FEATURE DRIVES, CONTROLS & MOTORS Taking the next step
Adding an encoder to a step motor system enables the drive to monitor the motor’s actual position directly, closing the feedback loop and avoiding many of the limitations traditionally associated with stepper systems, explains Jeff Kordik, chief technology officer, Applied Motion Products
L
ow in cost and easy to use, step motors are often a popular choice.
These have a permanent magnet motor that rotates on bearings, called the rotor; plus the stator – a series of electro magnets surrounding the motor. When the drive sends electric current through the stator, the resulting stator field forces the rotor to react. When stator current is properly controlled, the step motor operates appropriately, hitting its targeted position. However, if the step motor produces less torque at high speeds or becomes overloaded, it can stall. By adding an encoder to a step motor
system, the drive can know the position and velocity of the rotor and, therefore, the load at all times. This can enhance the performance by providing several closed-loop functions: 1. Position and velocity verification Conducted using a basic algorithm and with a standard controller or microprocessor, this function requires completion of the motion profile to indicate if the step motor stalled before reaching the end position. A PC or PLC can monitor position and velocity by connecting to a drive by RS232, RS485, CANopen, Ethernet or EtherNet/IP. Based on feedback, the system corrects the
motion profile to hit its mark. As corrective action may take time, position and velocity verification is best for low volume or low cycle applications that do not have high cycle times,
such as in a lab or test area. 2. Stall detection and reporting Encoder feedback can immediately detect and report stalls due to motor overloads, enabling a system to react to a stall faster. The driver can be configured to signal other equipment by the state change of a digital output or by providing this information over the communication interface. It does not, however, prevent a stall. 3. Stall prevention This eliminates stalls by dynamically controlling the stator current and speed of the motor to maintain torque in all situations. This enables otherwise impossible control schemes in stepper systems, such as torque-limited velocity control and torque-limited position control. In this scenario, the encoder monitors in real
time the torque producing lead/lag angle – the angular difference between the magnetic fields of the stator and rotor. Drives use this information to precisely control the motor’s torque and prevent unexpected motor stalls. While stall prevention offers advantages over stall detection, the closed loop system takes longer to complete moves that could negatively impact machine operations.
SENSOR HOMING Applying user logic often supplied by motor suppliers provides greater control of the system current in a near-stall situation to ride through or prevent most stall conditions. These programs provide the ability to detect a stall situation relative to position and reverse off of a hard stop to a predictable and repeatable home position. User-selectable idle and running currents provide a quiet and smooth homing routine, eliminating noise and vibration at end stops, and improving motion control accuracy and reliability through intelligent fault recovery.
CHOOSING AN ENCODER
Optical encoders offer accurate and reliable performance along with a wide range of resolutions. However, they are susceptible to signal degradation and loss when exposed to dust, oil or similar contaminants. Capacitive encoders, meanwhile, utilise
newer technology, offer similar benefits, and ultimately provide the same position and speed information as optical encoders. These are also immune to environmental contaminants. While you can add encoders to your motors,
some manufacturers offer motors already preconfigured with a high-resolution incremental encoder and closed-loop servo control firmware.
Applied Motion Products
www.applied-motion.com
DRIVES AND MOTORS MEET APPLICATION DEMANDS
To meet application requirements around the world, the range of electrical motors and drive systems from Oriental Motor is extensive. One example is the closed loop stepper
motor. This has a sensor in the motor that feeds information to the driver, allowing the driver to know exactly what position the motor is in and at what speed it is operating. They are able to complete accurate positioning faster and more accurately than a servo motor, do not require tuning and can be
manufactured at reduced cost. Some have high-speed and accurate return- to-home and, thanks to a mechanical absolute encoder, eliminate the need for a battery backup. These also support various industry comms systems and protocols, have intelligent drivers, and allow for remote control and monitoring. A versatile motor, these are said to be particularly effective for rapid indexing and applications with complex motion profiles, as well as robotics, testing and inspection systems, and applications that require
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smooth operation, short settling times and precision. Another product is the brushless DC motor (BLDC). These, the
company explains, offer 85-90% efficiency compared to 75-80% for brushed DC motors. More of the power used by the motor results in rotational force and less is lost as heat; and their speed ratio can be as great as 1:50. The permanent magnet employed allows for great and very accurate speed as well as torque control and stability. These also cost significantly less to manufacture than a servo. The BLDC is perfect for any application that requires a motor to rotate
at a stable speed, from low to high, even when the load fluctuates. Ideal applications include more advanced conveyor systems, checkweigher systems, AGVs, certain axis on factory automation machinery, as well as tensioning and winding applications. Being compact, these can help in downsizing equipment and saving energy. They also significantly reduce power consumption as the use of permanent magnets in the rotor portion prevents secondary loss from the rotor.
Oriental Motor
www.oriental-motor.co.uk
JUNE 2019 | DESIGN SOLUTIONS
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