Silicon Driving Mechatronic Designs

16 September 2008

Silicon technology is helping designers to deliver stepper motor designs in applications ranging from vehicle headlight positioning to surveillance cameras

The challenge of developing robust, efficient and cost-effective ‘mechatronic’ designs is becoming
increasingly important as OEMs seek to incorporate advanced motion control into a range of products and systems.

When it comes to implementing mechatronic applications requiring precise and efficient control, designers generally turn to stepper motors.

As a result, these motors can now be found in applications ranging from headlamp positioning and CCTV cameras to actuator and valve control, factory automation and medical dosimeters. In many of these applications, space is at a premium and the need for reliability is high. It is because of this that in addition to identifying the best motor, engineers often find that they must also minimise the size and the component count of the drive, control, and feedback circuitry. It is here that the latest IC developments can play a key role.

Stepper motor control
Many modern stepper motor designs use closed loop control techniques that feedback the position of the rotor to the driver circuitry. This provides the link between the actual and expected (or electrical) position,
leading to better control of the drive currents to produce the required acceleration and torque. Closed loop
schemes also have the ability to detect stalls and eliminate missed steps. While closed loop schemes have traditionally employed Hall sensors or optical encoders, sensorless closed-loop designs have recently become popular. These sense the actual drive currents and back EMF (electromagnetic force) created by the motor to calculate the mechanical position of the rotor.

As control algorithms and drive circuitry have evolved, so too has the range of integrated drive and control semiconductor technologies available to the designer. For example, the latest ASSPs (applicationspecific
standard products) can integrate a translator into compact SOIC and QFN packages that converts consecutive steps to the required coil current, driver transistors in H-bridge configuration, flyback diodes, onchip current regulation via PWM, and a variety of protection circuitry. Starting from high-level ‘next step’ position commands received via a logic-type interface such as an SPI bus, such devices can directly drive a
stepper motor. Furthermore, by offering integral microstepping functionality they can significantly improve resolution, increase torque at low velocities, reduce audible noise, and eliminate step-loss.

Drive and control ICs
Stepper motor ASSPs can be divided into two broad categories, typified by the ON Semiconductor AMIS-305xx and AMIS-306xx series. The latter is fully integrated and accepts high-level commands via an I2C or
LIN interface. A typical application for an AMIS-306xx device is shown in figure 1. The control algorithm is embedded within the IC in the form of a state machine and the designer provides an input that ‘tells’ the
device to move the motor to a certain position, with defined acceleration and maximum speed, using the desired microstep size.

This approach is well suited to applications such as positioning surveillance cameras where engineers need to quickly construct a working design, and details of the control algorithm are not critical. Standard motion dynamics are sufficient, and the IC may implement advanced features such as sensorless stall detection to further simplify the designer’s task. However, devices such as the AMIS-305xx series sacrifice a little of this turnkey approach and time-to-market advantage, to give the designer more flexible control over system behaviour.

Designers using such intelligent drivers employ more traditional control circuit architecture, with a microcontroller, DSP or programmable state machine running control software and delivering the input to the driver IC via SPI. This interface can be used to specify parameters such as current amplitude, step mode, PWM frequency and EMC slope control. In return, the intelligent driver will typically provide information back to the controller, including status flags, open and short-circuit alerts, and status of the internal step-to-current translation subsystem.

Back EMF measurement
To eliminate BOM costs and design complexity, the driver will also typically provide enough feedback to implement closed loop control. In the AMIS-306xx series, this feedback is provided directly to the input of the integral state machine, although in the AMIS-305xx series, it is externally available via the SLA (speed and
load angle) output pin. This gives designers direct access to a measure of the back EMF induced in the motor coils as it is passed by the magnetic poles of the rotor.

Measurement of back EMF offers a number of benefits. By enabling the rotor position and speed to be known, the MCU can perform a real-time comparison between the electrical and expected position of the rotor. This not only facilitates stall detection functionality, but also allows the circuit to ascertain when missed steps may be imminent. More than that, the difference between the actual and expected positions
gives an indication of the torque applied by the motor, opening up possibilities for complex torque control algorithms.

The ability to measure back EMF can also have an impact in motor choice, with improved control strategies being used to stretch the motor’s operational limits. Motors are typically characterised with a torque versus velocity curve, which gives a speed above which the motor cannot be used. However, running a characterisation of the motor’s behaviour and deducing the torque delivered by looking at the SLA output
may reveal a subtler situation.

Typically, motors are specified for use in full-step mode. As speed rises, a point is reached at which torque drops dramatically. However, the drop in torque may be much less significant if the motor is to be used at
the same speed, but in microstepping mode. As speed increases further, it is not uncommon for the full-step torque to return to values comparable to those attainable at lower frequencies. The velocityversus-
torque curve looks more like a ‘notch’ function than a ‘low-pass’ function.

Running an active characterisation on the motor allows the designer to implement a control algorithm that uses full stepping at the extremes of low and highspeed operation, and switches to microstepping in the measured narrow central range of speed values. Such a facility is useful not only for end users devising
system-level solutions; it also allows manufacturers of smart motors (those with built-in driver electronics) to extend the specified range of their products substantially.

Back EMF measurement via the intelligent driver’s SLA pin can also be used in determining end system behaviour, helping designers to avoid operating at forbidden, resonant Eigen-frequencies. These frequencies are properties of the entire motor-driver-load system and cannot be easily deduced from a datasheet. However, they are easily identified when monitoring the SLA pin because they show up as vibrations. The problem can then be solved by accelerating the motor through the Eigen frequency range as quickly as possible.

Auto speed functions
Using a suitable ASSP with back EMF measurement capabilities makes it relatively simple to adapt torque delivered to instantaneous need. This is useful in the situation where the controller identifies the possibility of imminent step loss as it can respond by delivering more torque. However, it also makes it possible to rapidly implement an ‘auto-speed’ function whereby the microcontroller asks the intelligent driver circuit to take the motor to a desired position as fast as possible. The back EMF feedback mechanism is then used during the move to determine

The benefits to this are that the power required to execute the specified motion is halved, as is the time taken for the motion to take place. These two results are connected; the motor delivers exactly the right amount of instantaneous torque to move the rotor as quickly as possible, and this means that the system makes maximum use of the energy provided to it.

Such benefits are particularly attractive in highly dynamic applications that do not involve continuous motion. Systems such as weaving and pick-and-place equipment need to complete movements accurately and as
quickly as possible before moving to the next set position. Although these are not typically high-power applications, the benefits of power savings may well be such that it is possible to choose a smaller, more efficient and lower cost motor than might originally have been envisaged.

PETER COX is product application engineer, industrial products, and GUIDO REMMERIE is director of motion control and energy management products, ON Semiconductor.

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