AN2501
Application note
ST10 electric motor control library
Current sensing driver
Introduction
STMicroelectronics provides a library of electric motor control functions that can be used as
base blocks for electric motor control applications.
This application note describes the set of API to implement a current sensing strategy in
electric motor control applications.
March 2007 Rev 1 1/13
www.st.com
Table of contents AN2501
Table of contents
1 Functions set overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1 Overview on current sensing strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2 API specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1 Synchronization of signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2 Functions definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.2.1 Current_sensing_Init . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.2.2 Current_sensing_Start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.2.3 Current_sensing_Register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.3 Resource . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
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AN2501 List of tables
List of tables
Table 1. Current_sensing_Init parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Table 2. Current_sensing_Start parameters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Table 3. Current_sensing_Register parameters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Table 4. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
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List of figures AN2501
List of figures
Figure 1. ST10 EMCL structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Figure 2. Block diagram of the flux oriented control scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Figure 3. Measurement strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Figure 4. Current sensing timing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
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AN2501 Functions set overview
1 Functions set overview
The current sensing driver is a set of functions included in ST10 electric motor control library
(EMCL) (see Figure 1) useful to process the measurements of current in electric motor
control. This document explains how to use two ADC channels to sample the currents on 2
shunt resistors at specific instants. In particular on one shunt resistor the current is
measured as instantaneous value and on the other shunt resistor as average value.
A brief overview on current sensing strategy is given in section Chapter 1.1.
Figure 1. ST10 EMCL structure
ST10 EMCL
Flux
Oriented
Control
Sinusoidal
Control
Observer
Leunberger
SPI
Driver
Encoder
Driver
Hall sensors
current_sensing.c
current_sensing.h
1.1 Overview on current sensing strategy
The flux oriented control (FOC) is based on a current controlled voltage source inverter
(VSI) structure to generate a desired voltage space vector that the inverter applies to the
motor phases by means of the measurement of the electric motor phase currents (see
Figure 2) and a continuous modulation method (modulation period T) .
Figure 2. Block diagram of the flux oriented control scheme
Driver
Current sensing
Driver
PWM
Driver
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Functions set overview AN2501
The transformations from a stator frame to a rotor frame that are the core of FOC need the
sampling of three phase currents in the modulation period T. The sampling can be reduced
to only two phase currents if the motor windings are star-connected.
In this case the acquisition of two current signals can be done with two different A/D
converters or using only one A/D converter multiplexed on two (or more) channels, a
solution less expensive than the first one where the acquisitions are sequential.
Independently from the used current sensor, this second choice introduces some
measurement errors. In order to reduce these errors, one conversion is done at the time 0
(or T/2) where the contribution of harmonic current components is minimum, while the
second current is calculated as average value sampling in two different instants [(0 ±δ) or
(T/2 ±δ)] centred compared with the acquisition on the other phase (see Figure 3) with a gap
δ bigger than the conversion time of the A/D converter. In particular δ is chosen equal to T/2.
So only two measurements for modulation period are needed.
In this document the current sensing strategy with one ADC is described.
Figure 3. Measurement strategy
0 (T/2)
-
x
x
+
x
t
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AN2501 API specification
2 API specification
The following API allows easy access to the described current sensing strategy configuring
the ADC channels (c1 and c2) in Injection Mode to sample the motor currents from each
channel sequentially, reading and processing these values.
The use of these functions eliminates the need to access the ST10 registers directly.
The available functions are:
1. void Current_sensing_Init(CS_UINT16 sPeriod)
2. void Current_sensing_Start(void)
3. void Current_sensing_Register(CSFUNCTION csfunction)
see file current_sensing.c in the EMSW_CS Library.
2.1 Synchronization of signals
The sampling of the ADC is synchronized with the PWM. The CAPCOM timer T7 is
synchronized with the PWM semiperiod (i.e. PWM timer and CAPCOM T7 are started in the
same instant with consecutive instructions) so that the Injection Request bit for ADC is set
by CAPCPOM channel CC31 in the instants 0 (channel c1 acquisition) and T/2 (channel c2
acquisition). The current on channel c2 is calculated as average value of the present value
and the value sampled in the previous period.
This means that the current values are available for the control only in the 2nd semiperiod of
the PWM period. If a control function is registered, this function will be performed at each
mid point of the PWM period (t = T/2), i.e. at each end of conversion for the channel c2.
Control Function performing if registered.
Figure 4. Current sensing timing
PWM counter
t
T
m
INT_PWM
A
DC end of conversion interrupt (t = 0) (channel c1)
T7 interrupt to start ADC conversion (hardware)
pw
T
7
ADC end of conversion interrupt (t = T/2) (channel c2)
Control Function performing if registered
T
pwm
t
t
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API specification AN2501
2.2 Functions definition
In the following the functions for configuring two ADC channels in Injection Mode, to read
and to process the results of ADC conversions so as to implement the currents sensing
strategy with one ADC are described.
2.2.1 Current_sensing_Init
Prototype definition:
void Current_sensing_Init(CS_UINT16 sPeriod)
Parameters:
Table 1. Current_sensing_Init parameters
Configuration Parameters Description
c1 channel number (0, ..., 9)
c2 channel number (0, ..., 9)
Global Parameters
csValue variable to store current value (0)
csValue_temp static variable to store current value (T/2)
csFunction pointer to the control algorithm function
Function Parameters
sPeriod sampling period (number of mcu clock tics)
Description:
The Current_sensing_Init function is used to configure two ADC channels (c1 and c2) in
Injection Mode and to convert a specific analog channel triggered via software at every
compare event of compare register CC31 on Timer 7, which sets Injection Request bit
(ADCRQ), synchronizing the sampling of the motor currents with PWM period.
It is needed to set the two ADC channels (0...9) connected to the current sensors by means
of parameters (c1 and c2) inside the function.
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AN2501 API specification
2.2.2 Current_sensing_Start
Prototype definition:
void Current_sensing_Start(void)
Parameters:
Table 2. Current_sensing_Start parameters
Configuration Parameters Description
c1 channel number (0, ..., 9)
c2 channel number (0, ..., 9)
Global Parameters
csValue variable to store current value (0)
csValue_temp static variable to store current value (T/2)
csFunction pointer to the control algorithm function
Function Parameters
none
Description:
The Current_sensing_Start function is used to start the Timer 7 and the current sensing by
interrupt.
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API specification AN2501
2.2.3 Current_sensing_Register
Prototype definition:
void Current_sensing_Register(CSFUNCTION csfunction)
Parameters:
Table 3. Current_sensing_Register parameters
Configuration Parameters Description
c1 channel number (0, ..., 9)
c2 channel number (0, ..., 9)
Global Parameters
csValue variable to store current value (0)
csValue_temp static variable to store current value (T/2)
csFunction pointer to the control algorithm function
Function Parameters
none
Description:
The Current_sensing_Register function is used to point to the control algorithm function. It is
needed to declare in the main a user function with the algorithm control and register it so
that the algorithm can be executed at every end of conversion on channel c2.
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AN2501 API specification
2.3 Resource
For the use of Current sensing Driver the needed resources are:
ADC (2 channels) ADC
Compare Timer (Timer 7) CAPCOM Timer
Compare register (CC31) CAPCOM register
Bank register (RB08) ADC Interrupt Service Routine
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Revision history AN2501
3 Revision history
Table 4. Document revision history
Date Revision Changes
12-Mar-2007 1 Initial release.
12/13
AN2501
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