As prices come down, servo drives are being used more and more in
mainstream engineering. Simultaneously, closed loop control has improved
induction motor performance to near-servo standards. So how do you now
decide between servo and standard technology? Stuart Calvert offers some
pointers
Not so long ago servo systems were so expensive that they were only used
in those very special cases where the costs could be justified. But it is
not like that any more. Advancing technology has brought prices down,
opening up huge new opportunities for machine builders, OEMs and end
users. This has created economies of scale for the servo manufacturers,
which reduce prices still further to open up yet more possibilities.
At much the same time vector control has been developed for standard ac
motors, enhancing their performance by an order of magnitude. Now, ten
years after the trend started, open loop vector drives are barely more
expensive that conventional V/f or PWM drives. And while closed loop
vector systems costs are notably higher than open, due to the need for an
encoder and the associated extra engineering, they are still
significantly less than servos. Another barrier has also diminished, that
of technical inaccessibility. Previously high tech drives required
considerable support from specialist engineers, but now with features
like auto-tuning and Windows-based programming packages they are very
much less intimidating and the 'black art' of servo set-up has largely
disappeared.
So, today, machine builders and drive engineers have a seamless continuum
of technologies to choose from: V/f, open loop vector, closed loop
vector, servo, with performance increasing with price. But this has given
rise to a new problem - technology overlap and knowing whether to use one
solution or the next, whether or not to spend the extra money on the
extra performance!
Good engineers should never allow themselves to be seduced by purchase
price, as this may prove a false economy when total life costs of
ownership should be considered. The reason for this is simply that the
vast majority of drive systems will be expected to run for many years,
possibly in difficult conditions, with too little long term maintenance,
or have performance requirements increased after a while. A cheaply built
axis will almost certainly fail under these conditions, leading to the
massive costs associated with breakdowns and repairs, whereas properly
engineered, it will give good service for many years and lower the total
cost of ownership. It is also good practice to size a drive such that it
can cope with an increased load, as this is often required if production
has to be increased.
In terms of hardware, the rule of thumb would be that a closed loop
vector system costs two or three times as much an open loop system, and a
servo system two or three time as much again. Engineering and
installation costs would be almost the same for a drive or servo
solution, but there can be extra costs associated with programming,
particularly if there is a positioning or motion control requirement.
Long term maintenance costs are basically the same for open and closed
loop control. Servo solutions, then, appear relatively expensive; but
they gain because they can do things that are impossible with an
induction motor, such as rapid acceleration and deceleration and
extremely accurate speed and position control.
But good engineering is usually simple engineering, because complicated
designs are more likely to fail. So perhaps the first rule for drive
technology selection is to use open loop if possible, it's cheaper and
simpler. Choosing between V/f, and open loop vector is relatively
straightforward - simple drives for fine-tuning and machine setting type
requirements, sophisticated drives for more demanding speed profile,
multi-axis and signal following applications.
The next stage