8.0 CONSIDERATIONS FOR THE CONTROL OF DC MICROMOTORS

8.1 General Comments

Due to its inherent qualities the Escap^® micromotor is very suitable for a wide range of application types. The principal modes of operation are:

• Open loop - direct drive
• Closed loop - velocity servo
• Closed loop - position servo

8.2 Summary of Feedback Techniques and Components

8.2.1 Position feedback is required when critical positioning of the load is required such as in:

• Welding robot head location
• Daisy wheel printer positioning
• Aircraft indicator drive

The most frequently used position feedback transducers (sensors) in use today are:

• Potentiometers
• Synchros and resolvers
• Linear synchro
• Optical encoders
• Hall effect transistors
• Linear variation differential transformer
• Magnetic pickup

8.2.2 Velocity feedback is utilized when precise speed control of the motor is required or where a prescribed velocity profile is required to obtain controlled acceleration and deceleration: Examples of motor applications where velocity feedback might be required are:

• Daisy wheel printer - ramping up and down of print wheel velocity to obtain minimum character positioning time.
• Tape drive applications requiring constant tape speed or maximum acceleration and deceleration of the tape.
• Carriage drive to obtain maximum accelerate/decelerate characteristics.
• Plotter pen drive where the pen is required to trace a prescribed path in a given time.

The most frequently used velocity feedback devices are:

• Tachometers
• Optical encoders (velocity decoding)
• Motor back-emf sensing
• Phase locked quartz-based frequency comparator system

8.2.3 In summary, position control systems can be categorized into three primary functional areas:

• Fixed displacement (point-to-point position)
• Translation/velocity profile
• A combination of both velocity and position

8.2.4 The most elementary components of a servo system are:

• Motor
• Position transducer
• Velocity transducer
• Amplifier
• Controller (logic & comparator)
• Power source

These are arranged in block diagram form in figure 17.

The feedback transducers are either analog or digital sensors. Typically, tachometers and synchros are examples of analog feedback devices whereas optical encoders and pulse counters are examples of digital devices.

Controllers also fall into either the analog or digital category or a combination of both. The operational amplifier is the basic analog element while the microprocessor is a digital control device.

8.3 Ironless Rotor Micromotor Model With Laplace Transform Notation

Figure 18 ironless Rotor Micromotor Model With Laplace Notation

Note that with negligible inductance the classic Laplace term of reduces to . 

This property is reflected in a negligible electrical time constant for ironless rotor motors which, in practice, never exceeds 1% of the motor unloaded mechanical time constant.

8.4 Open Loop Control Considerations  

The typical open loop operating mode for an Escap^® micromotor is a battery powered drive motor application. The high efficiency and excellent regulation makes the micromotor well suited for these applications. We know from the torque speed characteristics that a motors operating velocity is given by:


Thus, the regulation term defines how much the motor rpm will fall off with increasing torque. Proper choice of R and K is critical especially in an open loop mode.

Generally speaking, over a certain operating range, constant torque can be obtained by using a constant current power supply. This simply makes use of the fact that motor torque (M) = Kl. Within its current rating, a battery is well suited for use with a micromotor as a constant current source.

Conversely, over a low torque operating range, a constant, motor output RPM can be maintained by using a constant voltage power supply since  

This mode of operation is only successful in low torque ( l ≅ l_NL ) applications where the lR term is kept at a minimum. Obviously, as the torque demand increases the current must also increase and thus, the lR term increases and so does motor power dissipation l²R. 

It should be remembered that

Therefore for "good" regulation the power dissipation is greatly reduced for any given load torque. We again see that the regulation is critical to the proper selection of a motor.

8.5 Closed Loop Control Considerations 

8.5.1 ln an incremental motion control servo system the sensed position and/or velocity parameters, whether they be in digital or analog form, are ultimately converted into control feedback signals and then into voltage and current form as drive power to the motor. The motor thus executes this power command and the resulting motion is once again detected by the sensors. Thus, the feedback loop is "closed". This is illustrated in figure 17 in block diagram form.

8.5.2 Velocity profile 

A key consideration in designing a servo system for Incremental motion is the velocity profile of the motor/load. The design goal is to optimize this profile, while minimizing some other parameter such as peak current or power dissipation. A common design problem may require an optimum velocity profile which will provide for minimum power dissipation (P_d) and minimum time (t_θ) to perform a certain displacement (θ). 

Equations for this incremental motion are:

Torque:  
Displacement:  
Power Dissipation:

The simultaneous satisfaction of these equations while optimizing the desired conditions will result in a parabolic velocity profile.

Such a profile has an efficiency of 1.0. Other velocity profiles such as the trapezoid and triangle have efficiencies of 0.89 and 0.75 respectively.

In practice the trapezoidal velocity profile, being reasonably close to ideal, is generally used. Also, the ramp-up and ramp-down constant acceleration is a simple matter to accomplish from a controls point of view. The trapezoidal velocity profile and its associated drive current profile are shown in figure 19. The drive circuit for the trapezoidal profile must provide a constant current pulse. Both positive and negative.


