Transcript Servos
DC Motor Control The material presented is taken from a variety of sources including: http://www.compworks.faithweb.com/electronics/com ponents/inductor001.html#howworks, and Building Robot Drive Trains by Clark and Owings Voltage A motor requires a power source within its operating voltage, i.e., the recommended voltage range for best efficiency of the motor. Lower voltages will usually turn the motor (but provide less power). Higher voltages, in some cases, can increase the power output but almost always at the expense of the operating life of the motor. Current When constant voltage is applied, a DC motor draws current in the amount proportional to the work it is doing. For example, if a robot is pushing against an obstacle, it is drawing more current than when it is moving freely in open space. The reason is the resistance to the motor motion introduced by the obstacle. If the resistance is very high the motor draws a maximum amount of power, and stalls. This is defined as the stall current of the motor: the most current it can draw at its specified voltage. Control A microprocessor cannot drive the motor directly (Not enough current supply) The motor power must come from another source; only control signals come from the microprocessor Control Topics: Basic H-Bridges Isolation Pulse-width modulation H-bridge The basic circuit for driving DC motors in both directions is an Hbridge. This circuit enables the motor to spin in either direction from a single power supply. Noise A DC motor can create a tremendous amount of power supply noise. Why? Current demand: When a motor starts or changes direction, it draws a great deal of current almost behaving as a short circuit. Commutator brush noise: As the brushes make and break contact with the communtator, power to the coils is switched on and off. As a result of inductance, the coils generate a brief high voltage spike as the current is switched off. Inductance An inductor resists change in current flow. You learned that when current flows through a conductor, a magnetic field surrounds the conductive wire. The more current traveling through the wire the greater the amount of flux. What you didn't learn is that these lines of flux can generate voltage on surrounding conductors. Induced voltage results from change in current flow. At steady state, the induced EMF collapses. The voltage that appears in the inductor (i.e., the motor) is of opposite polarity to the original voltage and is called Counter Electro Motive Force (CEMF). The faster the current changes, the larger the CEMF voltage. Spike of 20 times the original voltage can appear. A Better H-bridge 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. PWM 75 50 25 Which PWM frequency is best A wide range of frequencies could be used for the pulse width modulation signal. Frequencies above 1K Hz are recommended. Lower frequencies may resonate and cause your motor to vibrate. Lynxmotion DC Motor Controller Control Truth Table BS2 Code ' -----[ I/O Definitions ]----------------------------------Aenable PIN 0 Aminus PIN 2 Aplus PIN 3 Benable PIN 1 Bminus PIN 4 Bplus PIN 5 ' -----[ Variables ]----------------------------------------loopCnt VAR Byte '------[ turn both motors CCW: B fast and A slow ]----------' set motor B to turn fast CCW HIGH Benable ' set B enable HIGH Bminus ' set BLOW Bplus ' set B+ ' set motor A to turn slow CCW HIGH Aminus ' set ALOW Aplus ' set A+ ' loop used to pluse Aenable FOR loopcnt = 0 TO 1000 PULSOUT Aenable,1000 PAUSE 80 ' the duration of the pause determines the motor speed NEXT