Beginners Robotics Guide : Motors
by, 03-05-2010 at 08:52 PM (16736 Views)
This article covers the 3 main types of motors which are used in robotics namely the DC motor , Stepper motor and the Servo motor. The article also gives various circuits which can be used to drive the motors . This article is a must for all those starting out or are confused about motors .
Nearly all the robots we make have motors be it servo , stepper or some other unless you are using stuff like pneumatics , synthetic muscle etc . Any way coming back to the point there are mainly 3 types of motors
DC motors are widely used in robotics because of their small size and high energy output. They are excellent for powering the drive wheels of a mobile robot as well as powering other mechanical assemblies.
Ratings and Specifications
Several characteristics are important in selecting a DC motor. The first two are its input ratings that specify the electrical characteristics of the motor.
If batteries are the source of power for the motor, low operating voltages are desirable because fewer cells are needed to obtain the specified voltage. However, the electronics to drive motors are typically more efficient at higher voltages. Typical DC motors may operate on as few as 1.5 Volts or up to 100 Volts or more. Robotics often use motors that operate on 6, 12, or 24 volts because most robots are battery powered, and batteries are typically available with these values.
The ideal motor would produce a great deal of power while requiring a minimum of current. However, the current rating (in conjunction with the voltage rating) is usually a good indication of the power output capacity of a motor. The power input (current times voltage) is a good indicator of the mechanical power output. Also, a given motor draws more current as it delivers more output torque. Thus current ratings are often given when the motor is stalled. At this point it is drawing the maximum amount of current and applying maximum torque. A low voltage (e.g., 12 Volt or less) DC motor may draw from 100 mA to several amperes at stall, depending on its design.
Usually this is specified as the speed in rotations per minute (RPM) of the motor when it is unloaded, or running freely, at its specified operating voltage. Typical DC motors run at speeds from one to twenty thousand RPM. Motor speed can be measured easily by mounting a disk or LEGO pulley wheel with one hole on the motor, and using a slotted optical switch and oscilloscope to measure the time between the switch openings.
The torque of a motor is the rotary force produced on its output shaft. When a motor is stalled it is producing the maximum amount of torque that it can produce. Hence the torque rating is usually taken when the motor has stalled and is called the stall torque. The motor torque is measured in ounce-inches (in the English system) or Newton-meters (metric). The torque of small electric motors is often given in milli-Newton-meters (mN-m) or 1/1000 of a N-m. A rating of one ounce-inch means that the motor is exerting a tangential force of one ounce at a radius of one inch from the center of its shaft. Torque ratings may vary from less than one ounce-inch to several dozen ounce-inches for large motors.
The power of a motor is the product of its speed and torque. The power output is greatest at about half way between the unloaded speed (maximum speed, no torque) and the stalled state (maximum torque, no speed). The output power in watts is about (torque) x (rpm) / 9.57.
DC Motor Control
To control a DC motor we have to first convert the Digital 01 output into one which can drive the motor for this we use the HBridge . Given below is a simple HBridge circuit
When both the points A & B are "HIGH" Q1 and Q2 are in saturation. Hence the bases of Q3 to Q6 are grounded. Hence Q3,Q5 are OFF and Q4,Q6 are ON . The voltages at both the motor terminals is the same and hence the motor is OFF. Similarly when both A and B are "LOW" the motor is OFF. When A is HIGH and B is LOW, Q1 saturates ,Q2 is OFF. The bases of Q3 and Q4 are grounded and that of Q4 and Q5 are HIGH. Hence Q4 and Q5 conduct making the right terminal of the motor more positive than the left and the motor is ON. When A is LOW and B is HIGH ,the left terminal of the motor is more positive than the right and the motor rotates in the reverse direction. You could have used only the SL/SK100s ,but BC148 used have a very low hFE ~70 and they would enter the active region for 3V(2.9V was what I got from the computer for a HIGH) . You can ditch the BC148 if you have a SL/SK100 with a decent value of hFE ( like 150).The diodes protect the transistors from surge produced due to the sudden reversal of the motor. The approx. cost of the circuit without the motor is around Rs.40.
After the Hbridge is you wish to control the power of the motor this can be easily done with a PWM based control
Pulse Width Modulation
Pulse width modulation is a technique for reducing the amount of power delivered to a DC motor. Instead of reducing the voltage operating the motor (which would reduce its power), the motor's power supply is rapidly switched on and off. The percentage of time that the power is on determines the percentage of full operating power that is accomplished. This type of motor speed control is easier to implement with digital circuitry. It is typically used in mechanical systems that will not need to be operated at full power all of the time.
Figure illustrates this concept, showing pulse width modulation signals to operate a motor at 75%, 50%, and 25% of the full power potential.
The shaft of a stepper motor moves between discrete rotary positions typically separated by a few degrees. Because of this precise position controllability, stepper motors are excellent for applications that require high positioning accuracy. Stepper motors are used in X-Y scanners, plotters, and machine tools, floppy and hard disk drive head positioning, computer printer head positioning, and numerous other applications.
Stepper motors have several electromagnetic coils that must be powered sequentially to make the motor turn, or step, from one position, to the next. By reversing the order that the coils are powered, a stepper motor can be made to reverse direction. The rate at which the coils are respectively energized determines the velocity of the motor up to a physical limit. Typical stepper motors have two or four coils. For more information on stepper motors you can read the stepper motor tutorial on the website . Any way here is a very simple stepper controller
Servo motors incorporate several components into one device package:
* a small DC motor;
* a gear reduction drive for torque increase;
* an electronic shaft position sensing and control circuit.
The output shaft of a servo motor does not rotate freely, but rather is commanded to move to a particular angular position. The electronic sensing and control circuitry -- the servo feedback control loop -- drives the motor to move the shaft to the commanded position. If the position is outside the range of movement of the shaft, or if the resisting torque on the shaft is too great, the motor will continue trying to attain the commanded position.
Servo Motor Control
A servo motor has three wires: power, ground, and control. The power and ground wires are simply connected to a power supply. Most servo motors operate from five volts.
The servo controller receives position commands through a serial connection which can be provided by using one I/O pin of another microcontroller, or a PCs serial port! The communication protocol, that is used for this controller, is the same with the protocol of all the famous servo controllers of Scott Edwards Electronics Inc., this makes this new controller 100% compatible with all the programs that have been written for the "SSC" controllers...! However, if you want to write your own software, it is as easy as sending positioning data to the serial port as follows:
Byte1 = Sync (255)
Byte2 = Servo #(0-15)
Byte3 = Position (0-254)
So sending a 255,4,150 would move servo 4 to position 150, sending 255,12,35 would move servo 12 to position 35.
The standards of the serial communication should be the following: 9600 baud, 8 data bits, 1 stop bit and no parity.
The control signal consists of a series of pulses that indicate the desired position of the shaft. Each pulse represents one position command. The length of a pulse in time corresponds to the angular position. Typical pulse times range from 0.7 to 2.0 milliseconds for the full range of travel of a servo shaft. Most servo shafts have a 180 degree range of rotation. The control pulse must repeat every 20 milliseconds. This pulse signal will cause the shaft to locate itself at the midway position +/-90 degrees. The shaft rotation on a servo motor is limited to approximately 180 degrees (+/-90 degrees from center position). A 1-ms pulse will rotate the shaft all the way to the left, while a 2-ms pulse will turn the shaft all the way to the right. By varying the pulse width between 1 and 2 ms, the servo motor shaft can be rotated to any degree position within its range.
To build your own servo controller for servo motors go to 16 channel serial servo controller for robotic applications
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