Figure 19 Trapezoidal Velocity Profile and Drive Current Profile

8.6 Servo Amplifiers and Control Techniques8.6.1 General commentsTo thoroughly cover the topic of servo control systems and their design would be an exhaustive exercise and certainly beyond the scope of this text. Whole chapters and even books have been written on individual areas of the total topic such as: damping, torsional resonance, amplifier design, etc. The intent of this text is to cover, in summary fashion, some of the salient issues in order to provide the reader with an overview of servo amplifiers and control techniques. The reader is encouraged to study further, his areas of interest. The references at the conclusion of this section represent an excellent library for further study.

Amplifiers fall into two major categories: class A (linear) amplifiers, and switching amplifiers. Transistors may be used for either type but SCR's are often used in high power switching amplifiers.

Simple speed control amplifiers are usually operated in a single quadrant and therefore are not bidirectional (no negative torque). The true servo amplifier however, is capable of bidirectional drive and is able to operate in all four quadrants.

8.6.2 Linear amplifiers

The linear operating characteristics of a class A amplifier with no significant control lag within its designed bandwidth, make it the obvious choice for motor control applications. A single transistor speed control system is shown in figure 20.  

A single transistor amplifier has a relatively low gain and therefore requires a large error signal before it will react. Higher gain can easily be obtained by adding additional stages of amplification. Figure 21 gives an example of a speed control circuit having two stages of current amplification and an integrated circuit (lC) incorporating a dual voltage amplifier.

Transistor servo amplifiers are characterized by two primary output (drive) stages and consist of two basic circuit configurations. They are:

(a) The bridge or "H" consisting of four transistors and requiring a single dc power source; and,

(b) The "T" consisting of two complementary transistors and requiring two dc power sources.

   See figure 22 for "H" and "T" circuits.
   The advantages and disadvantages of the bridge and "T" servo output stages are listed in the following tables.

Bridge "H" Stage

Advantages                         · single power supply · some voltage protection

Disadvantages                             · difficult to drive in linear class · difficult to obtain feedback

"H" & "T" Servo Amplifiers

Both the "T" and "H" circuits may be operated as linear amplifiers or in the switching or "bang bang" mode. However, the "T" is most often used as a linear amplifier while the "H" circuit seems to be more common in switching amplifiers.

8.6.3 Switching amplifiers 

Switching amplifiers are the most versatile and perhaps most widely used servo drivers. There are three basic schemes used to control power by switching amplifiers.

These methods are:

• Pulse width modulation (PWM)
• Pulse frequency modulation (PFM)
• Silicon controlled rectifier (SCR)

The output waveforms of these three schemes are shown in figure 23.

All three of these control methods vary the power delivered to the motor by modulating the average power output over a given time period. This can be observed in figure 23 where the average power is represented by the area under the curve during a given period. The wider the pulse or the more frequent the pulse rate the greater the power.

The generation of the pulses is performed by the controller logic and pulse generator circuits which in turn drive the output amplifiers. Command signals and feedback signals of course control the pulse generation.

Technically speaking, the SCR amplifier is a PWM amplifier but with a part sine wave output instead of a square wave. The SCR is also a lower frequency device and must be accompanied by a separate turn off circuit. SCR's are limited to high current and low switching rates. These features generally make SCR controls less suitable for Escap^® micromotors.

8.6.4 Selection of switching frequency

The major considerations in the selection of the switching frequency (f_s) are:

• f_s must be above the servos dynamic response capability.
  
• Rule of thumb: f_s > 10f_BW where few is system bandwidth.
• f_s must be above the major system resonance points.
  f_s>f_Res.
• The switching period must be greater than the transistor switching delay time (T_d)

        

8.6.6 The ramp generator

One important element in a switching servo drive system is the ramp generator. This is the circuit responsible for generating various sloping waveforms required for a particular velocity profile. Figure 24 shows a typical ramp generating circuit.

8.6.6 Phase locked loop servos

Phase locked loop servo (PLS) has become a very popular means of obtaining precise velocity control. This method exhibits excellent speed regulation. The principal of operation is simple.

Basically the frequency of a feedback pulse is compared with a command frequency. The system adjusts itself until the feedback is identical to the command at which time the system is said to be "phase locked". In this way the system output velocity is stabilized to the speed corresponding to the command.

8.6.7 Back emf servo control

A back-emf servo utilizes the generated back-emf of the motor as a means of gaining velocity feedback information: The linearity and symmetry of the Escap^® mircromotor make it an excellent candidate for use in a back-emf servo. This scheme is low cost and is well suited where low to medium performance is required. Please refer to the reference material for a detailed treatment of the subject.

8.7 Additional Comments

The subject of dc servo systems is vast. Thoroughly covering all aspects of the subject is beyond the scope of this text. The reader is encouraged to conduct his own research to gain wider knowledge of the topic. Please consult the references at the end of this section for further reading on the following aspects of dc micromotor control:

• Damping of ironless dc motors
• System instability
• Damping of ironless dc motors
• System instability
• System bandwidth
• Torsional resonance
• Circuit design
• Back-emf servos

page 5 - Tutorial Selection of Micromotor Application Problems