AN894

M
AN894
Motor Control Sensor Feedback Circuits
Author:
A list of the sensors that can be used to feedback
information to a microcontroller are listed below:
Jim Lepkowski
Microchip Technology Inc.
INTRODUCTION
Sensors are a critical component in a motor control
system. They are used to sense the current, position,
speed and direction of the rotating motor. Recent
advancements in sensor technology have improved
the accuracy and reliability of sensors, while reducing
the cost. Many sensors are now available that integrate
the sensor and signal-conditioning circuitry into a single
package.
• Current sensors
- Shunt resistor
- Current-sensing transformer
- Hall effect current sensor
• Speed/position sensors
- Quadrature encoder
- Hall efect tachometer
• Back EMF/Sensorless control method
In most motor control systems, several sensors are
used to provide feedback information on the motor.
These sensors are used in the control loop and to
improve the reliability by detecting fault conditions that
may damage the motor. As an example, Figure 1 provides a block diagram of a DC motor control system to
show the sensor feedback provided for a typical motor
control.
Power Management
Torque
PICmicro®
Speed
Input
Microcontroller
Direction
Motor
Current
Sensor
Feedback
FIGURE 1:
Driver
Sensors
* Speed
* Shaft Position
* Rotation Direction
Typical DC Motor Block Diagram.
 2003 Microchip Technology Inc.
DS00894A-page 1
AN894
CURRENT SENSORS
applications. A summary of the advantages and
disadvantages of each of the current sensors is
provided in Table 1.
The three most popular current sensors in motor
control applications are:
Figure 2 shows an example of an AC motor powered by
a three-phase inverter bridge circuit. This example
shows that the composite current of all three Insulated
Gate Bipolar Transistor (IGBT) circuit legs can be
measured with a single shunt resistor, or that the
current in each individual leg can be determined with
three shunt resistors. Figure 2 shows a system that
uses shunt resistors. However, Hall effect and currentsensing transformers can also be used to provide the
current measurement.
• Shunt resistors
• Hall effect sensors
• Current transformers
Shunt resistors are popular current sensors because
they provide an accurate measurement at a low cost.
Hall effect current sensors are widely used because
they provide a non-intrusive measurement and are
available in a small IC package that combines the
sensor and signal-conditioning circuit. Current-sensing
transformers are also a popular sensor technology,
especially in high-current or AC line-monitoring
TABLE 1:
COMPARISON OF CURRENT SENSING METHODS
Current Sensing Method
Shunt Resistor
Hall Effect
Current Sensing Transformer
Accuracy
Good
Good
Medium
Accuracy vs.Temperature
Good
Poor
Good
Low
High
Medium
Cost
Isolation
No
Yes
Yes
High Current-Measuring
Capability
Poor
Good
Good
DC Offset Problem
Yes
No
No
Saturation/Hysteresis
Problem
No
Yes
Yes
Power Consumption
High
Low
Low
Intrusive Measurement
Yes
No
No
AC/DC Measurements
Both
Both
Only AC
Current Measurement with
a Single Shunt Resistor
Current Measurement with
Three Shunt Resistors
AC
Motor
VDC
IB
IA
I = IA+ IB + IC
FIGURE 2:
DS00894A-page 2
VDC
IC
IA
VOUT
RSENSE
AC
Motor
IB
IC
VOUT_A
VOUT_B
RSENSE_B
RSENSE_C
VOUT_C
R SENSE_A
AC Motor Current Measurement.
 2003 Microchip Technology Inc.
AN894
Shunt Resistors
• Special-purpose, low inductance resistors are
required if the current has a high-frequency
content.
• The power rating of RSENSE must be evaluated
because the I2 x R power dissipation can produce
self heating and a change in the nominal
resistance of the shunt.
Shunt resistors are a popular current-sensing sensor
because of their low cost and good accuracy. The
voltage drop across a known low value resistor is
monitored in order to determine the current flowing
through the load. If the resistor is small in magnitude,
the voltage drop will be small and the measurement will
not have a major effect on the motor circuit. The power
dissipation of the resistance makes current shunts
impractical for measurements of more than
approximately 20 amperes.
Special-purpose, shunt current measurement resistors
are available from a number of vendors. If standard
resistors are used, it is recommended that metal-film
resistors be used rather than wire-wound resistors that
have a relatively large inductance.
The selection criteria of a shunt current resistor
requires the evaluation of several trade-offs, including:
A shunt resistor can also be created from the trace
resistance on a PCB, as shown in Figure 3. PCB shunt
resistors offer a low cost alternative to discrete resistors. However, their accuracy over a wide temperature
range is poor when compared to a discrete resistor.
The temperature coefficient of a copper PCB trace
shunt resistor is equal to approximately +0.39%/°C.
