Embedded Systems October 2000 Vol13_11

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Don Morgan Hijacked! Since I started thi s seri es, quite a lot of mail has come in with com- me nts and questions concerning motors, PID, motion control, fi eld-ori- e nted control, and pulse width modu- latio n (PWM). I've recieved enough questions, in fact, that I decided to review some of these areas before moving on. I'm going to an swer some of the questions by presenting the data that we've cove red from a diffe r- e nt perspective. I'll also provide addi- tional illustra tions. Specifically, we' ll take another look at the physical aspects of controlling brush and brush less permanent mag- net moto rs. Our coverage will also include PWM and a little bit on phase curre nts . Next month , I'll go into greater detail on the PID algorithms and coefficie nts. Following that, we will again continue witll algo.-ithms on resolve r and sinusoidal encode r con- ve rsions, which was my initial plan. PWM Pulse width modula tion is a digita l technique fo r the control of DC sup- pli e to provide vat·ying voltages into a load . I use the word digital because with PWM th e full DC supply is e i th e r turned o n to th e load o r turn d off. That is, power is supplied to the load by means o f a series of on and off pulses. The o n-time is th e pe riod during which the DC supply is placed o n the load and th e off- tim e is the pe t-iod durin g which that supply is removed , or cut off, from the load . In this manner, we can control the average voltage delive red to a load. Fo r example, If we have a switch between a light bulb and a 9V battery and we turn the switch on and off at half-second intervals, the bulb experi- e nces an average voltage of 4.5V. This process is illustrated in Figure l. To describe tllis situation, we say that the duty cycle is 50% and the frequency is 1Hz. Most loads, inductive or capaci ta- tive, require a higher frequency to achieve tighter control and resolution. ent windings as tlle rotor turns. This resul ts in two flux patterns. One gen- erated by the permanent magne ts in the stator (fi eld ) and the otllet· gene r- ated by the armature windings. Elecu-omagnetic torque is created by the inte raction of the fi eld flux and the armature flux. To motor, o r pro- duce to rque, the electrical a ngle between the field and armature is typ- ically 90 degrees (about 45 mechani- Reader questions prompt a more basic tl~eatmel1t of PWM and motor internals. Common frequencie for motion con- u·ol amplifiers range from 4kHz to 20kHz. Whatever the frequency, a 9V supply with a 50% duty cycle will always produce an average voltage of 4.5V. Brush pennanent magnet motors Like brush less motors, brush motors have a rotor (th e part that rotates) and a stator (the stationary pa rt). Unlike brush less motors howeve r, the mag- nets (or field) are mounted on the sta- to r, while the windings (armature) are located o n tlle roto r. The armature of a brush motor has many windings, or phases, with only one portion excited at anyone time, depending on the positio n of th e rotor relative to the stator. Brushes, acting as switches, commutate the motor by connecting current to differ- cal degree) . So o n a brush perman en t magne t motor, the brushes a re arranged so that current-flow in the armature windings produces flux that leads the stator flux by 90 degrees. So many phases are possible in such an arrangeme nt that each o ne represents only a few electrical degrees of rota- tion , making fo r mo tly ripple-free torque. The relationship between current and torque on a permanent magnet machine is linear. It's possible to cre- ate a current source for such a motor, but most often the control o utputs a voltage to the motor windings. This works because the loads we are dealing with in motion control are overwhelmingly inductive. In an inductor, current begi ns to flow after voltage is applied, and, depending upon the amount of inductance, that current will rise to a certain level in a Embedded Systems Programming OCTOBER 2000 149

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