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SimXperience Developers Blog

Racing Simulator Developers Blog

7 Feb 2016

Hybrid Stepper Servo vs Traditional Stepper vs Traditional Servo

Hybrid Stepper Servo vs Traditional Stepper vs Traditional Servo

Author: Berney Villers  /  Categories: AccuForce Wheel  /  Rate this article:
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Many folks mistakenly refer to the motor control technology in our AccuForce products as a "stepper system". It doesn't take much research to discover the many advantages of a "servo system" over a "stepper system", so many have falsely concluded that the AccuForce has an inferior motor control system. The simple fact is, a stepper system couldn't even be used for the purpose of steering simulation. It is not a suitable technology.

The AccuForce is not a "stepper system", nor is it a "servo system", The AccuForce is an is an intelligent hybrid. There doesn't seem to be an industry standard term for intelligent hybrid (stepper/servo) motor control, but the term "hybrid stepper servo" is somewhat common and an accurate description so I'll be using it until someone corrects me.

In this article, we'll be comparing traditional stepper motor control, traditional servo motor control, and hybrid servo stepper motor control in an effort to help you better understand the technology behind the AccuForce. This will not be a comprehensive comparison. My goal is to help you understand what the AccuForce is rather than lead you to a conclusion of which motor control style is best for our use case. Clearly, I'm of the opinion that the hybrid stepper servo control used in the AccuForce is the best choice for the sim steering market but this article will not name a winner. Again, the goal is to help you understand what the AccuForce offers.  We will also neglect the DC motors and brushless DC motors used in consumer grade sim steering systems as they are not relevant to helping you understand the AccuForce.

Before we get into the comparisons, I want to acknowledge the fact that many will have strong bias toward one system or the other and in order to describe the AccuForce motor control, I need to point out the differences in the motor control technologies that came before it. If you're not open to new ideas, this is probably a good place to stop reading.

Let's start with this simple truth: both traditional stepper motor control and traditional servo motor control are imperfect, leaving much to be desired. When you look at their performance characteristics and application suitability, one naturally wishes for the best of both worlds.

 

 

 Traditional Servo Control Traditional Stepper Control
 Not well suited to low speed operation < 2000RPM  Not well suited to high speed operation >2000RPM
 Low pole count results in low speed torque disadvantage in same size motor vs stepper  High pole count results in torque loss at higher speeds
 Poor sustained torque performance / Peaks available Excellent sustained torque performance / No Peaks Available
Quiet Noisy in open loop
 Not well suited to stability at standstill Stable at standstill
 Smooth Notchy Steps
 Little vibration Moderate vibration
 Requires encoder Encoder optional
Good with inertia loads Poor with Inertia loads
 Doesn't pose position Possible to lose position
 Fairly Efficient Inefficient

 

This list is not complete, but it paints a clear picture that traditional stepper and traditional servo are almost polar opposites in terms of their applicable use cases. It's also quite clear that a traditional stepper shouldn't be used for a simulation steering system.

The hybrid stepper servo was created to bridge the gap between stepper and servo technologies, borrowing some benefits of each. In simple terms, a fast and highly intelligent motor control logic drives a stepper motor as though it were a servo. The result truly is a hybrid. It's quiet, smooth, has little to no unwelcome vibration, never loses position, is good with inertia loads and variable loads, is stable at standstill and is well suited to low and mid-speed operation. Another key benefit is it's excellent efficiency, using the least possible power to produce the highest torque in the smallest size motor while generating the least heat.

The AccuForce controller expands on the "hybrid stepper servo" innovations by adding several racing simulation specific features intended to do things such as reduce game induced oscillations, smooth jagged data from slow game output rates, ensure VERY low lag torque control, facilitate multiple inbound torque or positioning streams, provide true linear performance and enhance safety.

I hope this makes clear that the AccuForce controller is much more innovative than the "stepper control" it's often mislabeled as, offering the best traits of traditional servo and stepper technologies in a single solution.

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