Robots do numerous tasks. Centered to all these tasks is movement. Movement in robotic terms is also known as actuation. Actuation can be linear, where robot or its parts moves in straight line; or rotational, where it rotates about a fixed axis; or a mixture of rotation and translation where the robotic parts move around an axis which itself makes a translation or rotation about other axis.
By far the most popular actuators are electric motors that spin a wheel or gear, and linear actuators that control industrial robots in factories. But there are some recent advances in alternative types of actuators, powered by electricity, chemicals, or compressed air. Motors are of various types which can be categorized into three basic ones.
- continuous DC motor
- Stepper motor
- Servo motor
I will be discussing each one of them in detail. The discussion will consist of their construction, working principle and the most important of all, the big question :
"Where to use what?"
Now that I have introduced the basic types let us dwell deep into each one of them.
Continuous DC Motor
Construction and Working Principle
Ironically the miniature motors has all the parts and construction of its big brother, employed in Electrical engineering as DC Machines.
It mainly consists of an armature core with armature winding laid down in the slots made in them. These wires end up in commutator slots. The commutator is brushed through copper brushes which carry the current. The external body is static and does not move mostly(until you use an Out-runner motor, which is a special version of DC motor, having no brushes, but an electronic commutator circuit for ensuring half cycle current reversal such that the motor can produce torque). It carries field coils in the protruding structures called pole shoe. They are, like armature coil supplied by external voltage.
The current flowing through field coil produces magnetic field, which interacts with the current in the armature coil to produce torque. The alternating nature of the current in the armature coil due to commutation by split rings ensures that torque is always in the positive direction in both half cycles.
T = K(I x B)
T = Torque produced by the motors that is coupled with the shaft and eventually makes it move
I = The current supplied to the armature coil = V/R
V = Voltage supplied to the motor.
R = Armature resistance of the motor (Mentioned in the motor ratings)
B = The magnetic field produced bu current in the field coil
K = constant of proportionality
Enough of the bookish stuff. Now let us quickly jump down to real knowledge which every robotics hobbyist and specialists alike must have. I would like to pose these in a question answer format.
Q. How do you control the direction of the motor? (Level - Basic)
A. It is easy. Just reverse the connections of the two wires.
Q. What to do if one wants to run the motor at half its rated speed? (Level - Basic)
A. Reduce the voltage, such that it equals to half the rated voltage.
Q. What to do if one wants to run the motor at thrice its rated speed? (Level - Basic)
A. One might be tempted to increase the voltage to thrice the rated voltage. But that is certainly wrong. If done likewise, the motor will be overburdened, get heated up and consequently melt down. It is a good and safe practice to operate the motor no more than its rated voltage or at maximum for 1.5 times its rated voltage(that too for a very short time)
Q.How is voltage related to torque in a DC motor? (Level - Basic)
A. They are proportional to each other. Increasing the voltage increases the current (= V/R) and thus increases the torque (K. I x B).
Q. I see two current ratings in my motor - Operating current and Stall current. What are they? (Level - Medium)
A.
Operating Current -
This is the average amount of current the motor is expected to draw under a typical torque. Multiply this number by the rated voltage and you will get the average power draw required to run the motor.
Stall Current - This is when you power up the motor, but you put enough torque on it to force it to stop rotating. This is the maximum amount of current the motor will ever draw, and hence the maximum amount of power too. So you must design all control circuitry capable of handling this stall current.
Q. What is motor heat sink? Is there a heat sink for motor as well? (Level - Medium)
A. It is something every electronic circuitry requires. It is not only used for regulators, as is a common misconception, but any device which gets heated up. If one wants to operate the motor above its rated voltage, one must certainly apply heat sink to the motor.
Q. What is Power Spikes? (Level - Hard)
A. There is a special case for DC motors that change directions. To reverse the direction of the motor, you must also reverse the voltage. However the motor has a built in inductance and momentum which resists this voltage change. So for the short period of time it takes for the motor to reverse direction, there is a large power spike. The voltage will spike double the operating voltage. The current will go to around stall current. Thus one must design the power regulation circuitry of the motor to handle such extreme cases of heating due to power, current and voltage overload.
