Stepper Motor Control with Low System Costs and High Functionality
March 14, 2016 by John Day
By Adriano De Rosa, IC Architect, Micronas
In the field of small drives, stepper motors continue to be a widely-used and popular drive solution. Owing to their reasonable price, they are often a good economical alternative to servo-systems, e.g. with BLDC motors.
When it comes to selecting the control electronics for drives in the range smaller than 10 W, solutions offering the highest possible degree of integration are normally the preferred choice. As the legal regulations for lowering CO2 emissions are becoming ever more stringent and restrictive, OEMs in the automobile electronics sector place the focus more and more on factors such weight, geometry, energy efficiency and networking. These factors also play a major role in the selection of a drive system. Similar considerations are also made in the industrial sector with a view to Industry 4.0, for instance.
Many makers of small actuators nonetheless feel the need to use different types of motors such as BLDC, PMSM, and stepper motors – but preferably with the same or minimally adapted control electronics. The aim is to accomplish a high degree of standardization for hardware and software development.
It is precisely for this purpose that Micronas developed a highly integrated one-chip solution, the HVC 4223F, for controlling different motors. The possibilities and options for its use in stepper motors are outlined in the following section.
Fig. 1: PCB with HVC 4223F and stepper motor
Stepper motors and operating modes for different applications
A stepper motor is fundamentally a synchronous motor where the rotor turns by a specific an-gle in dependence of the excitation of the stator coils. Moving in several steps, every rotation angle (which is a multiple of the minimum angle of rotation) can so be approached. Stepping motors come in various designs, with bipolar and unipolar stepper motors found in the small and micro motors sector. The bipolar motor has two coils and therefore four connections. The uni-polar motor has fundamentally the same structure, with the difference that an additional con-necting lead is tapped in the middle between both coils and the motor therefore having six con-nections. The idea in the past behind the unipolar types was mainly a less complex excitation electronics with only four low-side switches. However, this configuration is less important today because modern semiconductor integration allows complete H-bridges to be integrated easily together with the necessary control electronics. This is why we will take a closer look at bipolar stepper motors. The HVC 4223F supports both the voltage-controlled open-loop operating modes and the current-controlled closed-loop operating modes. Also, it allows the commutated excitation with the analysis of the back EMF voltage, which is as yet not very widely used for stepper motors.
Table 1 shows a summary of the operating modes supported by the HVC 4223F.
Table 1 Stepper motor operating modes with HVC 4223F Operating Mode Open Loop Closed Loop Remarks
Full and half step x 90° full steps always with simultaneous current feed for both phases (current phasor always located precisely between the x and the y axis) “Wave Drive” x As in full step with 90° steps, but phases are not individually controlled for each step (current phasor always located on the x or y axis)
Half step x Full step and Wave-Drive combined
Voltage-controlled Micro-stepping x PWM modulation of the phase voltage generating micro steps (e.g. with sin/cos look-up table)
Current-controlled Micro-stepping x Micro stepping with phase current measurement and comparison to programmable reference current via DAC for current shaping.
Commutated x Commutated excitation with analysis of the back EMF voltage (similar to BLDC motor operation)
Micronas one-chip solution and use
The HVC 4223F integrates a powerful ARM® Cortex® M3 core and all necessary peripherals for motor excitation in a single monolithic component, which uses a minimum of PCB space and which, in our application, realizes an efficient stepper motor driver. This makes work easier for system developers, cuts design cycles, and so reduces the total costs across the life cycle of an application.
Six n/n half-bridges (incl. charge pump) are integrated which, with the appropriate wiring of the output pins and configuration of the software, can be adapted to the type of motor. The EPWM module (Enhanced Pulse-Width Modulation) supports passive and active free-wheeling current patterns (asynchronous/synchronous rectification) for different operating modes and types of motors. The integrated current measurement and the D/A converter allow the programming of current setpoint values (e.g. for current-controlled micro-stepping).
Especially in the PMSM/BLDC, feedback of the rotor position can be given without sensors via the comparators and the integrated star-point reference, or alternatively via Hall sensors / encoders. Also, the commutated operating mode can be selected for stepper motors, e.g. for reaching higher rotational speeds. Stepper motors may be adapted to match different operating modes (see Table 1) which are programmable in the software.
The ARM Cortex M3 processor rapidly computes algorithms for rotational speed and voltage control, supported by the high-speed 12-bit A/D converter (1 µs conversion time) and the se-lectable signal paths for current and voltage measurements. The output stage includes overload protection (overvoltage/overcurrent) and diagnosis functions. The integrated peripheral modules for the motor excitation (EPWM, comparators, star-point reference, D/A converter, diagnosis and overvoltage / overcurrent protection, temperature monitor, etc.) can be programmed for the operating modes listed in Table 1.
Figure 2 shows a minimal wiring option, e.g. for a typical small actuator application with only eight external components. But the chip also offers numerous options of connecting more components such as external sensors (Hall sensor, NTC sensor, etc.) or similar. Figure 1 shows a Micronas demo PCB which can be configured for different types of motors (BLDC, BDC, steppers) and which includes a number of other components such as LEDs, shunt (for BLDC operation), etc.
Fig. 2: One-chip minimal solution with Micronas HVC 4223F
More efficient drive design with an intelligent and highly integrated controller chip
Normally, drive solutions involving stepper motors do not include cost-intensive sensor technol-ogy for position return. This is why stepper motors provide a cost-effective solution for positioning. But the drive electronics must rely on the motor steps being executed correctly.
In practice, peak loads will occur during operation, with the motor experiencing wear and tear across the life cycle, resulting in mechanical sluggishness. This is why stepper motor systems are usually designed with operative safety margins to make sure that the actuator limits are not exceeded during operation. A smart combination of the above described operating modes helps to limit such “over-dimensioning”. For example, calibration travels for stop recognition can be made with commutated excitation to obtain position details through the analysis of the back EMF voltage and in order to respond very quickly at the actuator by stopping the motor. Calibration cycles will therefore generate far less mechanical stress, and the parts of the drive can be designed with greater cost effectiveness. The motor easily responds to different load sce-narios during operation because the programmability of the chip offers the full range of freedom with the selection of the mode of excitation. This is a substantial advantage, e.g. compared with fixed-programmed chip solutions.
The closed-loop excitation permits the adapted motor control by analyzing the motor phase voltage. The rotor position is detected by analyzing the Back EMF voltage or by the zero cross-ings of an open phase. Basically, a commutated stepper motor is nothing but a multi-pole BLDC motor. Drawbacks such as resonances and excessive heat generation associated with stepper motor technology will not arise. System designs can therefore expect optimized torques, with high rotational speeds to be realized which are untypical for stepper motors.
The D/A converters included in the chip may be used to realize programmable current limits. This allows ramping current to be limited, for instance to improve electromagnetic radiance.
Micronas presents a highly integrated solution for the excitation of a wide variety of different electric motors. The application presented here illustrates the efficiency created by the low number of external discrete components. The high degree of integration accomplished with this configuration allows maximum miniaturization and therefore economical solutions with the add-ed advantage of realizing different types of motors based on the same platform with just a few minor changes. This means that many functions and the appropriate applications can be ad-dressed by adapting the software, giving customers the freedom to efficiently equip a complete platform of actuators with one and the same chip. The high level of hardware and software re-usability guarantees a fast response to changes in customer needs and requirements.