msd                                                     Pulse-width modulation
                      101
                       ulse-width modulation (PWM), as it applies to motor         words, energy flows into the load not so much the
                       control, is a way of delivering energy through a            switching frequency, but at the reference frequency.
                       succession of pulses rather than a continuously               PWM is somewhat like pushing a playground-style
                P
                varying (analog) signal. By increasing or decreasing pulse         merry-go-round. The energy of each push is stored in the
                width, the controller regulates energy flow to the motor           inertia of the heavy platform, which accelerates gradually
                shaft. The motor’s own inductance acts like a filter, storing      with harder, more frequent, or longer-lasting pushes. The
                energy during the “on” cycle while releasing it at a rate          riders receive the kinetic energy in a very different manner
                corresponding to the input or reference signal. In other           than how it’s applied.
               questions                                                                               How PWM works
                  &answers
                Q: What’s the main advantage?                                      Command
                A: Efficiency. PWM amplifiers run cooler                           (modulating) signal                                     High
                than standard linear power amps,                                                      +
                requiring substantially less heat sink       A simple
                mass. At about 90% efficiency, PWM           comparator with a                        Comparator                           Low
                makes electromagnetic motion feasible        sawtooth carrier                                              PWM signal
                at power levels where hydraulics used to     can turn a                               -
                be the only option.                          sinusoidal             Chopping
                                                             command into a         (carrier) signal
                Q: What’s the downside?                      pulse-width
                A: Nature doesn’t like abrupt changes;       modulated output.                                           Output stays high as
                high-current switching generates             In general, the                                              long as the command
                electromagnetic noise as well as voltage     larger the                                                   is greater than the carrier.
                spikes. This calls for special measures      command signal,            H     H     H
                like filtering, shielding, and the use of    the wider the                                H
                spike-hardened components.                   pulse.               H                            H                        H
                Q: What’s the effect on bandwidth?
                A: As a rule of thumb, the usable                                                                  L                 L
                bandwidth of the command signal is                                                                       L     L           High
                about one decade (10X) below the
                switching frequency.                                                                                                       Low
                                           Linear Pass element                                                 PWM
                               Supply                                                            Supply
                               voltage                                                           voltage
                                                     R               RL             Command                                           Load
                                                                     Load              input        PWM 
                                                                                                   control
                   Command                                                                                                                  Duty
                      input           Driver                                                95%                 50%                5%       cycle
                Linear amplifiers vary the resistance of a pass element to
                regulate power. Efficiency is fine at the extremes — losses are
                minimal when                                                     The output of a PWM amplifier is either zero or tied to the supply
                              R= 0 or ∞ — but suffers elsewhere, bottoming
                out at midrange (                                                voltage, holding losses to a minimum. As the duty cycle changes to
                                 R= R ) where the amount of energy wasted as
                                      L
                heat in the amplifier equals that delivered to the load.         deliver more or less power, efficiency remains essentially constant.
                 66 ●  PTdesign ●  www.motionsystemdesign.com ●  October 2000