Open-loop stepper motors have been the industry standard, but closed-loop systems better address today’s needs

By Eric Rice, National Marketing Manager, Applied Motion Products

Open-loop
stepper motors have held a prominent position in automation applications over
the years due to their simplicity, excellent positioning, and
low cost. However, as manufacturers utilize faster machines to increase
throughput, open-loop steppers are being challenged. They may need to yield
ground to closed-loop stepper systems, which tests show provide greater
performance advantages than their open-loop counterparts.

High-throughput
production machinery requires high-performance motion control. As such, motors
using closed-loop feedback and advanced control technology create more torque, provide
faster dynamic response, and exhibit higher operating efficiency than open-loop
systems. In fact, closed-loop stepper systems create peak torques up to 50%
higher than the rated holding torque, which enables higher acceleration rates.
Closed-loop also means that the motor only draws current when it needs it, so such
step motors run cooler and more quietly than their open-loop counterparts. Results
from the following tests comparing closed-loop versus open-loop step motors
support these claims.

Meeting higher acceleration requirements
High-throughput
processes require motors to start and stop quickly. A motor’s available torque
determines how quickly it starts and stops. Closed-loop step motors produce
more torque and, therefore, accelerate and decelerate faster.

Results from a
comparison test that Applied Motion Products conducted between a closed-loop versus
open-loop stepper system (Fig. 1) indicate
that closed-loop steppers not only excel in acceleration and efficiency but
operate at much cooler temperatures and with less audible noise. The test
utilized equally-sized open-loop and closed-loop stepper systems with an
external inertial load of 460 g/cm2 to compare acceleration. Curves show
the peak and continuous torque of a closed-loop stepper system and the useable
torque of a typical open-loop stepper system.

Fig. 1: A torque comparison of a closed-loop versus open-loop
system under the same operating conditions.

Torque, in turn,
empowers acceleration. The open-loop system achieved a maximum acceleration
rate of 1,000 revs/s2 up to 10 revs/s. The closed-loop stepper
system produced much higher peak torque and higher torque at higher speeds than
the open-loop stepper system, driving the same load at an acceleration rate of
2,000 revs/s2 and to a speed of 20 revs/s. Move duration also reduced
to 60 ms. The faster move achieved nearly double the cycles/minute (see Fig. 2).

graphic2_open-loop-vs-closed-loop-speed

Fig. 2: Greater acceleration in the closed-loop system allows faster
moves, achieving nearly double the cycles/minute.

Lower energy needs mean lower power consumption, less heat
Typically, increasing
the speed and throughput of a machine means consuming more power. In the case
of closed-loop stepper systems, however, the opposite is true. Closed-loop step
motors actually consume less energy and waste less power as heat than their open-loop
counterparts.

A side-by-side
comparison using the same operating parameters measured the relative efficiency
of an open-loop versus closed-loop stepper motor system. The test implemented the
same move profile (see parameters listed below), load inertia, and rotor
inertia for both systems. Both operated from a 48-Vdc power supply.

Move parameters
included:

  • Accel = 100 revs/s2
  • Decel = 100 revs/s2
  • Distance = 5 revs
  • Speed = 10 revs/s
  • Dwell time = 0.1 s

 

The test results (Fig. 3) show that the average power
consumption of the open-loop stepper system was 43.8 W, while the closed-loop stepper
system consumed only 14.2 W.

graphic3_open-loop-vs-closed-loop-power

Fig. 3: The closed-loop stepper system requires less current
consumption than an open-loop stepper system.

This reduced power consumption also means that less
energy has been wasted as heat. Not only is this an energy savings, it improves
reliability and simplifies thermal design. Heat affects the operation and life
of machinery. Maintaining cool operating temperatures is often challenging for open-loop
steppers that run at currents higher than necessary to produce the torque to
run a motor.

The key to the
energy reduction is the control that a closed-loop system provides. When
setting the drive for a motor’s rated current, open-loop steppers continuously
put that current into the motor whether or not it requires all the torque. When the load is not demanding
torque, the additional current is lost as heat. Closed-loop step motor systems
automatically reduce current to the motor when the load is not demanding
torque. In fact, they only apply the current needed to drive the load.

A test that Applied
Motion Products conducted compared the differences between an open-loop and closed-loop
stepper system when commanded to continuously execute the same move. Both motors
drove inertial loads of 460 g/cm2, which is equal to their rotor
inertia.

Move parameters
included:

  • Accel = 100 revs/s2
  • Decel = 100 revs/s2
  • Distance = 5 revs
  • Speed = 10 revs/s
  • Dwell time = 1 s

 

After running the
test for 30 minutes, the case temperature of the open-loop stepper system rose
to 76°C, while the closed-loop stepper system ran at a cool 36.9°C.

Reduction in noise
One feature of
motors that many do not think about is the sounds that they make. Noisy
machinery can be disruptive and adversely affect worker health and safety. Open-loop
step motors make audible noise due to the high electrical frequency and rapid
flux changes in the stator teeth and because they operate the motor at full
rated current regardless of load. As closed-loop systems supply the motor with
just enough current to control the load, its acoustic levels are lower. In
fact, closed-loop systems provide a reduction of 10 dB in noise due to less
current in the motor windings.

Based
on these tests, results indicate that closed-loop step motor systems provide a
better option for high-throughput machines due to faster acceleration, quieter
operation, and higher overall system accuracy due to the elimination of stall
conditions. Available in the same NEMA frame sizes as open-loop motors, they
easily replace existing open-loop motors in both new and existing applications.