Pcond ≈ D · RON · IRM S
(1)
where Pcond is the power loss due to the conduction losses, D is the duty cycle, RON is the total internal resistance when the circuit is conducting, and IRM S is the average input current to the ESC.
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The second type, switching losses, occur as a result of the rapid electronic switching that occurs to convert the DC input power into the three-phase AC required by the brushless DC motor. The switching losses PSW can be approximated by the formula5 1 · IRM S · V · (ton + tof f ) · f (2) 2 and tof f are the on and off time for switching and f is the switching
PSW ≈
where V is the input voltage, ton frequency. The final loss that occurs is the standby power draw Pidle . For simplicity this is assumed to vary bilinearly with voltage and current. The efficiency can thus be modelled as the output power divided by the input power (V · IRM S ), where the output power is the input power subtracted by the losses. Thus. η=
D · RON 1 Pidle Pin − Pcond − PSW − Pidle =1− · IRM S − · (ton + tof f ) · f − Pin V 2 (V · IRM S )
(3)
Since the duty cycle D is linearly proportional to the input current IRM S , and RON , ton , tof f , and f are constants, the equation can be rewritten in the form η≈
2 a · IRM c d S +b+ + V IRM S V
(4)
where a, b,c and d are constants.
(a) SuperBrain 40
(b) Aerostar 30A
(c) Aerostar 50A
(d) Aerostar 80A
Figure 9: ESC Efficiency Map The test results for each ESC and each input voltage from 7 - 12 V were curve fitted to this equation to generate empirical models of the ESCs. These results were then used to create maps of the ESC efficiency as functions of input voltage and current. The models are shown by the surface plots in Figure 9, with the test data shown by the blue dots. The constants used in the model are given in Table 2. Each of the line fits has an R2 value of at least 0.91. Contour plots of the ESC efficiency model are given in Figure 10. The ESC
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efficiency maps can be used to accurately model the performance of the electronic speed controller under a range of input voltages and output power settings. 12
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Figure 10: ESC Efficiency Map
Table 2: Model Parameters ESC SuperBrain40 Aerostar 30 A Aerostar 50 A Aerostar 80 A
a 0.00703 0.008198 -0.1295 0.04742
V.
b 83.79 80.19 86.63 84.12
c -14.73 -17.67 -20.40 -15.48
d 21.56 45.62 15.28 12.64
R2 0.9268 0.9196 0.9466 0.9194
Conclusion
This paper introduces the use of electronic speed controllers (ESCs) in UAV propulsion systems and need for detailed data on the performance of these components. An overview is given of the test bench architecture and experimental set-up, as well as results outlining the impact of input voltage, input current and output power on the efficiency of ESCs. A reduced order model for ESC efficiency is also presented to allow more accurate modelling and prediction of ESC efficiency.
Acknowledgments This research has been supported by the Australian Defence Science and Technology Group through its Strategic Research Initiative on Signatures, Materials, and Energy. A student scholarship was provided to
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Andrew Gong by Northrop Grumman Australia and The University of Sydney.
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