Q. What is Operating Torque and Stall Torque? (Level - Medium)
A. Operating Torque is the torque that the motor will produce at the rated conditions of the voltage supplied. The motor generally gives high efficiency for operation around operating condition.
Stall Torque is the torque required externally (like a hand - if you want to hurt yourself really bad) to stop the motor from rotating.
Added fact - When struggling with the choice of which motor to choose, always go with one having high torque rating and not high velocity rating.
Q. What are other ways to reduce the velocity of the motor? (Level - Hard)
A. I think the question should be , how to reduce the velocity, while ensuring at the same time that the efficiency of the motor is the maximum. There is a general rule for DC motors.
"Motors run the most efficient when run at the highest possible speeds. "
One can't run the motor at full speed all the time. One trick to ensure higher efficiency as well as reduced speed is to use gears.
w = the velocity of each gear
D = Diameter of each gear
N = Number of teeth of each gear
But gearing automatically reduces efficiency. Each gear reduces the efficiency by 10%. So, say you have 3 gears in a gear train(you can't attain all the speeds with just one gear, as it will lead to huge diameter of gears and thus hurt miniature motors. Instead we use two to three gears in cascade to achieve the equivalent effect.). Then the total efficiency of the motor, now, is
Motor efficiency after three gears = (0.9)x(0.9)x(0.9)x Motor efficiency of original motor
Q. How to control the DC motors?
A. You need to know three aspects of controlling a DC motor:
1. Encoders
2. H- Bridge
3. DC Motor braking mechanism
These three are topics by themselves. I will be providing detailed explanations to them in next set of blogs. So stay tuned.
Feel free to Google them for the time being.
Stepper Motors
Construction and Working
A stepper motor is a type of DC motor which has a full rotation divided in an equal number of steps. It is a type of actuator highly compatible with numerical control means, as it is essentially an electro-mechanical converter of digital impulses into proportional movement of its shaft, providing precise speed, position and direction control in an open-loop fashion, without requiring encoders, end-of-line switches or other types of sensors as conventional electric motors require.
Working
DC brushed motors rotate continuously when voltage is applied to their terminals. The stepper motor is known by its important property to convert a train of input pulses (typically square wave pulses) into a precisely defined increment in the shaft position. Each pulse moves the shaft through a fixed angle. Stepper motors effectively have multiple "toothed" electromagnets arranged around a central gear-shaped piece of iron. The electromagnets are energized by an external control circuit, such as a micro-controller. To make the motor shaft turn, first, one electromagnet is given power, which magnetically attracts the gear's teeth. When the gear's teeth are aligned to the first electromagnet, they are slightly offset from the next electromagnet. This means that when the next electromagnet is turned on and the first is turned off, the gear rotates slightly to align with the next one. From there the process is repeated. Each of those rotations is called a "step", with an integer number of steps making a full rotation. In that way, the motor can be turned by a precise angle.
Now, since we have the basics of Stepper motor in our bag, lets quickly move through the question answer session. The session consists of the most common questions one asks for when trying to work with stepper motors.
Q. What are the three wires for in the stepper motor? What does each of the color of the wire mean? (Level - Medium)
A. This is one of the most common problems a hobbyist encounters. Let me explain you, that the colors of the wires have a meaning and follow a color code.
The number of wires in a stepper motors can be 4,6 or 8. The respective circuit diagram of a stepper motor is:
A. When functioning correctly the number of steps performed must be equal to the control impulses applied to the phases of the motor.
Q. Does stepper motor employ closed loop or open loop control system? (Level - Basic)
A. To understand this question, one must know, first, that what is difference between open loop and closed loop control system.
Open-loop control system employs no feedback mechanism, that is at no stage, final or intermediate output is fed back as input to the system. Stepper motor is perfect example of open loop control system as its output only depends on the train of pulses which are provided as input, at no stage the output position, velocity or acceleration are fed into the system.
