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Continuous actuators allow a system to position or adjust outputs over a wide range of values. Even in their simplest form, continuous actuators tend to be mechanically complex devices. For example, a linear slide system might be composed of a motor with an electronic controller driving a mechanical slide with a ball screw. The cost for such an actuators can easily be higher than for the control system itself. These actuators also require sophisticated control techniques that will be discussed in later chapters. In general, when there is a choice, it is better to use discrete actuators to reduce costs and complexity.
An electric motor is composed of a rotating center, called the rotor, and a stationary outside, called the stator. These motors use the attraction and repulsion of magnetic fields to induce forces, and hence motion. Typical electric motors use at least one electromagnetic coil, and sometimes permanent magnets to set up opposing fields. When a voltage is applied to these coils the result is a torque and rotation of an output shaft. There are a variety of motor configuration the yields motors suitable for different applications. Most notably, as the voltages supplied to the motors will vary the speeds and torques that they will provide.
A control system is required when a motor is used for an application that requires continuous position or velocity. A typical controller is shown in See A Typical Feedback Motor Controller. In any controlled system a command generator is required to specify a desired position. The controller will compare the feedback from the encoder to the desired position or velocity to determine the system error. The controller with then generate an output, based on the system error. The output is then passed through a power amplifier, which in turn drives the motor. The encoder is connected directly to the motor shaft to provide feedback of position.
A Typical Feedback Motor Controller
In a DC motor there is normally a set of coils on the rotor that turn inside a stator populated with permanent magnets. See A Simplified Rotor shows a simplified model of a motor. The magnetics provide a permanent magnetic field for the rotor to push against. When current is run through the wire loop it creates a magnetic field.
The power is delivered to the rotor using a commutator and brushes, as shown in See A Split Ring Commutator. In the figure the power is supplied to the rotor through graphite brushes rubbing against the commutator. The commutator is split so that every half revolution the polarity of the voltage on the rotor, and the induced magnetic field reverses to push against the permanent magnets.
The direction of rotation will be determined by the polarity of the applied voltage, and the speed is proportional to the voltage. A feedback controller is used with these motors to provide motor positioning and velocity control.
These motors are losing popularity to brushless motors. The brushes are subject to wear, which increases maintenance costs. In addition, the use of brushes increases resistance, and lowers the motors efficiency.
Pulse Width Modulation (PWM) For Control
PWM Unidirectional Motor Control Circuit
PWM Bidirectional Motor Control Circuit
A synchronous motor has the windings on the stator. The rotor is normally a squirrel cage design. The squirrel cage is a cast aluminum core that when exposed to a changing magnetic field will set up an opposing field. When an AC voltage is applied to the stator coils an AC magnetic field is created, the squirrel cage sets up an opposing magnetic field and the resulting torque causes the motor to turn.
The motor is called synchronous because the rotor will turn at a frequency close to that of the applied voltage, but there is always some slip. It is possible to control the speed of the motor by controlling the frequency of the AC voltage. Synchronous motor drives control the speed of the motors by synthesizing a variable frequency AC waveform, as shown in See Synchronous AC Motor Speed Control.
Synchronous AC Motor Speed Control
These drives should be used for applications that only require a single rotational direction. The torque speed curve for a typical induction motor is shown in See Torque Speed Curve for an Induction Motor. When the motor is used with a fixed frequency AC source the synchronous speed of the motor will be the frequency of AC voltage divided by the number of poles in the motor. The motor actually has the maximum torque below the synchronous speed. For example a motor 2 pole motor might have a synchronous speed of (2*60*60/2) 3600 RPM, but be rated for 3520 RPM. When a feedback controller is used the issue of slip becomes insignificant.
Torque Speed Curve for an Induction Motor
Brushless motors use a permanent magnet on the rotor, and user wire windings on the stator. Therefore there is no need to use brushes and a commutator to switch the polarity of the voltage on the coil. The lack of brushes means that these motors require less maintenance than the brushed DC motors.
To continuously rotate these motors the current in the outer coils must alternate continuously. If the power supplied to the coils is an AC sinusoidal waveform, the motor will behave like an AC motor. The applied voltage can also be trapezoidal, which will give a similar effect. The changing waveforms are controller using position feedback from the motor to select switching times. The speed of the motor is proportional to the frequency of the signal.
A typical torque speed curve for a brushless motor is shown in See Torque Speed Curve for a Brushless DC Motor.
Torque Speed Curve for a Brushless DC Motor
Stepper motors are designed for positioning. They move one step at a time with a typical step size of 1.8 degrees giving 200 steps per revolution. Other motors are designed for step sizes of 2, 2.5, 5, 15 and 30 degrees.
There are two basic types of stepper motors, unipolar and bipolar, as shown in See Unipolar and Bipolar Stepper Motor Windings. The unipolar uses center tapped windings and can use a single power supply. The bipolar motor is simpler but requires a positive and negative supply and more complex switching circuitry.
