Machine performance data trace

Frequently, the answer to this question begins with accurate and sophisticated data measurement, also called data trace. Most modern motion controllers allow the user to record various parameters of the external system, most commonly position (typically from an incremental encoder), but also occasionally velocity, vibration, and other parameters.

However, measuring external parameters is not enough. What if we want to know the motion position compared to the desired (commanded) position at each moment in time (this is called the position error, or the servo lag). The desired position at each instant of motion is only available from the motion controller itself. It can not be measured. There are in fact several internal parameters that may be useful in determining not only how the machine is performing, but more importantly why it is performing the way that it is.

What characteristics of the trace system are important? The list below gives additional information on what may be important in your efforts to optimize machine performance:

What variables to capture: Beyond the external parameters such as encoder position and the status of externals signals (analog and digital) some other useful parameters of the motion controller to trace are:

• Commanded (instantaneous desired) position
• Commanded velocity
• Commanded acceleration
• Motor command (output command to amplifier)
• Position error (servo lag)
• Servo integral (amount of windup)
• Servo derivative

Example motion product with trace

Number of simultaneous trace variables: The more the merrier. Two is a minimum, but three or four can be useful. Trace period: The user should be able to select how often data is captured, up to a data point every servo cycle. This may generate a lot of data but for certain precise motions this high level of accuracy is critical.

Trace length: How much data is to be stored should be selectable. Typical traces are at least 1,000 points. Often they are 10,000 or more. With 4 simultaneous traces at 4 bytes per point this means at least 16Kbytes, but often up to 256Kbytes of RAM is dedicated to trace storage.

When to start the trace: We will discuss this in more detail below but it is important to be able to specify when (upon what conditions) the trace will start.

When to stop the trace: Same as for start of trace. Sophisticated motion controllers allow the end of the trace to be programmed as well as the start.

How to capture: Capture modes are typically one-time (fill up the buffer and stop) or continuous (treat the buffer as circular and keep the latest data available).

Autonomous capture: This is very important. It is not sufficient that the motion controller allow the host to query for various parameters. To keep all the data synchronized the motion controller “engine” should directly support the trace function, usually using a RAM that it writes to while the trace is active.

Magellan Motion Processor