Difference between Closed Loop and Open Loop Stepper Motor

This article details closed loop stepper motor systems vs. traditional open loop systems. The test monitors power consumption of both motors when turning a dynamometer at a fixed speed of 10 rev/sec (600 rpm). Results show that closed loop systems offer greater motor efficiency, cooler operations and lower energy consumption for greater operational savings.

The simplicity, excellent positioning and low cost of step motors make them popular for use in automation applications such as indexing and positioning. Because traditional step motor systems run open loop, the drive electronics constantly supply current to the motor windings, regardless of torque demand from the load. Configured to provide the motor’s rated current, the drive will do its best to power that rated current into the motor at all times, whether the motor needs it or not. Expending unnecessary energy during operation makes open loop step motor systems inefficient. Excessive power consumption wastes money and step motors operate “hot” due to this excess of power.

Closing the loop makes a more efficient step motor system. By employing a common feedback device such as an encoder to monitor actual shaft position versus commanded position, closed loop step motor systems automatically reduce current to the motor when torque is no longer demanded by the load. Only the amount of current needed to drive the load powers the motor. This saves energy, especially when the torque demand is low.

This simple change to the stepper system is extremely powerful and greatly improves the efficiency of the step motor. A closed loop stepper system will consume much less power than a traditional step motor system. Increasing motor efficiency while decreasing power consumption translates to lower energy bills and greener operations.

Closed Loop vs. Open Loop Stepper Motors

In the open loop system, a step motor is driven open loop by a basic microstepping drive. The closed loop system is an ATO integrated stepper motor which combines the motor, encoder, drive and controller into a single package, as shown in figure 1.

ATO stepper motor closed loop system
Figure 1: ATO stepper motor closed loop system

Figure 1 shown is a cutaway of a closed loop ATO integrated stepper motor. It combines motor, drive and controller with a high-resolution incremental encoder and closed-loop servo firmware to operate cooler and more quietly than open loop motors, by drawing just enough current to control the load. Because motor power consumption is an issue for many machines, ATO integrated stepper motors provide the benefits of less power consumption and reduced operating costs.

The test shown in the video monitors the power consumption of two equivalent step motors, one open loop and one closed loop, when turning a dynamometer at a fixed speed of 10 rev/sec (600 rpm). At a torque load of 50 oz-in, the open loop system draws 0.73 amps and consumes on average 87.2 Watts of power. The closed loop system draws 0.42 amps and consumes on average just 50.0 Watts of power. That’s more than a 40% reduction in power consumption with the closed loop motor, which is doing the same amount of work as the open loop motor.

In addition to increasing motor efficiency, a closed loop stepper system operates more quietly. Stepper motors are known to make audible noise. This is due to the fact that open loop step motors operate at full rated current regardless of load. Because the closed loop stepper system runs with less current, it operates more quietly, especially at speeds in the range of 0 to 20 revs/sec (0 to 1200 rpm).

While closed loop step motor systems may cost more initially, reductions in energy consumption, heating and audible noise, along with increases in torque and accuracy justify the switch from open loop. The use of closed loop step motor systems will reduce energy consumption and provide other long-term benefits in many applications.

Closed loop vs. open loop stepper motor performance indicators
Figure 2: Closed loop vs. open loop stepper motor performance indicators

Figure 2: The application of low-cost, high-resolution feedback devices and advanced digital signal processing to close the loop on stepper motion results in an efficient and high-performance ATO closed loop stepper system. This table compares the relative benefits of an ATO closed loop stepper system compared to a traditional open loop stepper system.

ATO Closed loop stepper motors

These are closed Loop stepper motors from ATO that offer many advantages over traditional step motor systems, including higher torque and acceleration, higher machine throughput, better efficiency and quieter operation. With encoder feedback and advanced control loops that incorporate servo control, ATO stepper motors provide a cost-effective motion control system that operates faster and cooler than competitive technology.

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