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FEATURE DRIVES, CONTROLS & MOTORS


CLOSING THE LOOP


In a host of different applications with different operational requirements, there can be good reasons to explore the potential of operating a stepper motor in a closed loop architecture, as Gerard Bush of INMOCO and Peter Vandermeulen of PMD Corp. explain


P


roving capable in a vast range of applications, stepper motors offer an inexpensive solution when compared to brushless motors


and are frequently chosen for cost-sensitive applications that can tolerate less accurate positioning. Often, stepper motors are operated in open loop architectures,


with engineers tending to look at alternative motor solutions in critical applications where care has to be taken to ensure that, when a command is given to the motor controller, the motor actually ends up where the engineer wants it to go. But could the design engineer be missing a trick? A stepper motor


operating in closed loop mode can offer some advantages over the use of a brushless motor even in critical applications. The servo stepper motor will almost always be lower in cost and offer a higher torque density than a brushless motor. At the same time, even in more standard applications where one


would typically use a stepper motor in an open loop mode, there can be advantages to closing the loop with an encoder. The net cost savings from using a smaller motor could outweigh the cost of the encoder, while delivering performance advantages.


THE OPTIONS So let us consider the various options for stepper motor architectures, which include the open loop stepper motor, the sensorless stepper motor, the stepper motor with encoder, and the fully closed loop stepper motor. The typical open loop stepper motor architecture drives the motor with a constant winding current without regard to loading or true motor position. With no true position feedback, the motor is generally over-sized to ensure that it will always overcome torque requirements, but there is still no guarantee that the motor’s end position will be close to the target position. Encoderless stall detection technologies exist that purport to overcome


this lack of information. This architecture does not employ an encoder but instead attempts to derive the rotor location by others means. Various techniques exist such as detecting back EMF voltage on a passive phase or measuring current rise times. However, there is still no guarantee that the motor is actually at the proper position; and encoderless stall detection techniques are notoriously difficult to implement as dependencies exist on both minimum speed required and the load on the motor. In a typical stepper motor, the position error is proportional to the


loading, which will be dominated by friction at steady state speeds and by inertia during acceleration and deceleration. However, over time and with dynamic load conditions, this situation can change due to the ageing of belts, bearings and other secondary effects. Adding a position loop to the stepper motor architecture requires that position feedback – through some form of a position encoder at some minimal resolution – is added. In the simplest configuration, operating the stepper motor with an encoder, the feedback ensures that the controller can verify that the motor is in the desired position.


22 OCTOBER 2018 | DESIGN SOLUTIONS If not, the controller can adjust by providing additional steps, either


in real-time during the move when position errors are detected, or post move. This allows the motor size to be reduced as the encoder gives a positive indication of the position problem. But we can go a stage further with this encoder by using the stepper


motor in a fully closed loop configuration, which adjusts the torque requirements in real time and is able to calculate the appropriate motor current and current phase angle. The ability to calculate the appropriate motor current (torque) enables the system to compensate for position error resulting from frictional or inertial loads. Since the rotor angle is known, the controller can calculate a motor current phase angle which optimises the torque response and makes the delivered torque predictable.


THE CLOSED LOOP ARCHITECTURE This new closed loop stepper motor architecture - also referred to as the servo stepper – can decrease motor size and cost, improve motor efficiency and accuracy, and reduce noise, heat and maintenance. Why can motor size be reduced in close loop architectures? Open loop


architectures cannot detect a lost step, so the motor will be oversized in order to increase torque margin to ensure there are no lost steps. The existence of an encoder which can detect a lost step will reduce the torque margin, allowing for a much smaller motor. There are efficiency benefits too, since the controller’s optimisation of the motor current phase means the greatest possible torque is being generated by the motor for a given current. Much less electrical power will be used to generate the same amount of mechanical power. In terms of accuracy, the position error in a closed loop architecture


will be smaller since the position loop will continuously command a torque in an attempt to compensate for any position error. This will have a positive effect on both positional and velocity accuracy. Furthermore, not only will the motor run much cooler in a closed loop


system (meaning a longer service life), but it also reduces vibration and will result in quieter operation. This is not to say that the closed loop architecture will be the best or the right solution in every application. However, even if the accuracy of an open loop system is deemed to be acceptable, there might still be advantages in turning to a closed loop architecture if motor efficiency is a factor. For example, if the system uses a battery as a power supply, the improved efficiency from a closed loop solution will be beneficial even though the position accuracy of the open loop solution may be adequate. Another example would be if the motor is in an environment where the increased thermal energy or noise levels emitted from the open loop system are not tolerable or desirable.


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