lecture 8 motors, actuators, and power drives forrest brewer
TRANSCRIPT
Lecture 8Lecture 8Motors, Actuators, and Power DrivesMotors, Actuators, and Power Drives
Forrest Brewer
Motors, Actuators, ServosMotors, Actuators, Servos
Actuators are the means for embedded systems to modify the physical world– Macroscopic Currents and power levels– Thermal Management– Power Efficiency (often vs. Performance)
Motor Types– DC Brush/Brushless– AC (shaded pole and induction)– Stepper Motors– Servo (variety of DC motor)– Peisio-electric (Kynar, Canon ultra-sonic)– Magnetic Solenoid– Electro-static (MEMS)
DC Motor ModelDC Motor Model
•Torque (force) ~ Current
•Max Current = V/R
•Max RPM = V/Bemf
Bemf = L dI/dt
In general:
Torque ~ (V – Bemf)/R
R
+
-
Torque
Bemf
In-oz/Amp
V/krpm
L
Output
RPM
Vo
ltag
e
torque
speed
ke V
R kV
max speed
Linear mechanical power Pm = F v
Rotational version of Pm =
power output
speed vs. torque
Jizhong Xiao
speed vs. torque, fixed voltage
stall torque
kke
Controlling speed with voltage
DC motor model
V e
R
• The back emf depends only on the motor speed.
• The motor’s torque depends only on the current, I.
e = ke
= k I
• Consider this circuit’s V: V = IR + eIstall = V/Rcurrent when
motor is stalledspeed = 0
torque = max
How is V related to
V = + ke R k
= - + R ke V
Speed is proportional to voltage.
Jizhong Xiao
Electrostatic MEMS ActuationElectrostatic MEMS Actuation
Electrostatic Drives (MEMS)– Basic equations– Rotation Drive– Comb Drive
Electrostatic Actuator AnalysisElectrostatic Actuator Analysis
Consider the capacitance of the two figures:– To good approximation, the
capacitance is double in the second figure:
Imagine that the charge is fixed in the top figure:
The stored energy is not the same!
The difference must be the work done by the motion of the plate:
Plate-1
Plate-2
Plate-3
V (volts)d
Plate-1
Plate-3
V (volts)d
Plate-2
2211 VCVCQ
2/21 CC
)2
1(
2
1
2
1 2222
2111 VCEVCE
2Vdx
dEFFdxE
Electrostatic ActuatorsElectrostatic Actuators
Plate-1
Plate-2
Plate-3
V (volts)
x
yd Consider parallel plate 1 & 2
Force of attraction (along y direction)
Fp = (½ V2A/g2)
1nN@15Vdimensionless
Consider plate 2 inserted between plate 1 and 3(Popularly known as a COMB DRIVE)
Fc = (½ V2 )(2t/g)Force of attraction (along x direction)
Constant with x-directional translation
Paschen CurvePaschen Curve
Distance*pressure(meter*atm)
Breakdown voltage
Ebd~100MV/m
~200V
~2um@1atm
Side view of SDM Top view of SDM
First polysilicon motors were made at UCB (Fan, Tai, Muller), MIT, ATTTypical starting voltages were >100V, operating >50V
Side Drive MotorsSide Drive Motors
A Rotary Electrostatic Micromotor 1A Rotary Electrostatic Micromotor 18 Optical Switch8 Optical Switch
Micro Electro Mechanical SystemsJan., 1998 Heidelberg, Germany
A Rotary Electrostatic Micromotor 18 Optical Switch
A. Yasseen, J. Mitchell, T. Streit, D. A. Smith, and M. MehreganyMicrofabrication Laboratory
Dept. of Electrical Engineering and Applied PhysicsCase Western Reserve University
Cleveland, Ohio 44106
Fig. 3 SEM photo of an assembled microswitch with vertical 200 m-tall reflective mirror plate. Fig. 4 Insertion loss and crosstalk measurements for multi-
mode optics at 850 m.