Closed-loop control systems are those systems that employ such feedback mechanism. For instance a DC motor using encoders for position or velocity feedback to control the position or velocity is an example of closed loop control system.
Q. Does a stepper motor fall back i.e lose steps i.e does not move for one or more pulses? (Level - Basic)
A. A stepper motor does not lose steps, i.e. no slippage occurs, it remains synchronous to control impulses even from standstill or when braked, thanks to this characteristic a stepper motor can be started, stopped or reversed in a sudden fashion without losing steps throughout its operation.
Q. Where should one use stepper motors ans what are its limitations? (Level - Basic)
A. The power range of a stepper motor varies from few micro-watts (uW) to no more than few kilowatts(kW). Thus they are used for low to medium power applications.
They are used mostly where high speed precision movements are required, than at places of heavy duty applications where torque is the point of concern.
Areas of applications - Plotters, Disc drivers, Robotic arm, Printers, CNC machines.
Q. You told us a lot of advantages. So is it all dream come true motor? (Level - Medium)
A. No! I think i got 80% optimistic and just 20% realistic
The stepper motor has its own set of disadvantages :-
1. Low torque capacity
2. Low power efficiency
3. Fixed step value for a given motor.
Q. What is unipolar and bipolar motors? What type is Stepper motors?
A. Unipolar motors : When the direction of the current supplied to the motor is not changed to change the direction of the motor. Then it is a Unipolar motor.
Bipolar motors: When the direction of the current must be changed to ensure a reversal in the direction of current, it is called bipolar motor. Eg. - Continuous DC motors
Stepper motors can be either of the two types depending on the types of stepper motor. Please follow the next question to find out the exact answer.
Q. What are the types of Stepper motors? (Level - Hard)
A. There are three types of stepper motors:
- Variable Reluctance type - Reactive type
- Permanent magnet type - Active type
- Hybrid - Mixture of (1) and (2)
Variable Reluctance type
The rotor has teeth as shown in the figure. It is passive in nature as the control winding (the coils), are not on the rotor, but on the stator. By energizing one or more phases, the rotor will turn in such a manner that it settles on a minimum reluctance path.
There are two such paths:-
- When the rotor teeth align with the stator teeth (also called stator pole).
- When the rotor teeth align with the bisector of the stator pole.
Pros:
- Helps in achieving small to medium step angles
- Feasible to operate at high control frequencies.
Cons:
- It can't hold its position i.e when no current flows through the stator winding, there is no stopping torque.
It is an example of
Unipolar motor.The change in the direction of the motor is achieved through change in the impulse sequence.
Permanent Magnet Type
Stay tuned for the explanation.
Hybrid Type
Stay tuned for the explanation.
Q. Can you please help me with Stepper motor jargon? (Level - Medium)
A. Sure! why not. Here are few most important ones, one should be aware about.
- Step Angle - It is the angle turned by the motor for one input impulse.
- Pull-in torque - Represents the maximum torque load at the shaft, for which the motor can start without losing steps.
- Pull-out torque - Maximum torque that the motor can maintain at a particular speed, without losing steps
- Detent torque - It is the holding torque that the motor torque offers when it is not energized.
- Angular speed - Angular speed = (Stepping Angle) x (Control Frequency)
- Maximum Frequency - It is the maximum frequency at which a motor can remain synchronized with the input train of pulses.
Q. How to control a stepper motor? (Level - Hard)
A. This is a topic in itself. I will be posting the link to this question in a separate blog. So stay tuned for the link to that blog under this question at a later stage. But for tha sake of introduction,
There are actually 4 types of control system for the stepper motor:
- Half step drive
- Full step drive
- Wave drive
- Micro-stepping drive
Q. What is Ringing and Resonance in Stepper motor? (Level - Medium)
A. When the motor moves a single step it overshoots the final resting point and oscillates round this point as it comes to rest. This undesirable ringing is experienced as motor vibration and is more pronounced in unloaded motors. An unloaded or under loaded motor may, and often will, stall if the vibration experienced is enough to cause loss of synchronisation.
