Title: Stepper motors
1- Stepper motors
- Presented by
- Vijay Mahadevan
- Pratik Majmundar
- Sameer Vaidya
- Under the guidance of Prof. Rajab Chaloo
2Basics of stepper motors
- Incremental motion devices with precise open loop
positioning capability. - Applications probers, plotters, robots, computer
peripherals and manufacturing systems. - Desired motion is obtained through discrete
increments generated by series of current pulses.
3- Advantages low cost, high reliability and
robustness. - When operated in open loop mode, they are prone
to missing steps, resonance effects and low
settling times. - Therefore closed loop control of stepper motor is
desired for high performance.
4Classification of stepper motors
- Based on construction
- Based on type of motion
5Classification based on construction
- Variable reluctance
- Permanent magnet
- Hybrid type
- Variable reluctance are characterized by absence
of permanent magnet. - Permanent magnet incorporates a permanent magnet
rotor. - Hybrid type makes use of both permanent magnet
and variable reluctance principles for
development of torque.
6Based on type of motion
- Rotary stepper motors
- Linear stepper motors
7Types of linear stepper motors
- Single axis type motion in two axial direction.
- Dual axes type along with motion in two axial
direction, a small rotational (yaw) degree of
freedom is present.
8Why do we use stepper motor?
- Positioning something accurately by using DC
motor is pretty difficult. - DC motors have a very gradual acceleration and
deceleration curves, stabilization is slow. - Adding gearing can help but overshoot is still
present and will throw off the anticipated stop
position.
9- The only effective solution to use DC motor for
precise positioning is using servo. - But this can get fairly complex and expensive for
most hobby applications. - So what is the solution?
10- Stepper motor is the solution to this problem.
- Stepper motors differ from DC motors in their
torque speed relationship. - Stepper motors can produce highest torque at low
speed which is not the case in DC motors.
11- Another characteristic of stepper motor is
holding torque which is not present in DC motors.
- This eliminates the need for gearing mechanism.
- Stepper motors have several windings which needs
to be energized in correct sequence.
12- If the control signals are not sent in correct
order, motor may not turn in a rough or jerky
manner. - Circuit which is used to convert the step signal
into winding energizing patterns is called a
translator. - A driver is also used along with translator to
handle current drawn by the motor.
13- A translator circuit along with driver
14Common characteristics of stepper motors
- Voltage It is printed on unit or mentioned in
the datasheet. It may be exceeded sometimes to
obtain desired torque. - Resistance resistance per winding will determine
the current draw of the motor and will affect
torque curve and maximum operating speed.
15- Degree per step It specifies the degrees the
shaft will rotate for each full step. It can be
calculated by dividing 360 by the number of steps
in one complete rotation. - Common degrees per step numbers include 0.72,
1.8, 3.6, 7.5, 15, and even 90. - Degree per step is referred to as the
resolution of the motor.
16Open-Loop Control of Stepping Motors
- In general, open loop control in a stepper motor
is adequate. Why? - An exception is under transient conditions near
rated torque.
17Block Diagram
- Hardware approach
- Software approach
18Stepper Motor Response
-It is useful to observe the response to a single
pulse input before studying the behavior under
general stepping conditions. - Why do we have
dynamic torque? -The static torque is positive at
the switching point, but is generally not maximum
at that point.
19Consider the following three phase VR stepper
motor
20Stepper Motor response to a sequence of pulses.
- Single stepping cascaded sequence of single
pulse responses. - Slewing motion to achieve slewing motion, motor
has to be accelerated from a low speed ramping.
This is accomplished by applying a sequence of
pulses with a continuously increasing pulse rate
R(t).
21Typical slewing response.
22- In practice, step rate is often increased
beyond the slew rate, in a time interval shorter
than what is specified for acceleration and then
decelerated to the slew rate by pulse subtraction
at the end.
23-In transient operation, non uniform stepping
sequences might be necessary depending on the
complexity of the motion trajectory and the
required accuracy.
-The Electrical Time Constant is given by L/R.
Electrical Time constant L/(R R).
24Static Position Error
25Damping in Stepper Motors -
- Mechanical damping.
- Electrical Damping.
- Electronic Damping.
-Advantages and disadvantages.
26Feedback Control of Stepper Motor
27Why Feedback?
- Open loop operation is adequate for low speed and
steady state operation. - Actual response can not be measured.
- In open loop control significant errors can not
be measured.
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29Models of Stepper Motor
- Simple Model
- Under steady state operation most of its
parameters remains constant. - Modified Model
- Under transient and high speed operation assumed
constant in the torque source model will vary
with time as well as rotor position.
30Selection of stepper motor
- Main requirement of particular application.
- eg. Speed, acceleration, required accuracy and
resolution and load characteristics, such as
size, resistance torques. - Compute operating torque and stepping rate for
particular application. - Using torque versus stepping rate curves select
suitable motor.
31Applications
- Printing
- Disk drives
- Robotics
- Automotives
- Medical and Metallurgical lens drives
-
32Advantages
- Position error is noncumulative.
- High accuracy is possible.
- Easy to adopt to digital controlled applications.
- No serious stability problem occurs.
- Torque and power requirements can be optimized
and response can be controlled by electronic
switching.
33Disadvantages
- Low torque
- Limited speed
- High vibration
34 Conclusion
35References-Control sensors and actuators
by Clarence W. deSilva.-http//www.eio.c
om/jasstep.htm
36 Thank you !!!
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