Comb DrivesComb Drives
Sandia cascaded comb drive(High force)
Close-upTang/Nguyen/Howe
Layout of electrostatic-combdrive
Ground plate
Folded beams (movable comb
suspension)
Anchors
Stationarycomb
Movingcomb
Tang, Nguyen, Howe, MEMS89
Electrostatic instability
Parallel-Plate Electrostatic Actuator Pull-inParallel-Plate Electrostatic Actuator Pull-in
s0
0
30
0
00
20
20
27
8
30
2
2
A
ksV
sx
x
V
xskx
V
kxxs
VF
snap
snap
k
m
x
V
V
x
s0
1/3 s0
Vsnap
Electrostatic springElectrostatic spring
00
02
00
20
300
20
00
02
00
20
00
02
00
20
20
20
21
2
212
2122
xs
x
xs
Vx
xs
Vkxm
xs
xx
xs
VkxxmF
xs
xx
xs
V
xs
VF
Adjustable stiffness (sensitivity) and resonance frequency
Stepper MotorsStepper Motors
Overview Operation – full and half step Drive Characteristics
VR Stepper MotorVR Stepper Motor
M. G. Morrow, P.E.
Actual Motor ConstructionActual Motor Construction
ROTOR
STATOR
A
S
N
N
N
S
A
A
/A
/A
/B
/B
B
B
M. G. Morrow, P.E.
Multi-pole Rotation, Full-StepMulti-pole Rotation, Full-Step
ROTOR
STATOR
A
S
N
N
N
S
A
A
/A
/A
/B
/B
B
B
A = S
B = OFF
M. G. Morrow, P.E.
ROTOR
STATOR
A
S
N
N
N
S
A
A
/A
/A
/B
/B
B
B
A = OFF
B = S
M. G. Morrow, P.E.
Multi-pole Rotation, Full StepMulti-pole Rotation, Full Step
ROTOR
STATOR
A
S
N
N
N
S
A
A
/A
/A
/B
/B
B
B A = N
B = OFF
M. G. Morrow, P.E.
Multi-pole Rotation, Full StepMulti-pole Rotation, Full Step
A = OFF
B = N
ROTOR
STATOR
/A
/A
/B
/B
AS
N
N
N
S
A
A
B
B
M. G. Morrow, P.E.
Multi-pole Rotation, Full StepMulti-pole Rotation, Full Step
A = S
B = OFF
Multi-pole Rotation, Full StepMulti-pole Rotation, Full Step
ROTOR
STATOR
A
S
N
N
N
S
A
A
/A
/A
/B
/B
B
B
M. G. Morrow, P.E.
““Full-Step” SteppingFull-Step” Stepping
““Full-Step, 2-on” SteppingFull-Step, 2-on” Stepping
Half-steppingHalf-stepping
M. G. Morrow, P.E.
Unipolar motor Unipolar motor
M. G. Morrow, P.E.
Bipolar motorBipolar motor
M. G. Morrow, P.E.
Torque v.s. Angular DisplacementTorque v.s. Angular Displacement
Stepping DynamicsStepping Dynamics
Load Affects the Step DynamicsLoad Affects the Step Dynamics
Drive Affects the Step DynamicsDrive Affects the Step Dynamics
a
Stepper Motor Performance CurvesStepper Motor Performance Curves
Current DynamicsCurrent Dynamics
Drive CircuitsDrive Circuits
Inductive Loads AC Motor Drive (Triac) H-bridge Snubbing and L/nR Stepper Drive PWM Micro-Stepping