Further details on PCB trace resistors are given in reference (2).
• Increasing R SENSE increases the VSENSE voltage,
which makes the voltage offset (VOS) and input
bias current offset (IOS) amplifier errors less
significant.
• A large RSENSE value causes a voltage loss and a
reduction in the power efficiency due to the I2 x R
loss of the resistor.
• A large RSENSE value will cause a voltage offset to
the load in a low-side measurement that may
impact the EMI characteristics and noise
sensitivity of the system.
.
Trace resistance is based on:
* Length (L)
* Thickness (t)
* Width (w)
* Resistivity (ρ)
* 1 oz. Copper (Cu) is defined to be a layer
with 1 oz. of Cu per square foot.
t ≈ 1.37 mil./oz. Copper
ρ ≈ 0.68 µΩ-inch
R… ≈ (0.50 mΩ / …) x [(1 oz. Cu) / (# oz. Cu)]
w
FIGURE 3:
≈ 10 mΩ
P = I2 x R
= (5A)2 x (0.010Ω)
= 0.25 Watt
⇔
L
t
Example: What is the resistance of the PCB shunt resistor
using the parameters listed below?
Given: 1 oz Cu PCB
w = 50 mils (0.050 in)
L = 1 inch
I = 5 ampere
L / w = number of squares (…)
= 1 in / 0.050 in
= 20 squares
R ≈ (L / w) x R…
≈ (20 squares) x 0.50 mΩ/…
RPCB
PCB Trace Resistor
PCB Shunt Resistor.
 2003 Microchip Technology Inc.
DS00894A-page 3
AN894
High-Side vs. Low-Side Current Shunt
Measurements
SYSTEM INTEGRATION ISSUES
Shunt resistors can provide either a high-side or lowside measurement of the current through the load, as
shown in Figure 4. A high-side monitor has the resistor
connected in series with the power source, while the
low-side monitor locates the resistor between the load
and the ground current return path. Both approaches
pose a trade-off to the designer. The attributes of the
two methods, along with the typical monitor circuits, will
be shown in the following sections. Reference (3)
provides more details on high-side and low-side
shunts.
High-side current measurements are the preferred
method from a system-integration standpoint because
they are less intrusive than low-side measurements.
The trade-off with the high-side measurement is that
the circuitry is more complex than the low-side method.
RSENSE
VS
High-side resistive shunt measurements will not have a
significant impact on the system if the sensing resistor
is small and the resulting voltage drop across the shunt
is small compared to the supply voltage. In contrast,
low-side monitoring disrupts the ground path of the
load, which can cause noise and EMI problems in the
system.
Low-side current measurements are often chosen
because low voltage op amps can be used to sense the
voltage across the shunt resistor. Note that low-side
monitoring is not possible in some applications
because the ground connection is made via the
mechanical mounting of the motor on the chassis or
metal frame. For systems powered via a single wire
connection, it may not be practical to insert a shunt
resistor between the device and the chassis that
functions as the ground wire.
ILOAD
ILOAD
VSENSE
+
-
Measurement
Circuit
High-Side Current Measurement
ILOAD = VSENSE / RSENSE
FIGURE 4:
DS00894A-page 4
Load
VS
+
-
Load
Measurement
Circuit
VSENSE
RSENSE
Low-Side Current Measurement
ILOAD = VSENSE / RSENSE
High-Side and Low-Side Resistive Current Shunts.
 2003 Microchip Technology Inc.
AN894
HIGH-SIDE CURRENT SHUNT
MEASUREMENTS
Disadvantages:
High-side current measurements can be implemented
with a differential amplifier circuit that produces an
output voltage that is proportional to VSENSE or the
current flowing through the load. Figure 5 provides an
example of a high-side shunt circuit. The differential
amplifier circuit can be implemented with an op amp
and discrete resistors or with an integrated IC device.
Integrated differential amplifier ICs are available from a
number of semiconductor vendors and offer a
convenient solution because the amplifier and wellmatched resistors are combined in a single device.
The attributes of high-side monitoring are listed below:
Advantages:
• The VSENSE voltage is approximately equal to the
supply voltage, which may be beyond the
maximum input voltage range of the operational
amplifier.
• A differential amplifier’s CMRR will be degraded
by mismatches in the amplifier resistors.
• The input impedance of the differential circuit is
relatively low and is asymmetrical. The input
impedance at the amplifier’s non-inverting input is
equal to R IN + R*, while the impedance at the
inverting terminal is equal to R IN.
• May require rail-to-rail-input op amps because of
the high voltage level of the input signal.
