Tuning Analysis Tools: Step and Frequency Response

Adept SmartMotion Developer's Guide

Tuning Analysis Tools: Step and Frequency Response

Tuning refers to the process of adjusting the Adept SmartMotion control system parameters for a particular amplifier, motor, and load. It is performed using the Adept SmartMotion Specification Utility (SPEC) program, and must be done one motor at a time. Read this section thoroughly before attempting to move the motors under program control.

CAUTION: Before attempting to tune the servo loop, it is imperative to verify the proper operation of all motor, drive, and encoder hardware via the diagnostic utilities provided. It is of particular importance to verify proper operation of the Emergency Stop circuit.

During the early stages of any installation, it is easy to miss common hardware problems such as miswired or intermittent connections. In many cases, the motor will run in spite of such problems, but will perform very poorly. The user should be aware that many hours can be spent trying to "tune away" what is really a simple difficulty with the hardware. If a system appears very difficult to tune, it may therefore be worth double checking that the hardware installation is problem-free.

Step Response Performance Measures

Time Responses to a Step Command shows three typical motor responses to a step position command, with many of the key features exaggerated for clarity. The figure should be used as guide to understanding the displays provided by SPEC. Some of the key features are: Overshoot, Rise Time, Settling Time, and Steady State Error. These terms are defined on the next page.

When under V+ control, the robot will be commanded with a trajectory considerably smoother than the step command used during tuning. As a result, the motor response to the step command should be considered a "worst case" response that is not likely to be duplicated in the application. The step command is useful during tuning precisely because it excites a full range of frequency responses from the motor.

Time Responses to a Step Command

Overshoot. This is defined as the distance the motor moves past the final setpoint. It is indicative of an under-damped system, and can be undesirable because it increases settling time. It does produce shorter rise times, however.

Rise Time. This is defined as the amount of time required to first reach some percent of the final value. Rise time will increase with increased damping.

Settling Time. This is defined as the amount of time to get within some envelope near the final value. Every attempt should be made to minimize this value, since it directly affects robot cycle time.

Steady State Error. This is defined as the minimum error in position after settling. Usually the Integrator Path can be used to bring this value to zero, but probably at the expense of increasing overshoot somewhat.

Frequency Response Performance Measures

The frequency response analysis tools in the SPEC utility program (V+ 11.2 and later) can be used to understand the dynamics and performance limits of your mechanism. For example, if you excite the mechanism with sine waves swept over a frequency range, any resonances in the system should show up as peaks in the chart. If you then stiffen the mechanism, you should see that peak increase in frequency. The frequency response analysis selections available in the SPEC program are listed in the following sections.

Open-loop Frequency Response (Open-loop Excitation)

WARNING: An open or closed loop excitation will command a sinusoidal torque or position to the axis. This may cause the axis to wave back and forth violently, and bang into the axis hardstops. Use extreme care using this feature and be sure to start with small amplitudes.

Be sure to take all precautions necessary to avoid equipment damage or injury to personnel. Stand clear of all mechanisms and be prepared to depress the Emergency Stop button. It is imperative that operation of the Emergency Stop circuit be verified BEFORE attempting to start up the system.

Sine wave excitation is applied directly to the DAC controlling the specified motor, and motor position is measured as an output, as shown in Open-loop Frequency Response with Open-loop Excitation. This method usually provides the cleanest measurement of the open-loop frequency response of the mechanism; however, you have no control over the position of the mechanism during the excitation stage, and you need to be careful to make sure it doesn't hit anything during this stage.

Open-loop Frequency Response with Open-loop Excitation

A typical plot of open-loop frequency response results is shown in Typical Open-loop Frequency Response. As you would expect at low frequencies, the oscillations for a given torque command are quite large. As the frequency increases, the oscillations reduce. If the mechanism is flexible, a small peak may occur at the resonant frequency.

Typical Open-loop Frequency Response

Open-loop Frequency Response (Closed-loop Excitation)
Open-loop Frequency Response with Closed-loop Excitation below shows how the same open-loop results can be obtained by measuring the input command and output position even during closed-loop control. This method should yield the same plot as shown in Typical Open-loop Frequency Response , but can be a safer method since the robot remains under position control during the test period. However, the results tend to be a bit noisier, since a clean sine wave position command does not mean the DAC command will also be a clean sine wave.

Open-loop Frequency Response with Closed-loop Excitation

Closed-loop Frequency Response (Closed-loop Excitation)
The ratio between the commanded position and the output position should ideally be 1, but at higher frequencies this becomes impossible to achieve. The closed-loop frequency response is useful for measuring the highest frequency that the mechanism can accurately track, which is called the "bandwidth" of the system (Typical Closed-loop Frequency Response).

Closed-loop Frequency Response with Closed-loop Excitation

Typical Closed-loop Frequency Response

Related Topics
Step-by-Step Tuning Process
Effects of Servo Tuning Parameters
Edit Motor Tuning Servo Parameters