Choosing the right driver/controller combination for your stepper motor design
Maximizing existing IP
Although such complete integrated solutions provide the smallest system size and lowest cost of construction, many designers prefer to opt for an intermediate stage, retaining the core controller but relying on an intelligent driver chip such as the AMIS-30522 (Figure 2 below) for other functions.
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| Figure 2: Some designers prefer to retain the core controller together with an intelligent driver chip for other functions. |
The motivation behind this twochip strategy is two-fold. First, some applications require more current drive than can be provided from a single chip. But much more commonly, designers opt for a two-chip solution because they are aiming to maximize the value of their existing IP.
They may have developed a high level of expertise and associated software that they use with their preferred standard microcontroller or DSP. Quite naturally, they wish to re-use and improve that resource.
The intelligent integrated motor driver chip is designed for such users. It requires only a next micro-step command from a microcontroller as its input, and delivers the required PWM at the coils of the motor. BOM is dramatically reduced, and the loading on the microcontroller is minimized, potentially to such a degree that one micro-step command can control more than one motor.
The use of an integrated driver allows the host controller functions to be as simple - or as complex - as required. The driver directly implements microstepping, reducing audible noise and step-loss due to resonance, while improving torque at low speeds.
Processing burden is further off-loaded from the host controller by the integration of a current translation table and incorporation of a proprietary PWM algorithm for reliable current control.
Interfacing is via I/O pins and a simple SPI, providing control over a wide range of parameters, including current amplitude, step-mode, PWM frequency, EMC slope control and sleep mode. The driver can also be chosen to provide the controller (again via SPI) with all of the information it requires on speed, position and coil current as well as diagnostics such as open and short detection or overheating.
Further integration
As we have already observed, however, further integration is possible. A device such as the AMIS-30624 implements all of the capabilities of an intelligent driver. It also has the capability of a programmable state machine that translates a target position into the sequence of (micro) steps required to get to that position with the specified acceleration, speed and deceleration.
The target position and other high level information is dictated by a remote host and is communicated via a bus-level interface such as I2C or LIN. Such an architecture has the particular advantage that it scales well to accommodate more axes of movement: the hardware and software design are extended in a modular way, and bus-based communication is inherently scalable.
In addition to simplifying hardware design, the use of integrated controllers significantly eases development and implementation of an appropriate motion control algorithm. In practice, this often boils down to running a characterization algorithm that returns the required parameter setting.
Motor dynamics
Working out how to drive the motor without losing steps follows a defined sequence. Torque and velocity are generally defined system requirements that can be used to determine the required motor current.
The next step is to consider the motor dynamics. Of particular interest is the resonance or forbidden frequency. During acceleration and deceleration, this must be crossed as quickly as possible. The AMIS-30624 allows the configuration of "minimum" and "normal" operating velocities as well as acceleration and deceleration times to achieve the correct motion profile for the motor being used.
Once all of the relevant parameters have been calculated, they are sent to the device via I2C bus. They can be iteratively honed to demonstrate stability and finally burned into non-volatile memory as the final operating parameters.
Besides reducing BOM and simplifying design, the ASSP approach to stepper motor control yields more sophisticated control strategies and designs that are more closely tailored to application requirements. Two of the key techniques for achieving such improvements are sensorless stall detection and dynamic torque conditioning.
Stepper motors are mostly used in open loop systems. Although such systems are simple and - by definition - stable, they have the disadvantage of lacking absolute positional feedback.
If the motor is blocked, there is a danger that the driver/positioner will continue to drive the coils as though the motor were still moving. This creates noise, and more importantly, breaks the link between the real position and the information stored in the positioner.
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