The high-side shunt circuit requires a high-voltage
amplifier that can withstand a high common mode
voltage. In addition, the key amplifier specifications are
a high CMRR and a low VOS because of the relatively
small magnitude of VSENSE. High voltage op amps and
integrated differential amplifier ICs are available for
systems that have a maximum voltage of
approximately 60V. For voltage requirements beyond
60V, a current mirror circuit can be used to sense the
current. A current mirror can be implemented with
readily available, high-voltage transistors. References
(1) and (5) provide examples of high-voltage, high-side
current monitor circuits.
• Less intrusive than low-side monitors and will not
affect the EMI characteristics of the system.
• Can detect overcurrent faults that can occur by
short circuits or inadvertent ground paths that can
increase the load current to a dangerous level.
• A differential amplifier circuit will filter undesirable
noise via the common-mode-rejection-ratio
(CMRR) of the amplifier.
• A resistive network can be used to reduce the
voltage at the amplifier’s input terminals. For
example, if RIN = R*, the input voltage will be
reduced in half and the amplifier will be biased at
VS/2. Note that the amplifier gain will be equal to
one and that a second amplifier may be needed to
increase the sensor’s output voltage.
Table 2 provides a list of the recommended Microchip
op amps that can be used in a high-side circuit.
VSENSE
RSENSE
RIN R*
RIN
VS
ILOAD
+
VOUT
ADC
PICmicro®
Microcontroller
Load
VOUT = VSENSE x (R*/RIN )
R*
= (ILOAD x RSENSE) x (R*/R IN)
FIGURE 5:
TABLE 2:
High-Side Resistive Current Measurement Circuit.
RECOMMENDED MICROCHIP OP AMPS FOR HIGH-SIDE CURRENT SHUNTS
Product
Operating Voltage
CMRR (Typ.)
VOS (Max.)
TC7652
6.5 to 16V
140 dB
10 µV
• Low Noise
• Chopper Stabilized
TC913A
6.5 to 16V
116 dB
15 µV
• Auto-zeroed Op Amp
TC913B
6.5 to 16V
110 dB
30 µV
• Auto-zeroed Op Amp
 2003 Microchip Technology Inc.
Features
DS00894A-page 5
AN894
LOW-SIDE CURRENT MEASUREMENT
Disadvantages
Low-side current measurements offer the advantage
that the circuitry can be implemented with a low voltage
op amp because the measurement is referenced to
ground. The low-side measurement circuit can use a
non-inverting amplifier, as shown in Figure 6.
• The low-side resistor disrupts the ground path
and the added resistance to the grounding system
produces an offset voltage which can cause EMI
noise problems.
• Low-side current monitors are unable to detect a
fault where the load is accidently connected to
ground via an alternative ground path.
The low-side current monitor can also be implemented
with a differential amplifier. The advantages of
differential amplification are limited because RSENSE is
connected to ground and the common mode voltage is
very small. Note that integrated IC low-side monitors
that combine the op amp and resistors are not readily
available because of the simplicity of the circuit that can
be implemented with a few discrete resistors and low
voltage op amp.
Table 3 provides a list of the recommended Microchip
op amps that can be used in a low-side circuit. The key
op amp specifications for selecting a low-side amplifier
are rail-to-rail input and a low offset voltage (VOS).
The attributes of low-side monitoring are:
Advantages
• VSENSE is referenced to ground. Therefore, a low
voltage amplifier can be used.
• A non-inverting amplifier can be used and the
input impedance of the circuit will be equal to the
large input impedance of the amplifier.
Load
VS
+
-
ILOAD
VSENSE
RSENSE
FIGURE 6:
TABLE 3:
R1
VOUT
ADC
PICmicro®
Microcontroller
VOUT = (VSENSE) x (1 + R 2/R1)
= (ILOAD x RSENSE ) x (1 + R2/R 1)
R2
Low-Side Resistive Current Measurement Circuit.
RECOMMENDED MICROCHIP OP AMPS FOR LOW-SIDE CURRENT SHUNTS
Product
Operating Voltage
CMRR (Typ.)
VOS (Max.)
TC913A
6.5 to 16V
116 dB
15 µV
Features
• Auto-zeroed Op Amp
TC913B
6.5 to 16V
110 dB
30 µV
• Auto-zeroed Op Amp
MCP606
2.5 to 5.5V
91 dB
250 µV
• Rail-to-Rail Output
• Low Operating Current
MCP616
2.3 to 5.5V
100 dB
150 µV
• Rail-to-Rail Output
• Low Operating Current
DS00894A-page 6
 2003 Microchip Technology Inc.
AN894
SHUNT OFFSET ADJUSTMENT CIRCUIT
NOISE REDUCTION TECHNIQUES
The circuit shown in Figure 7 can be used to provide an
offset to the amplification of the VSENSE signal.
Resistor R 1 is used to prevent the offset voltage
provided by resistors R4 and R5 from changing the
value of VSENSE. The offset can be used to center the
amplifier’s output to the midpoint of the voltage supply
(VDD/2). The VSENSE signal is typically only 10 to
100 mV above ground and the offset often is needed if
the amplifier is connected to an ADC.