Stepper motors have a natural frequency of operation. When the excitation frequency matches this resonance the ringing is more pronounced, steps may be missed, and stalling is more likely. Motor resonance frequency can be calculated from the formula:
Mh = Holding torque cN·m
p = Number of pole pairs
Jr = Rotor inertia kg·cm²
Servo Motor
Construction and Working Principle
This is nothing but a simple electric motor, controlled with the help of servomechanism. If the motor as controlled device, associated with servomechanism is DC servo, then it is commonly known DC Servo Motor. If the controlled motor is operated by AC, it is called AC Servo Motor.
The working principle is same as the DC motor (as visible in the break apart motor shown above), but with an added velocity or position feedback achieved using a potentiometer.
Control Mechanism
The entire control mechanism can be very easily understood using the figure cum circuit illustration shown below.
ServoMechanism
The position of the output, measured by the potentiometer, is continually compared to the commanded position from the control (i.e., the radio control). Any difference gives rise to an error signal in the appropriate direction, which drives the electric motor either forwards or backwards, and moving the output shaft to the commanded position. When the servo reaches this position, the error signal reduces and then becomes zero, at which point the servo stops moving.
If the servo position changes from that commanded, whether this is because the command changes, or because the servo is mechanically pushed from its set position, the error signal will re-appear and cause the motor to restore the servo output shaft to the position needed.
Now let us come down to few typical questions that comes in mind of a robotics aspirant.
Q. How many wires are used and is there a color code for the wires? (Level - Basic)
A. Yes, there are three wires in a servo motor.
- Black or Brown - Ground
- Red - Live
- Yellow or White - Control Signal wire
Q. What are the types of Servomotors? (Level - Basic)
A
. There are basically three types of servomotors.
- Positional rotation servo: This is the most common type of servo motor. The output shaft rotates in about half of a circle, or 180 degrees. It has physical stops placed in the gear mechanism to prevent turning beyond these limits to protect the rotational sensor. These common servos are found in radio-controlled cars and water- and aircraft, toys, robots, and many other applications.
- Continuous rotation servo: This is quite similar to the common positional rotation servo motor, except it can turn in either direction indefinitely. The control signal, rather than setting the static position of the servo, is interpreted as the direction and speed of rotation. The range of possible commands causes the servo to rotate clockwise or counterclockwise as desired, at varying speed, depending on the command signal. You might use a servo of this type on a radar dish if you mounted one on a robot. Or you could use one as a drive motor on a mobile robot. In such servos the input pulse results in a rotational speed, and the typical 1.5 ms center value is the stop position. A smaller value should turn the servo clockwise and a higher one anticlockwise.
- Linear servo: This is also like the positional rotation servo motor described above, but with additional gears (usually a rack and pinion mechanism) to change the output from circular to back-and-forth. These servos are not easy to find, but you can sometimes find them at hobby stores where they are used as actuators in larger model airplanes.
Q. What is PWM and how is it used to run the servomotor? (Level - Hard)
A. PWM stands for Pulse Width Modulation. It is a control signal that is fed into the control wire of the servo motor.
In general a PWM signal and its corresponding servo position looks like this:
Each servo's requirements vary slightly, but a pulse train (as in Figure above) of about 50 to 60 Hz works well for most models. The pulse width will vary from approximately 1 millisecond to 2 or 3 milliseconds (one millisecond is 1/1000 of a second).
NOTE :
- I will be updating this blog in near future to make it more complete. So kindly stay tuned with this page.
- This is a very introductory blog for beginners to come at scale with the experts. The level of difficulty will keep on increasing
- I will be posting blogs related to
- Under-actuated Robotics
- Computer Vision
- Artificial Intelligence
- Machine Learning
- Mobile Robotics
I will try to post one blog a day, so you can expect a lot of in depth information related to robotics at a place.
Do like and comment, Your suggestions for topics to cover are most welcomed.
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