Stepper: Relics or Reality by Schneider Electric

In the early days of electronic positioning control, stepper technology was the savior of the factory. By the mid-1970s, mechanical linkages, handwheels and hydraulic applications were quickly being replaced by these revolutionary devices, which offered predicable motor movement based on a simple electric pulse input. The failures and errors induced by adjustable hard stops and manual measurements were eliminated, spawning the electronic motion control and industrial automation industry.

03/30/2004


Steppers: Relics or Reality?

By Robb Dussault
Manager, Motion Product Marketing
Schneider Electric
North Andover, MA

In the early days of electronic positioning control, stepper technology was the savior of the factory. By the mid-1970s, mechanical linkages, handwheels and hydraulic applications were quickly being replaced by these revolutionary devices, which offered predicable motor movement based on a simple electric pulse input. The failures and errors induced by adjustable hard stops and manual measurements were eliminated, spawning the electronic motion control and industrial automation industry.

Though closed-loop servo technology has been available for just as long, only a PhD in physics had the skill-set to be able to apply and tune the complex control systems. Only in the 1980s, when modern solid-state computing was able to take over these calculations, and with development of advanced magnetic materials, did electric servos enter the factory environment.

At first, only the applications that demanded the highest degree of precision and speed could afford this expensive new technology. With continuing software advances and the general cost reduction of motors and electronics, however, the playing field had been leveled. Servos have now become as simple to use as steppers. This equality also extends to the cost of manufacture. Where a servo motor used to cost many times more than an equivalently powered stepper motor, the cost differential for a typical 2 hp industrial-grade system is now around 40%1. That gap is narrowing quickly.

Steppers still have a place in industrial automation applications. In 2001, an estimated 300,000 axis were sold in North America, roughly equivalent to the number of servo axis2. As the cost and complexity gap between the two technologies continues to shrink, the controls or motion engineer must rely more on technical comparisons to determine the best fit for an application.

1. Cost comparisons include an equivalent encoder feedback device on both stepper and servo system.
2. “Updated Forecasts for IMS Research’s Study‘The North American Market for Motion Controls-2001 Edition’”, July 2002.

There are several important technical application differences between stepper and servo-based motion systems:

  • Stepper motors provide full holding torque at stand-still, regardless of load, while servo motors need error to produce torque. This attribute makes steppers ideal for applications that require the load to be stopped and “locked” down without the component and maintenance costs of a holding brake.

There is, however, a price to pay in electrical consumption. Steppers require full current at all times to maintain full holding torque, while servos deliver only the power required to overcome error and maintain position. This also explains why steppers are usually limited to low-power applications.

  • Stepper motors and drives can be used without a feedback device, providing a very inexpensive but highly accurate positioning system for applications which do not require this function. The price of a stepper is typically about 25 percent less than a servo, since it saves money in three ways:
    1. No encoder needed on motor
    2. No rotational monitor circuit on drive
    3. No feedback cable
    Stepper systems are usually appropriate for low-power applications or applications which will not cause damage to personnel or to machinery if a failure causes the axis to move unexpectedly. Stepper systems are quick and easy to setup, since no tuning is required.

  • Servo systems can better accommodate variances in load, such as in material transfer or machining applications. Stepper systems, on the other hand, can slow down (lose torque) when there is a significant change in load. This makes stepper systems most appropriate for well defined and consistent loads.

  • Stepper systems are easy to control with only the need of a pulse and direction signal. In fact, a stepper axis can be completely controlled with just two discrete outputs on a PLC. A high-speed “pulse” output is provided on many PLC platforms for this specific purpose. This attribute provides substantial system cost savings by eliminating the need for a motion controller in many cases.

  • Stepper motors provide excellent torque output, but only for speeds below 2000 rpm and limited acceleration/deceleration rates, while servos can typically maintain their torque at 6000-8000 rpm with peak to continuous ratings of 5:1 and much higher acceleration / deceleration rates.

  • Steppers are inherently digital (moved with “pulses” of current instead of a sinusoidal wave) and thus may introduce mechanical resonance into the mechanical system.

Applications


Distinguishing Attributes of ideal Stepper Applications

  • Lower torque (&16NM or 140 in/lbs) Note that gearboxes are available to facilitate higher torque at lower speeds

  • Lower speed (max 500-2000 RPM , depending on size)

  • Approximately constant loads

  • Cost-sensitive

  • May not need feedback (substantial cost savings if this can be eliminated from application) Distinguishing Attributes of ideal Servo Applications


Distinguishing Attributes of ideal Stepper Applications

  • Higher torque (up to 160NM or 118 ft/lbs)

  • Higher speed (up to 8000rpm)

  • Variable loads

  • High acceleration rates


Stepper Application Examples

  • Constant-load conveyors.

  • Labeling and packaging systems

  • X-Y tables

  • Cut-to-length (light torque) and rotary knife applications

  • Film processing and printing

  • Traversing winding applications

  • Medical equipment

  • Product dispensers


Servo Application Examples

  • Drilling

  • Milling

  • Grinding

  • Gantries for heavy parts

  • Palletizers

  • Saw (heavy torque)

  • Paper converting

With careful consideration of these technical differences, an engineer is able to make a qualified decision not solely on price but on which motion technology can best improve the overall functionality, cost, and simplicity of his machine. Mixing and matching of the two technologies, however, can add additional complexity because they typically involve different software and hardware platforms. A few full-line motion vendors such as Schneider Electric have solved this problem with dual platform drives, which offer the same feature set and programming interface for both technologies. Solutions like this will enable stepper technology to continue to thrive in modern factories.





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