The combination of a differential amplifier with a high
CMRR and discrete RC filters can be used to minimize
the effect of EMI noise. The effect of EMI on a
measurement typically results in poor DC performance
and a large DC offset at the output of the op amp.
Figure 8 provides an example of a circuit that can be
used in a motor application to reduce noise.
VS
VDD
R4
Load
ILOAD
VSENSE R1
R5
VDD
VOUT
RSENSE
R2
R3
RSENSE << R1
Amplifier Gain = (1 + (R3 / R2))
VOUT = [(VSENSE (1 + (R3/R2)) + ((R5 / (R 4+R5)VDD)]
FIGURE 7:
Circuit.
The addition of the common mode filters formed by the
RC combinations of R1C1 and R2C2 are used to reduce
the noise that is imposed on the two input lines of the
amplifier. Discrete RC networks lower the voltage level
of the noise signal by functioning as a low pass filter.
However, an EMI filter, such as a TVS zener diode, is
required to ensure that the input noise is clamped to a
safe voltage level that will not damage the amplifier.
The common mode resistors and capacitors should be
matched as close as possible. The resistors should
have a tolerance of 1% or better, while the capacitors
should have a tolerance of 5% or better. Capacitor C3
is used to add a RC differential filter that compensates
for any mismatch of R1C1 and R2C2. Any difference in
the RC combinations will result in a degradation of the
amplifier’s CMRR. The differential filter formed by R1C3
and R2C3 will attenuate the differential signal at the
amplifier caused by the tolerances of the common
mode filters.
Shunt Offset Adjustment
Providing an offset to the shunt resistor circuit can also
improve the linearity of the amplification, especially if
standard op amps are used. The linearity, accuracy
and power consumption of a standard single power
supply op amp is typically degraded when the output
signal is at, or near, the power supply rails. Thus, the
offset circuit can be used to avoid this problem. The
preferred op amps to use in a shunt circuit have a small
offset voltage (VOS) and a rail-to-rail, input-output
specification.
VS
R1
R4
ILOAD
C1
C3
RSENSE
VOUT
R2
C2
Load
R3
RSENSE << R 1 and R2
R1 = R2
C1 = C2
C3 >> C1 and C3 >> C2
Common Mode Filter
f-3dB = 1 / (2π R1 C1)
= 1 / (2π R2 C2)
Differential Mode Filter
f-3dB =1/ [2π (R1+R2) (((C1 x C2)/(C1+C2)) + C3)]
FIGURE 8:
 2003 Microchip Technology Inc.
RC Noise Reduction Circuit.
DS00894A-page 7
AN894
Figure 9 provides an example of a shunt amplifier
circuit that combines the filtering of the shunt current
signal with an offset adjustment. The RC components
R1C1, R2C 2 and C 3 are used to provide EMI and ESD
protection to the amplifier. The RC feedback networks
of R 7C5 and R6C4 are selected to provide a low pass
filter response to the differential amplifier.
A trade-off with discrete filter networks is that the
frequency response of the filter is dependent on the
source and load impedance. The filter equations shown
are only an approximation. A more detailed analysis or
SPICE simulation may be required to accurately model
the filter response of the circuit.
VDD
VS
EMI Filter
R1
ILOAD
RSENSE
C1
R2
Load
C2
R5
R3
R7
C5
VDD
C3
Hall effect sensors are a current-measuring sensor that
can be easily integrated into an embedded application.
Several vendors offer devices that combine the
magnetic sensor and conditioning circuit in a small IC
package. These IC sensors typically produce an
analog output voltage that can be input directly into the
microcontroller’s ADC. The main disadvantages of Hall
effect current sensors are that they are expensive and
their accuracy varies with temperature.
The Hall effect is based on the principle that a voltage
(VH) is created when current (IC) flows in a direction
perpendicular to a magnetic field (B), as shown in
Figure 10. Hall effect current sensors are available in
either an open-loop or closed-loop implementation.
The closed-loop Hall effect sensors offer the advantage
that their output linearity is better than an open-loop
sensor over a wider current measurement range.
Further details on Hall effect sensors are available in
references (4), (7) and (12).
VOUT
R4
R6
Hall Effect Current Sensors
VH-
C4
B
RSENSE << R 1 and R2
R3 = R4 = R IN
R1 = R2 = RIN*
RIN >> RIN*
R7 = R5 ll R6 = RF
C1 = C2
C3 >> C1 and C3 >> C2
C4 = C5 = CF
DC Amplifier Gain = -RF / (RIN* + R IN)
Amplifier Feedback Low Pass Filter
f-3dB @ 1 / (2π RF CF)
VOUT = [((ILOAD x RSENSE) x (RF/(R IN + RIN*))
+ ((R6/(R5+R6)VDD )]
FIGURE 9:
Combining the Offset and
Noise Reduction Circuit.