Unipolar and Bipolar Stepper Motor Windings
The motors are turned by applying different voltages at the motor terminals. The voltage change patterns for a unipolar motor are shown in See Stepper Motor Control Sequence for a Unipolar Motor. For example, when the motor is turned on we might apply the voltages as shown in line 1. To rotate the motor we would then output the voltages on line 2, then 3, then 4, then 1, etc. Reversing the sequence causes the motor to turn in the opposite direction. The dynamics of the motor and load limit the maximum speed of switching, this is normally a few thousand steps per second. When not turning the output voltages are held to keep the motor in position.
Stepper Motor Control Sequence for a Unipolar Motor
Stepper motors do not require feedback except when used in high reliability applications and when the dynamic conditions could lead to slip. A stepper motor slips when the holding torque is overcome, or it is accelerated too fast. When the motor slips it will move a number of degrees from the current position. The slip cannot be detected without position feedback.
Stepper motors are relatively weak compared to other motor types. The torque speed curve for the motors is shown in See Stepper Motor Torque Speed Curve. In addition they have different static and dynamic holding torques. These motors are also prone to resonant conditions because of the stepped motion control.
Stepper Motor Torque Speed Curve
The motors are used with controllers that perform many of the basic control functions. At the minimum a translator controller will take care of switching the coil voltages. A more sophisticated indexing controller will accept motion parameters, such as distance, and convert them to individual steps. Other types of controllers also provide finer step resolutions with a process known as microstepping. This effectively divides the logical steps described in See Stepper Motor Control Sequence for a Unipolar Motor and converts them to sinusoidal steps.
translators - the user indicates maximum velocity and acceleration and a distance to move
indexer - the user indicates direction and number of steps to take
microstepping - each step is subdivided into smaller steps to give more resolution
Hydraulic systems are used in applications requiring a large amount of force and slow speeds. When used for continuous actuation they are mainly used with position feedback. An example system is shown in See Hydraulic Servo System. The controller examines the position of the hydraulic system, and drivers a servo valve. This controls the flow of fluid to the actuator. The remainder of the provides the hydraulic power to drive the system.
The valve used in a hydraulic system is typically a solenoid controlled valve that is simply opened or closed. Newer, more expensive, valve designs use a scheme like pulse with modulation (PWM) which open/close the valve quickly to adjust the flow rate.
The continuous actuators discussed earlier in the chapter are the moroe common types. For the purposes of completeness additional actuators are listed and described briefly below.
Heaters - to control a heater with a continuous temperature a PWM scheme can be used to limit a DC voltage, or an SCR can be used to supply part of an AC waveform.
Pneumatics - air controlled systems can be used for positioning with suitable feedback. Velocities can also be controlled using fast acting valves.
Linear Motors - a linear motor works on the same principles as a normal rotary motor. The primary difference is that they have a limited travel and their cost is typically much higher than other linear actuators.
Ball Screws - rotation is converted to linear motion using balls screws. These are low friction screws that drive nuts filled with ball bearings. These are normally used with slides to bear mechanical loads.
· AC motors work at higher speeds
· DC motors work over a range of speeds
· Motion control introduces velocity and acceleration limits to servo control
· Hydraulics make positioning easy
1. A stepping motor is to be used to drive each of the three linear axes of a cartesian coordinate robot. The motor output shaft will be connected to a screw thread with a screw pitch of 0.125". It is desired that the control resolution of each of the axes be 0.025"
a) to achieve this control resolution how many step angles are required on the stepper motor?
b) What is the corresponding step angle?
c) Determine the pulse rate that will be required to drive a given joint at a velocity of 3.0"/sec.
2. For the stepper motor in the previous question, a pulse train is to be generated by the robot controller.
a) How many pulses are required to rotate the motor through three complete revolutions?
b) If it is desired to rotate the motor at a speed of 25 rev/min, what pulse rate must be generated by the robot controller?
3. A stepper motor is to be used to actuate one joint of a robot arm in a light duty pick and place application. The step angle of the motor is 10 degrees. For each pulse received from the pulse train source the motor rotates through a distance of one step angle.
a) What is the resolution of the stepper motor?
b) Relate this value to the definitions of control resolution, spatial resolution, and accuracy, as discussed in class.
c) For the stepper motor, a pulse train is to be generated by a motion controller. How many pulses are required to rotate the motor through three complete revolutions? If it is desired to rotate the motor at a speed of 25 rev/min, what pulse rate must be generated by the robot controller?
4. A stepping motor is to be used to actuate one joint of a robot arm in a light duty pick and place application. The step angle of the motor is 10 degrees. For each pulse received from the pulse train source the motor rotates through a distance of one step angle.
5. A stepping motor is to be used to drive each of the three linear axes of a cartesian coordinate robot. The motor output shaft will be connected to a screw thread with a screw pitch of 0.125". It is desired that the control resolution of each of the axes be 0.025"
a) To achieve this control resolution how many step angles are required on the stepper motor?
b) What is the corresponding step angle?
c) Determine the pulse rate that will be required to drive a given joint at a velocity of 3.0"/sec.
6. Explain the differences between stepper motors, variable frequency induction motors and DC motors using tables.
