Transcript Slide 1

DESIGN OF A PLL-BASED R/D CONVERTER
FOR SERVOMOTOR CONTROL
Ahmed Mahran and Ahmed Al-Homsi
Supervisor: Prof. Lazhar Ben-Brahim
Department of Electrical Engineering, College of Engineering
Spring 2009
The Resolver
What is an R/D Converter ?
A position sensor is often needed to measure the mechanical angle of the motor.
Rotary type position sensors generally produce information expressed as an
The resolver is a position sensor used to determine the angular position
of the rotating shaft by providing modulated sine & cosine of the mechanical
electrical signal that varies as the position of the motor (mechanical angle) changes. angle, as shown in figure 2. Since every angle has a unique combination of sine
& cosine values, the resolver is considered as an absolute encoder.
To transform these signals into a real mechanical angle an interface called
Resolver-to-Digital (R/D) converter is needed.
R/D converters are normally used to extract the mechanical angle from the
resolver output signals (sin(θ), cos(θ)).
Demodulator
θ≈ψ
R/D
Converter
Figure 1: R/D converter
Figure 2: Resolver structure
Simulation of the new PLL
A PLL Based R/D Converter
θ
ψ
Radian
The project is an implementation of a new method that overcomes the
disadvantages of the conventional PLL-based R/D converter.
The new PLL technique only requires the followings:
• Phase Detector (PD)
• integrator
• Comparators
• Sample and Hold (S/H)
• Phase Shifter
sin(θ)
sin(ψ)
Cos(θ)
Cos(ψ)
Time(x250us)
Figure 4: Simulation Results
Figure 3: The new PLL Type converter
Experimental Setup
Proposed R/D converter circuit
PD
PI
Switching
Switching
Integrator
Circuit
Circuit
θ≈ψ
Ψ≈θ
θ
Extraction Circuit
Interval Selection
Figure 5: Experimental Setup
Circuit
Conclusion
Experimental Results
In this project, a proposed R/D converter based on PLL was analyzed and
•Transient response
verified by simulation and experiment. The proposed PLL avoids using lookup
Sin(θ)
θ
ψ
table and takes the advantage of the available component in the resolver
Sin(ψ)
Mp=20%
ts=25ms
excitation circuitry. This R/D converter is robust to fluctuations in the
ε=Sin(θ-ψ)
ε=Sin(θ-ψ)
amplitude of the excitation signal.
Figure 6: Transient performance of the proposed PLL converter with two different controller gains
•Steady state response at different motor speeds
Sin(ψ)
Cos(ψ)
Cos(ψ)
Cos(θ)
Sin(θ)
Cos(θ)
Sin(ψ)
Sin(θ)
Future works on this project may address the following issues:


Detailed analysis of the proposed converter .
Techniques to increase the precision of the proposed converter.
Acknowledgment
This work was supported by Qatar Foundation through UREP grant #4- 4 - 24
Figure 7 : Tracking at low motor speed
Figure 8: Tracking at high motor speed