Integrated EMI filters can be used to simplify the circuit
shown in Figure 9 and reduce the number of discrete
components. Integrated Passive Device (IPD) EMI
filters that consist of resistors and transient
suppression (TVS) zener diodes are available from a
number of IC venders. IPD filters integrate the discrete
components in a small IC package, while providing
transient voltage protection.
TVS devices offer the advantage that the input signal is
clamped to a safe value that is equal to the breakdown
voltage of a zener diode. The zener diode functions as
a capacitor when the voltage is below the breakdown
voltage. Thus, the IPD filter is equivalent to a RC filter
when the input voltage is small. Further details on IPD
EMI filters and ESD protection devices are provided in
reference (8).
DS00894A-page 8
IC
IC
VH+
FIGURE 10:
Hall Effect Principle.
The Hall effect current sensor can be placed on the
PCB directly over the current trace that will be
monitored. The sensor functions by measuring the
magnetic flux that is created by the current flowing
through the trace. Figure 11 provides an example of a
PCB mounted Hall effect sensor that measures the
current through a wire placed on the top of the IC. Hall
effect current sensors are also available in a package
that is mounted on the PCB, with the current-carrying
wire passing through a hole in the sensor.
I
I
Printed Circuit Board
FIGURE 11:
Hall Effect Current Sensor.
 2003 Microchip Technology Inc.
AN894
Current-Sensing Transformers
Current-sensing transformers offer an alternative to
shunt resistors and Hall effect sensors to measure current. These sensors use the principle of a transformer,
where the ratio of the primary current to the secondary
current is a function of the turns ratio. The main advantage of current transformers is that they provide galvanic isolation and can be used in high-current
applications. The main disadvantage of current transformers is that an AC input signal is required to prevent
the transformer from saturating.
Figure 12 provides schematics of a single turn and a
multi-turn primary current-sensing transformers. The
single-turn primary transformer offers the advantage
that the measurement is non-intrusive and the currentcarrying wire can be passed directly through a hole in
the transformer. The multi-turn transformer offers the
advantages of improved magnetic coupling, since
many turns of the primary wire can be provided.
B
A
2
3 1 4 2
1
Ip
Is 1
A
1
Ip
Is 3
+
+
Np
Ns
VOUT
2
Single-Turn Primary
Np
Rt
-
B
FIGURE 12:
Ip
2
Ns
VOUT
Rt
-
4
Is = Ip / N where N = turns ratio
VOUT = Is x Rt
Multi-Turn Primary
Current-Sensing Transformers.
 2003 Microchip Technology Inc.
DS00894A-page 9
AN894
BACK EMF CONTROL METHOD
The back EMF is created when the motor’s armature
turns, which creates a electrical kickback or EMF that
is sensed as a voltage through a resistor. The
amplitude of the EMF signal increases with the speed
of the armature rotation. A limitation of the back EMF
method is that the amplitude of the signal is very small
at low shaft RPMs.
The back electro-magnetic-force (EMF) or sensorless
motor control method obtains the speed and position of
the motor directly from the voltage at the motor
windings. This method is typically used in brushless DC
motors to provide commutation. The back EMF control
method eliminates the requirement for relatively expensive sensors, such as Hall effect devices. The back
EMF voltage produces a sine or trapezoidal waveform
that is sensed at the motor’s winding and typically is
converted into a digital square wave by a zero-crossing
comparator circuit. The comparator signal is inputted to
the microcontroller, which calculates the commutation
sequence and motor position from the phase
relationship of the square wave representation of the
back EMF signals.
The zero-crossing circuit can be constructed from
either discrete comparator ICs or comparators that are
located inside the PICmicro® microcontroller. Figure 13
provides a block diagram of a sensorless control for a
Brushless Direct Current (BLDC) motor that uses
discrete comparator circuits.
VDC
PWM5
A
PWM4
PIC18FXX31
PWM3
PWM2
3-Phase
Inverter Bridge
PWM1
PWM0
B
C
VDC
BEMFA
VREF_A
VDC
BEMFB
VREF_B
VDC
BEMFC
VREF_C
BACK EMF
ZERO-CROSSING
COMPARATOR CIRCUITS
FIGURE 13:
DS00894A-page 10
Block Diagram of a Sensorless BLDC Motor Control.
 2003 Microchip Technology Inc.
AN894
SELECTING A COMPARATOR
A comparator is designed to provide a logic-level
output signal that indicates whether the voltage at the
non-inverting input is larger or smaller than the voltage
at the inverting input. Figures 14 and 15 show the circuit topology and design equations for a non-inverting
and inverting comparator, respectively. The non-inverting circuit’s output is in phase with the sinewave input,
while the inverting circuit that has an output 180° out of
phase from the input signal. Reference (6) provides
further details on the comparator voltage transition and
hysteresis equations.
