In this article, we'll take a closer look at two types of stepper motors—high-resolution and encoder types—and explore how they can enhance precision in critical laboratory automation tasks like autosamplers.
Stepper motors are vital in ensuring accurate positioning, especially in applications where repetitive movements are required. This article focuses on how these motors can be optimized for better performance.
Jump straight to the sections below:
- What exactly is an autosampler?
- Examples of different autosampler models
- How to improve the positioning accuracy of stepper motors?
Let’s dive into what an autosampler is first. An autosampler is an automated device that delivers samples to an analytical instrument for examination. They’re commonly used in lab settings to test large batches of samples. For instance, an autosampler might collect samples from a carousel or bring a sampling tool directly to the sample location. A practical example includes analyzing blood samples for detecting viral infections.
Since precision is key when moving samples to specific locations repeatedly, servo motors and stepper motors are frequently utilized. The stepper motor’s ability to control and position accurately is particularly useful for axes involved in sample injection and syringe movement. For more demanding tasks, such as robotic arms that need higher torque and speed, servo motors are preferred. Regardless of the motor type, they are often paired with mechanisms that convert rotational motion into linear motion, such as belts and pulleys, ball screws, or rack and pinion systems. In this article, we’ll concentrate on stepper motors.
Now let’s check out some examples of autosamplers.
First, there’s the liquid or gas autosampler, which uses microsyringes to extract samples. These are widely used in chemical analysis. Then there’s the autosampler designed for solid samples, often used in gamma-ray measurements. Both types rely heavily on precise positioning, which is why the motors need to match their performance.
An autosampler demands positioning precision, so the motors must offer the same qualities. A typical 2-phase hybrid stepper motor usually steps at 1.8° and can achieve ±0.05° (±3 arc minutes) stop accuracy while operating in an open-loop system. Enhancing the motor’s positioning precision can significantly improve the overall performance of the machine.
So, how do you improve the positioning precision of stepper motors? You can either increase the motor resolution or add an encoder—or both. In this article, we’ll focus on two types of stepper motors that can help: high-resolution type and encoder type.
Option 1: Use a high-resolution type stepper motor to boost resolution and accuracy.
By replacing a standard 2-phase stepper motor with a high-resolution type 2-phase stepper motor, the motor’s resolution doubles because the tooth pitch is reduced from 7.2° to 3.6° (360° divided by 50 or 100 teeth). Since we step at a quarter of the tooth pitch each time, this means stepping at 0.9° instead of 1.8°. With finer resolution, the precision of movement improves.
In a high-resolution type stepper motor, the number of rotor teeth has doubled to 100 compared to 50 with the standard type. As a result, the tooth pitch is halved, and the basic step angle becomes 0.9°. Smaller step angles can be achieved by using half-step or microstep driving. However, these methods don’t increase accuracy. Better angle-torque characteristics improve the stop accuracy of the stepper motor by minimizing the negative effects of frictional loads, such as in a ball screw application.
Option 2: Use an encoder type stepper motor for reliability.
Although a standard hybrid stepper motor won't miss steps if appropriately sized for the load and speed, today’s designers of automated machines and robots require more reliability. Most designers now close the loop to ensure the motor doesn’t miss steps during operation. Eliminating missed steps immediately is crucial since it affects any subsequent motion (and other processes) after the missed steps occur.
By adding an encoder to a stepper motor's rear shaft and closing the feedback loop to the controller, a motor's travel distance can be monitored and verified by comparing the number of commanded steps to the number of pulses from the encoder, and missed steps can be programmed to be auto-corrected quickly to avoid delays.
Oriental Motor offers many types of encoders that can be preassembled. For our PKP Series, compact incremental encoders of different resolutions, number of channels, output types, and electrical specifications are available for maximum flexibility with host controllers or PLCs. For better noise immunity over longer distances, use a line driver output type encoder.
Here are the encoder specifications:
| Encoder Code | Type | Resolution | Output | Input Current | Input Voltage | Output Type | Output Voltage (Low) | Output Voltage (High) | Response Frequency |
|--------------|------------|------------|----------------|---------------|---------------|---------------|----------------------|-----------------------|--------------------|
| R2E | Incremental| 200 P/R | A-phase, B-phase, Z-phase (3ch)| 45 mA or less | 5 VDC ±10% | Voltage (TTL equivalent) | 0.5 VDC or less | 4.3 VDC or more (with no load) | 100 kHz |
| R2F | Incremental| 400 P/R | A-phase, B-phase, Z-phase (3ch)| 45 mA or less | 5 VDC ±10% | Voltage (TTL equivalent) | 0.5 VDC or less | 4.3 VDC or more (with no load) | 100 kHz |
| R3J | Incremental| 1000 P/R | A-phase, B-phase, Z-phase (3ch)| 45 mA or less | 5 VDC ±10% | Voltage (TTL equivalent) | 0.5 VDC or less | 4.3 VDC or more (with no load) | 100 kHz |
| R2EL | Incremental| 200 P/R | Line Driver | 30 mA or less | 5 VDC ±10% | Line Driver | 0.5 VDC or less | 2.5 VDC or more | 100 kHz |
| R2FL | Incremental| 400 P/R | Line Driver | 30 mA or less | 5 VDC ±10% | Line Driver | 0.5 VDC or less | 2.5 VDC or more | 100 kHz |
| R3JL | Incremental| 1000 P/R | Line Driver | 30 mA or less | 5 VDC ±10% | Line Driver | 0.5 VDC or less | 2.5 VDC or more | 200 kHz |
Oriental Motor's PKP Series stepper motors are renowned for their quality and include both high-resolution and encoder types. Watch a video to learn more about the PKP Series stepper motors, including its high torque design and other types. The PKP Series offers 2-phase bipolar type (4 wire), 2-phase unipolar type (6 wire), and 5-phase bipolar type (5 wire) to connect to your preferred driver. Use a 5-phase motor for applications requiring lower vibration or noise.
As always, feel free to reach out to our knowledgeable technical support team for assistance with our products.
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