AN1362 STMicroelectronics, AN1362 Datasheet - Page 7

• Manufacturer Part Number: AN1362
• Description: SWITCHED RELUCTANCE MOTOR CONTROL BY ST FIVE
• Manufacturer: STMicroelectronics

AN1362
(i.e. fig.2). This comparison is important in order to design some strategies, which implement the expected
outputs (fig. 3), by switching from one to another.
This technique may be implemented either via firmware (if the computational time allows it), or via external
logic devices in order to reach very high performances. The switch from one strategy to another, allows
the jump from a starting phase to the speed-up phase and/or to the necessary pre-injection phases to
reach the speed target.
These pre-injection phases are important to correct the “current tale action”.
By using a lead in the energizing phase, the current tale can be in the rising “L” zone (with dL/dt >0 and
consequentially torque positive), in this way the brake effect of the SRM motor at high speed is avoided
(fig.3).
SOFTWARE DESCRIPTION
The targets are: speed performances and a start-up sequence with a predetermined counter clockwise.
In the starting phase a “soft start procedure” is necessary in order to guarantee a start in the right sense
of rotation and to increase the Phase Current and the speed from zero to first step speed. After the start-
up, the program switches to the “low-speed” procedures to reach the 20000 rpm with a Bus of 220 VAC.
The implementation used in this application, performs the driving of the SRM motor, by analyzing the
encoders and by computing a logical function of these signals. This computation is carried out without an
external logic, to achieve a low cost target.
A patented pending technique is used to perform a variable pre-phase injection, in order to obtain a driving
signal of reduced timing. In this “Hybrid” control phase, a logical combination of the encoder is used to turn
on the phase of the SR Motor, while a timer is used to turn it off. The “counter” of the Timer decreases by
increasing the speed in order to obtain a firing signal, which is, in turn, synchronized with the correct
sequence. This sequence is less wide than the allowed minimum time from the simple combination of
encoder signals. The value of the “counter” could be chosen from an L.U.T., a standard linear function or
Fuzzy Block which is aimed to extract the correct timing in relation to the speed. In this application, an
L.U.T. is used to allow the Pre-phase timing.
A flow chart of the software is displayed in the following section, developed in Visual FIVE, the visual
Development Tool of the ST FIVE Family micros.
The flow chart could be divided into the following steps:
1 PERIPHERAL CONFIGURATION AND MICRO INITIALIZATION
In this step, micro is initialized to perform the Hw implementation, in particular with:
·timer configuration (Timer 2 in PWM mode @20 KHz, Timer 1 in timer mode to perform the pre-phase
switch off of the phase at high speed);
·the A/D configuration (current sensing);
·the initialization of the ports (analog input, digital input/output) in order to drive the L6386 and to
implement the over-current protection.
2 START-UP PROCEDURE (SOFT START)
Start up procedure is essential to allow the SRM motor to start correctly, in a direction or in the opposite,
and to speed up the motor from zero to 5000 rpm.
The correct sequence of phase ignition allows the counterclockwise direction. A variable Duty Cycle
allows the start-up @full DCBus from 0 rpm.
3 SPEED-UP ALGORITHM. (Low-Speed).
After the start up phase, the encoder signal, obtained in digital mode, is filtered by an algorithm which
performs a specified logic function, in order to express the drive sequence.
4 PRE-PHASE, HIGH SPEED ROUTINE (High-Speed)
There are lots of sequences of the manipulated encoder signals aimed to drive the motor, each of them
for a specific speed range. There are other two different sequences to get a speed of 20Krpm. There are
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