For example, the output voltage of a single voltage
supply, non-inverting comparator will be analyzed. The
output will be the same for a push-pull or an open-drain
output device that is connected to voltage VDD through
a pull-up resistor. If the voltage at the non-inverting (+)
terminal is larger than the voltage at the inverting (-)
terminal, the output will be equal to approximately VDD.
In contrast, if the voltage at the (+) terminal is less than
the voltage at the (-) terminal, the output will be equal
to approximately V SS or ground.
TABLE 4:
Though op amps can be used as a comparator, the
designer must consider the trade-offs of using an
amplifier in a non-linear mode. Op amps are designed
to linearly amplify a small signal and use negative
feedback to function in the linear region. By contrast,
comparators are designed to function in the non-linear
region and use positive feedback to force the output to
have a fast transition to the saturation region where the
output is at either the high or low power supply rail.
Though op amps can function as a comparator by
using positive feedback, the switching speed of the circuit is typically poor. The propagation delay of an op
amp comparator is large in comparison with a typical
comparator. In addition, the current consumption of an
op amp comparator usually is much larger than a
standard comparator.
Table 4 provides a list of recommended Microchip
comparators. A key specification for motor control
applications is the propagation delay of the comparator.
RECOMMENDED MICROCHIP COMPARATORS
Product
Operating
Voltage
IQ (Typ.)
Propagation Delay
(typ.)
TC1025
TC1027
TC1028
1.8 to 5.5V
8 µA
4 µs
• Rail-to-rail input and output
1.8 to 5.5V
1.8 to 5.5V
18 µA
10 µA
4 µs
4 µs
• On-board V REF
• Shutdown pin (TC1028)
TC1031
1.8 to 5.5V
6 µA
4 µs
• Prog. Hysteresis
• Shutdown pin
• On-board V REF
TC1037
TC1038
TC1039
1.8 to 5.5V
1.8 to 5.5V
1.8 to 5.5V
4 µA
4 µA
6 µA
4 µs
4 µs
4 µs
TC1040
TC1041
1.8 to 5.5V
1.8 to 5.5V
10 µA
10 µA
4 µs
4 µs
MCP6541/2/3/4
1.6 to 5.5V
0.6 µA
per comparator
4 µs
• Low IQ
• Push-pull output
MCP6546/7/8/9
1.6 to 5.5V
0.6 µA
per comparator
4 µs
• Low IQ
• Open-drain output
 2003 Microchip Technology Inc.
Features
• Shutdown pin (TC1038)
• On-board V REF (TC1039)
• On-board V REF
• Prog. Hysteresis (TC1041)
DS00894A-page 11
AN894
From
Motor Windings
VPULL-UP
VOUT
R3
VM
R1A
VIN
VOH
RPULL-UP
VDD
VOUT
VDD
R2
R1B
VOL
VREF
R4
VTL
VTH
VIN
Hysteresis Plot
Assume: VOH = VDD, VOL = 0, R3 >> R1 and R3 >> R PULL-UP
R 1B
V IN =  -------------------------- × V M
R + R 
1A
1B
R4 
V RE F = V DD ×  -------------------R + R 
2
R 1 = R 1A || R 1B
Design Procedure:
1. Select VREF, the “zero-crossing” voltage
2. Select VHYS to be equal to 10 to 100 mV
4
3. Select R3 >> R1
( R 1 + R 3 )V REF – ( R 1 × V DD )
V TL ≅ ---------------------------------------------------------------------R3
( R 1 + R 3 )V REF
V TH ≅ -----------------------------------R3
V HYS = V TH – V TL
R
V HYS ≅  -----1- × V DD
R 
Note: RPULL-UP is required for open drain outputs,
but is not required for push-pull output comparators.
3
FIGURE 14:
Single Supply Non-Inverting Comparator Circuit.
VPULL-UP
VDD
VOUT
R3
From
Motor Windings
R1
VM
R2
VDD
VOUT
VREF
R1A
VOH
RPULL-UP
VOL
VIN
VTL
R1B
VIN
Hysteresis Plot
Design Procedure:
Assume: VOH = VDD, VOL = 0, R3 >> R1 ll R2 and R3 >>RPULL-UP
V IN
VTH
R 1B
=  -------------------------- × V M
R + R 
1A
1B
1. Select VREF, the “zero-crossing” voltage
R 1 = R 1A || R 1B
2. Select VHYS to be equal to 10 to 100 mV
3. Select R3 >> R1 ll R2
R1 
V REF ≅ V DD ×  -----------------R + R 
1
2
R1 
× VD D
V TL ≅  -----------------R + R 
1
2
R1 
( R 1 || R 2 ) × V DD
× V DD +  --------------------------------------V TH ≅   ----------------- R + R 
 

R
1
2
3
V H YS = V TH – V TL
( R 1 || R 2 ) × V DD
V H YS ≅ --------------------------------------R3
FIGURE 15:
DS00894A-page 12
Note: RPULL-UP is required for open drain outputs,
but is not required for push-pull output
comparators.
