# THE NEED FOR SPEED – THE RIGHT MOTOR FOR THE JOB

Dec 052011

In these three blogs I want to share what my experience has told me about the interplay of the three critical factors that determine the best servomotor for a specific job. It will always be one of inertia, speed and torque that takes the lead in determining the right motor and the ideal of most ‘bang for buck’.

In my previous blog I gave my view on inertia as a criterion. Now it’s the turn of Speed, nothing to do with the film of that name although it can lead to similarly unpleasant kinetic results if miscalculated.

The problem the motion control engineer has to resolve is to deliver a repeating operation within a fixed time from start to stop while maintaining both cycle times and reliability. This may be in seconds or fractions of a second. This will drive the average motor speed (RPM) required and your ballpark motor speed rating. Err on the side of safety, so pick a 1600rpm motor if the apparent need is for 1300. It’s the starting point for some serious calculations

There are an infinite number of velocity profiles but these can usually be simmered down to five main velocity profiles, Step, Triangle, Trapezoidal and Parabola or Sine. Personally I would try and work with the trapezoidal profile as this gives the most reward with the least effort.

Generally step profiles require the lowest average speed but deliver two major impulsive forces that make a smooth process unlikely and durability questionable. When I find this velocity profile in use it is usually the result of being forced for whatever reason to choose a motor with too low a speed.

The triangle profile, conversely, is often the result of choosing a motor with insufficient torque, of which more later. While it has lower force when the motor starts and stops it has a huge change in force when it switches from acceleration to deceleration. It also may require higher peak speed than may be desirable due to longer acceleration and deceleration phases.

Sine, parabolic and higher order profiles often use the least energy or have smoother actions but often require complex maths to get the right result. Hence I will try and work with trapezoidal profiles as these can almost be worked mentally. Remember the profile will cover half the distance in one third of the time.

It may not be the most elegant but in the vast majority of projects will produce an optimal result in less time and so at less expense

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