Inductive Load Drive CircuitsInductive Load Drive Circuits
M M MM
i < ~30mAV M = V CC
V M V M V M V M
i < ~150mAi < ~1A
V CCPROTPROTPROT
PROT
BJTs– VCEsat ~ 0.4V
– PD = IC*VCE
MOSFETs– VDS = ID * RDSon
– PD = ID*VDS
M. G. Morrow, P.E.
Switching CharacteristicsSwitching Characteristics
V IN
V C
+5V
VIN
VC
M. G. Morrow, P.E.
Switching CharacteristicsSwitching Characteristics
V IN
V C
+5V
VIN
VC
M. G. Morrow, P.E.
AC Motor DriveAC Motor Drive
M
VAC
PROT
VCC
M. G. Morrow, P.E.
H-bridgeH-bridge
Q1 Q2 Q3 Q4
ON ON * * Don’t use - short circuit
* * ON ON Don’t use - short circuit
OFF OFF * * Motor off
* * OFF OFF Motor off
ON OFF OFF ON Forward
OFF ON ON OFF Reverse
ON OFF ON OFF Brake
OFF ON OFF ON Brake
+M-
V M
Q1
Q2 Q4
Q3
An H-bridge consists of two high-side switches (Q1,Q3) and two low-side switches (Q2,Q4)
BJTs or FETs
M. G. Morrow, P.E.
H-Bridge/Inductor operationH-Bridge/Inductor operation
V = IR V = V+
V = 0(V = -IR)
V = IR
L/nR DriveL/nR Drive
Current Rise in DetailCurrent Rise in Detail
0
Performance Improvement with L/nR DrivePerformance Improvement with L/nR Drive
Pulse-Width ModulationPulse-Width Modulation
Pulse-width ratio = ton/tperiod
Never in “linear mode”– Uses motor inductance to smooth out current– Saves power!
– Noise from Tperiod
ton
tperiod
Benjamin Kuipers
Pulse-Code Modulated SignalPulse-Code Modulated Signal
Some devices are controlled by the length of a pulse-code signal.– Position servo-motors, for example.
0.7ms
20ms
1.7ms
20ms Benjamin Kuipers
Back EMF Motor SensingBack EMF Motor Sensing
Motor torque is proportional to current. Generator voltage is proportional to velocity. The same physical device can be either a
motor or a generator. Alternate Drive and Sense (note issue of Coils
versus Induction)
Drive as a motor Sense as a generator
20ms
Benjamin Kuipers
Back EMF Motor ControlBack EMF Motor Control
Benjamin Kuipers
MicrosteppingMicrostepping
Use partial Drive to achieve fractional steps
Stepper is good approximation to Sine/Cosine Drive– 2 cycle is 1 full
step! Usually PWM to
reduce power loss– Fractional voltage
drop in driver electronics
IB
Full Step position
Half Step positionIB = -1, IA = 0
IA
IA = 2 IB
IA = IB
Microstepping Block DiagramMicrostepping Block Diagram
Easy to synthesize the PWM from Microcontroller– Lookup table + interpolation– /2 phase lag = table index
offset Need to monitor winding
current:– Winding L,R– Motor Back EMF
PWM Frequency tradeoff– Low Freq: resonance (singing)– High Freq: Winding Inductance
and switching loss
PWM HBridge
PWM HBridge
PWM IssuesPWM Issues
Noise/Fundamental Period– Low Freq: Singing of motor and resonance– High Freq: Switching Loss
Can we do better?– Not unless we add more transitions! (Switching Loss)
If we bite bullet and use MOS drives can switch in 2-50nS…
Then can choose code to optimize quantitization noise vs. switching efficiency– Modulated White Noise– Sigma-Delta D/A
Requires higher performance Controller
Motors and Actuators -- SoftwareMotors and Actuators -- Software
Embedded motor control is huge, growing application area– Need drive(sample) rates high enough to support quiet, efficient
operation
– Rates roughly inversely proportional to motor physical size
– Stepper Motors are usually micro-stepped to avoid ‘humming’ and step bounce
– Typical: 1kHz rate, 50kHz micro-step; slow HP motors 300Hz-1kHz Upshot- 1 channel “fast” or micro-stepped motor is substantial
fraction of 8-bit processor throughput and latency– Common to run up to 3 “slow” motors from single uP
– Common trend to control motor via single, cheap uP
– Control multiple via commands send to control uPs