Single-Supply Inverting Comparator Circuit.
 2003 Microchip Technology Inc.
AN894
COMPARATOR REFERENCE VOLTAGE
HYSTERESIS
In single supply comparators, a reference voltage must
be created. The circuits create VREF by using a resistor
voltage divider. The offset voltage of VREF enables the
circuit to function as a zero-crossing detector without
requiring a dual voltage power supply. The back EMF
voltage produces a sine or trapazoidal waveform that
swings above and below power ground. The back EMF
voltage can be sensed as a sine or trapazoidal waveform offset by a DC voltage if the comparator circuit is
either referenced to center point of the motor windings,
or if a resistor network is used. The resistor network
can either pull-up the floating signal to VDD or pulldown the signal to ground. Further details on the back
EMF comparator circuit used in a brushless DC motor
controller are provided in reference (11).
Hysteresis can be used to provide noise reduction and
prevent oscillation when the comparator switches
output states. A comparator provides hysteresis by
feeding back a small fraction of the output signal to the
positive input terminal. This additional voltage provides
for a polarity sensitive offset voltage, which either
increases or decreases the threshold value of the
switching voltage. Hysteresis produces two different
switching points that result in a transition voltage that is
dependent on whether the input voltage is rising or
falling in amplitude.
Frequency-dependent hysteresis can be provided by
placing a capacitor in the positive feedback network, as
shown in Figure 16. The capacitor adds an additional
pole that changes the amount of hysteresis as a
function of frequency. At frequencies below fp, the
hysteresis will be a constant voltage determined by
resistors R1 and R3. However, at frequencies above the
pole fp, the hysteresis will be increased as a function of
the frequency, as shown in the equations provided in
Figure 16.
C3
R3
VDD
R1
VIN
VOUT
VDD
Z3 = R3 ll C 3
= R3 / (sR3C3 +1)
where s = jω = j2πf
VHYS ≅ [(R1 / Z 3) x VDD]
R2
R4
FIGURE 16:
High Frequency Pole @ fp = 1 / (2π R3 C3)
VREF
≅ [(R1 x (sR3C3 +1)) / R3)] x VDD
Frequency Dependent Hysteresis for a Comparator.
 2003 Microchip Technology Inc.
DS00894A-page 13
AN894
QUADRATURE ENCODER
A quadrature encoder can be used to provide the
speed, direction and shaft position of a rotating motor.
A simplified block diagram of an optical quadrature
encoder is shown in Figure 17. The typical quadrature
encoder is packaged inside the motor assembly and
provides three logic-level signals that can be directly
connected to the microcontroller.
Motor speed is determined by the frequency of the
Channel A and B signals. Note that the counts-per-revolution (CPR) depends on the location of the encoder
and whether motor-gearing is used. The phase
relationship between Channel A and B can be used to
determine if the motor is turning in either a forward or
VDD
The quadrature encoder’s speed and direction
information can be determined either with discrete
logic, a quadrature encoder logic IC or a PICmicro®
microcontroller. Vendors, such as LSI Computer
Systems, offer an IC that converts the three encoder
signals to a signal that represents the velocity, position
and distance that the motor has moved. Alternatively,
the encoder information can be obtained from the
hardware registers and software logic inside a
PICmicro
microcontroller.
For
example,
the
PIC18FXX31 dsPIC® MCUs have a Quadrature
Encoder Interface logic integrated into the processor.
Motor (+)
VDD
Channel A
Quadrature
Encoder
Channel A
reverse direction. The Index signal provides the
position of the motor and, typically, a single pulse is
generated for every 360 degrees of shaft rotation.
0V
Motor
VDD
Channel B
Channel B
0V
Index
Forward
Motor (-)
Direction
Reverse
Change
Index
VDD
Codewheel
Photodiode
Channel A
Signal
LED
Lens
Processing
Channel B
Circuitry
Index
Ground
Simplified Block Diagram of a Quadrature Encoder
FIGURE 17:
DS00894A-page 14
Quadrature Encoder.
