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Smoother Constant-Current Stepper Control for Sharper Security Video
Tuesday, April 11, 2017

By Arthur Gonsky, Solutions Engineering Manager, ON Semiconductor

 

A new control strategy allows stepper motors not only to move as smoothly as in servo topology mode, but also helps ensure consistent positional accuracy and superior resolution.

Motion and positioning applications such as panning and tilting security cameras can be fulfilled using stepper motors or servo-controlled brush/brushless DC motors. Stepper motors can be controlled by counting steps. Since there is no need for position feedback, the control system can be both easier to design and less expensive to produce. With a stepper motor/ driver/ controller system design, it is assumed the stepper motor will follow digital instructions. One important aspect of stepper motors is the lack of feedback to maintain control of position, which classifies stepper motors as open-loop systems.

Stepper Motors and Control Strategies

Stepper motors are available in three basic types based on the structure of the rotors. Variable reluctance motors (VR) feature a toothed rotor and wound stator, and tend to be well suited to low-torque applications such as positioning micro mechanisms. Permanent magnet stepper motors (PM) are typically less expensive than variable reluctance motors, and incorporate a rotor that is magnetized with alternate north and south poles. Hybrid motors (HB) are also available, and feature a rotor that is both magnetic and has teeth to guide the flux resulting in generally stronger torque characteristics. Moreover, depending on the winding arrangements for the electromagnetic coils, they are classified as Unipolar stepper and Bipolar stepper. In this paper, we will discuss Bipolar stepper.

Stepper Motors have their own step angles. In general, PM type has 7.5~15deg, HB has 0.9~3.6deg. Cost of the HB type is very high compared to the PM type. Step angle is fixed by the number of magnetic poles the motor has. For example, the step angle of a motor that has 48 magnetic poles is 7.5 deg, has 400 poles is 0.9 deg. When we discuss Full step, Half step or Micro-step in this paper, we refer an electrical angle of the stator flux shown in Figure 1, and it needs to be differentiated from a step angle.

ONSAR2732A_Fig1 sm

Figure 1: Output current vector locus (one step is normalized to 90 degrees)

Figure1 shows there are 16 electrical angles in Micro-step between a magnetic pole and the other. The step angles (angle between a magnetic pole and the other) depend on motors, so that we show this figure referring a motor whose magnetic poles are located in 90 deg. In this case, θ0-16 show the rotor positions directly. If the step angle is 7.5 deg, you should transfer the figure into 7.5 deg from 90 deg.

Meanwhile, step angle means an angle of the rotor in 1 step when the motors run in Full step. For example, motors with 7.5deg step angle will rotate 360 degrees in 48 steps when it runs in Full step. It takes 2 times more steps in Half step, and 4 times more steps in ¼ step.

In other words, stepper motors can step with smaller angle by excitation mode (=Micro-step resolution). Therefore, Micro-step enables to reduce vibration or noise even though the motor is cheaper and the step angle is not so small.

Figure 2: Stepper motors - # of steps, an electrical angle and a step angle

Figure 2: Stepper motors – # of steps, an electrical angle and a step angle

Smaller steps control is required for the security camera equipped outside because the camera needs high powered zooming. The reason is when the camera moves in smaller angle, it can avoid dead angle. In the past, designers were using geared motors in order to control units with smaller steps in Full step or Half step. It is possible to reduce cost and size of the gears by Micro-step.

Figure 3: Micro-step covers dead angle

Figure 3: Micro-step covers dead angle

The oscillations produced in either of the Full step or in Half step can be enough to compromise the video quality of a pan/tilt security camera by causing blurring of the image. The problem is more acute with cameras using low-cost image sensors. Controlling the motor using Micro-step can overcome this issue.

Control Modes for Smoother Motion

The controller integrates four H-bridges for controlling the stepper motor phases. The outputs of each H-bridge are brought out to A and B pins on the IC. Two sets of A and B pins are used to control one bipolar stepper motor. The four H-bridges provide enough channels to control two bipolar motors. The motor coil current is set according to an external resistor and a voltage reference. A proprietary lossless current-sensing mechanism eliminates any need for an external power resistor, thereby enhancing overall energy efficiency.

When operating in constant-current PWM mode, voltage is applied to the coil during the charge stage until the current reaches the preset maximum value. This can be seen in the timing diagram of figure 4, where A = high and B = low. (1.CHARGE) At this point the controller enters slow-decay stage by shorting outputs A and B internally (A = low, B = low), causing the current to circulate through the lower side of the bridge. (3.SLOW) Finally, the bias applied to the coil is inverted (A = low, B = high) resulting in fast decay of the coil current. (5.FAST) The controller handles dead-time insertion to prevent shoot-through at the H-bridge switching transitions. (2, 4 & 6) Status 3 & 5 in the diagram below are called synchronous rectification.

Figure 4: MOSFET switching sequence for constant- current control

Figure 4: MOSFET switching sequence for constant- current control

Due to recent market expansion in IoT market, the demand for a small size surveillance camera/Network camera has become high. The demand requires less noise, low power consumption and smaller footprint.

Figure 5: Stepper motor drive waveforms and noise

Figure 5: Stepper motor drive waveforms and noise

In addition to the energy savings achieved through the lossless current sensing circuitry, the LV8714 H-bridges are built with special low RDS(ON) MOSFETs and uses synchronous rectification to deliver further efficiency improvement over alternative solutions. In addition, the controller enters a low-power standby mode when idle. The power savings achieved by these features can help in battery-powered or PoE-powered camera applications

Figure 6: Advantages of the LV8714

Figure 6: Advantages of the LV8714

The controller is also capable of over 256-step Micro-step and conventional Half step or Full step control modes. An evaluation kit and easy-to-use configuration software that simplifies controller setup help to streamline application development.

Conclusion

Surveillance cameras are a mainstay of security operations around the world. Operators keen to acquire more information – and particularly the ability to identify suspects – from captured images place a high value on achieving sharp, clear, high-resolution images. In the past, this has been difficult to achieve using low-cost mechanisms based on stepper motors to move the camera. The latest advances in stepper control, such as constant-current PWM mode, allow low-cost motion controls to deliver smooth camera action for clearer pictures.

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