UNCLASSIFIED//APPROVED FOR PUBLIC RELEASE
Variable Rail Voltage Control of
Brushless DC Motors
Yuan Chen
Mentors: Dr. William Nothwang and Dr. Joseph Conroy
Abstract
Brushless DC (BLDC) motors achieve higher efficiency and endure less wear and tear than brushed DC motors but require additional
drive circuitry and electronics to perform commutation. Due to their natural efficiency advantage, BLDC motors are used in small robotic
platforms where low power consumption is of concern. The standard drive circuit is a three phase inverter with fixed rail voltage, and
speed control is typically achieved by applying pulse width modulation signals to the inverter MOSFET gates. We implement a
proportional-integral feedback controller to achieve closed-loop speed control in the presence of variable rail voltage. We examine the
relation between rail voltage and steady state power consumption and rail voltage and controller transients for fixed motor angular velocity.
Background - PI Control
Results
•Treating the BLDC motor and commutation logic as a single
system gives the transfer function:
Power Consumption (W) vs Rail Voltage (V)
5000 rpm
6150 rpm
7300 rpm
8450 rpm
where K(v) is plant gain, a function of a applied rail voltage
•Closed loop system:
Power Consumption (W)
3.00
2.50
2.00
1.50
1.00
0.50
2.5
3
3.5
4
4.5
5
5.5
Rail Voltage (V)
Step Response - 5000 rpm to 7300 rpm
5V
•System transfer function:
4V
3.2V
8000
Tracking & Power Consumption
•System tracks step inputs with zero steady state error if and only
if K(v) > 0:
Angular Velocity (rpm)
7500
7000
6500
6000
5500
5000
•System poles:
4500
0
50
100
150
200
250
300
350
400
450
Time (ms)
•All states of angular velocity and its time integral are reachable if
and only if K(v) > 0
Conclusions
•Define power consumption as the product of rail voltage and
motor phase current
•There exists a distinct local minimum of power consumption as a
function of rail voltage – unique for each motor/driver/controller
combination
•Controller transients and power consumption in both transient and
steady state determined by rail voltage for fixed gains and fixed
reference
•Controller time-domain behavior varies with rail voltage: faster
rise time with increasing voltage
tradeoff between power
consumption and controller response
Implementation & Experimentation
Future Work
•Implement feedback system using PSoC-5; external Atmel based
commutation board
•Implement controller using ARM microcontroller
•Estimate speed from timing of commutation signals
•Optimal control: autonomously choose rail voltage based on
adjustable weights of power consumption and response time
•Measure power consumption in both transient and steady state
for fixed speed while experimentally varying rail voltage
•Develop variable gain controller to counteract the effect of varying
plant gain on controller transients
UNCLASSIFIED//APPROVED FOR PUBLIC RELEASE