 2003 Microchip Technology Inc.
AN894
HALL EFFECT TACHOMETERS
Hall effect sensors can be used to sense the speed and
position of a rotating motor. Further information on Hall
effect tachometer sensors are provided in references
(4) and (12). These sensors are based on using the
Hall element to sense the change influx in the air gap
between a magnet and a notch in a rotating shaft or a
passing ferrous gear tooth. The main advantage of Hall
effect tachometers is that they are a non-contact
sensor that is not limited by mechanical wear. Hall
effect tachometers that integrate the sensor and
sensor-conditioning circuit in a small IC package are
available from a number of vendors. The circuitry inside
the sensor typically consists of a comparator or Schmitt
trigger to provide a digital output signal that can be
directly connected to the microcontroller.
An example of a Hall effect rotary interrupt switch is
provided in Figure 18. A notch is placed in the rotating
shaft that provides a magnetic field to the sensor when
the notch is positioned directly in-line with the magnet
and the Hall effect sensor, turning the switch “ON”.
When the solid portion of the disk is between the Hall
effect sensor and the magnet, the magnetic field is
interrupted and the switch is in the “OFF” position.
Hall effect tachometers can also be used as a
geartooth sensor. A Hall effect geartooth sensor,
shown in Figure 19, senses the variation in the flux in
the air gap between the passing ferrous geartooth and
the magnet. Geartooth sensors typically provide a
digital output that can be directly connected to the I/O
port of the microcontroller. In addition to detecting the
speed of the rotation, some Hall effect tachometers
also detect the direction of the turning gears.
Magnet
Refer to Reference 12 for additional information
FIGURE 18:
Hall Effect Rotary Interrupt Switch Tachometer.
Hall Effect
Sensor
Refer to Reference 12 for additional information
FIGURE 19:
Hall Effect Geartooth Tachometer.
 2003 Microchip Technology Inc.
DS00894A-page 15
AN894
CONCLUSION
BIBLIOGRAPHY
Feedback sensors serve a critical role in a motor
control system. These sensors provide information on
the current, position, speed and direction of a rotating
motor. In addition, the sensors improve the reliability of
the motor by detecting fault conditions that may
damage the motor.
1.
The four major feedback sensors discussed in this
document are: current sensors, back EMF or
sensorless control, quadrature encoders and Hall effect
tachometers. Each of these sensors offer advantages
and disadvantages that the designer must evaluate in
order to provide a stable, reliable and cost-effective
control system.
Further details on motor control circuits and sensors
are provided in several books, including “Motor Control
Electronics” (10). In addition, please review Microchip’s
web site (www.microchip.com) for reference designs
and applications notes on motor control systems.
References (9) and (11) are just two of the many
documents available that demonstrate how Microchip’s
PICmicro microntrollers and analog products can be
used in a motor control system.
DS00894A-page 16
Bell, Bob and Hill, Jim, “Circuit Senses HighSide Current”, EDN, March 1, 2001.
2. Blake,
Kumen,
“Analog
PCB
Layout
Techniques”,
2002
Microchip
Master
Conference, Microchip Technology Inc., 2002.
3. Farley, Mike, “High-Side ICs Simplify Current
Measurements”, Power Electronics Technology,
September 2003.
4. Gilbert, Joe and Dewey, Ray, “Application Note
27702A, Linear Hall-Effect Sensors, Allegro
Microcsystems, Worcester, MA, 2002.
5. Klein, William, “Circuit Measures Small Currents
Referenced to High Voltage Rails”, Electronic
Design, January 7, 2002.
6. Moghimi, Reza, “Curing Comparator Instability
with Hysteresis”, Analog Dialogue 34-7, Analog
Devices, Norwood, MA, 2000.
7. Law, Lou, “Measuring Current with IMC Hall
Effect Technology”, Sensors, November 2003.
8. Lepkowski, Jim, “AND8027 - Zener Diode
Based Integrated Passive Device Filters, An
Alternative to Traditional I/O EMI Filter Devices”,
ON Semiconductor, Phoenix, AZ, 2001.
9. Parekh, Rakesh, “AN889 - VF Control of 3Phase Induction Motors using PIC16F7X7
Microcontrollers”, Microchip Technology Inc.,
2003.
10. Valentine, Richard, editor, “Motor Control Electronics Handbook”, McGraw-Hill, Boston, 1998.
11. Yedamale, Padmaraja, “AN885 - Brushless DC
(BLDC) Motor Fundamentals”, Microchip
Technology Inc., Chandler, AZ, 2003.
12. “Hall
Applications
Guide”,
Melexis
Microelectronics, Concord, N.H., 1997.
 2003 Microchip Technology Inc.
Note the following details of the code protection feature on Microchip devices:
•
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•
Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the
intended manner and under normal conditions.
•
There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our
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•
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•
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dsPIC, KEELOQ, MPLAB, PIC, PICmicro, PICSTART,
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© 2003, Microchip Technology Incorporated, Printed in the
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 2003 Microchip Technology Inc.
DS00894A-page 17
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