CLD-R Exam, Question 1

[Leisure_ksj]

1. Consider the following scenario: You must create a control system where a PID calculation determines the value of an analog output based upon an analog input and a double-precision setpoint. Which of the following is the most appropriate mechanism to transfer the setpoint from your user interface loop to your control loop?

a. A queue with a data type of analog waveform.

b. A functional global variable with logic to hold the maximum.

c. A global variable

d. A queue with a data type of double.

The answer is C. I know a global variable can transfer data from one VI to another. I don't know what the meanings of the user interface loop and control loop are? Are they two different kinds of VIs? Thanks!

[RavensFan]

They aren't two different VI's.  They are the description of whatever purpose a while has to exist.  In this case, one while loop was designed to handle the user interface.  The other while loop was designed to do the PID control.

Descriptions of other while loops might include:  data acquisition, data analysis, data logging, communication with other devices, .....

[johntrich1971]

Leisure_ksj,

What level of certification do you have? The CLD-R is a recertification exam for those who already have a CLD certification which is expiring or has recently expired. I would have thought that someone with a CLD would be familiar with multiple loops within a program as this is a rather basic concept.

[Leisure_ksj]

Thanks. And why is d wrong?

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@RavensFan  已写:

They aren't two different VI's.  They are the description of whatever purpose a while has to exist.  In this case, one while loop was designed to handle the user interface.  The other while loop was designed to do the PID control.

Descriptions of other while loops might include:  data acquisition, data analysis, data logging, communication with other devices, .....

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[crossrulz · Accepted solution]

It is a matter of the type of data you are dealing with.  In the simplest form, you have tags (single point, only care about the latest value), messages (intermittent, N to N communication, can be multiple kinds of data), and streams (constant data transfer, typically 1 to 1, single data type).  A queue is perfect for a stream.  But a simple PID set point is a tag.  Global Variables are one of the most efficient forms for tag communication.

[Leisure_ksj]

I passed the CLD in Chinese exam. In this question, if let me transfer data between two loops, I prefer to use queue or local variable. I chose D but the answer is C. So I asked the simple question. Maybe I need to improve my English too.

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@johntrich1971  已写:

Leisure_ksj,

What level of certification do you have? The CLD-R is a recertification exam for those who already have a CLD certification which is expiring or has recently expired. I would have thought that someone with a CLD would be familiar with multiple loops within a program as this is a rather basic concept.

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[Leisure_ksj]

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@crossrulz  已写:

It is a matter of the type of data you are dealing with.  In the simplest form, you have tags (single point, only care about the latest value), messages (intermittent, N to N communication, can be multiple kinds of data), and streams (constant data transfer, typically 1 to 1, single data type).  A queue is perfect for a stream.  But a simple PID set point is a tag.  Global Variables are one of the most efficient forms for tag communication.

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Thanks very much! That's what I want.

[johntrich1971]

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@Leisure_ksj wrote:

I passed the CLD in Chinese exam. In this question, if let me transfer data between two loops, I prefer to use queue or local variable. I chose D but the answer is C. So I asked the simple question. Maybe I need to improve my English too.

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@johntrich1971  已写:

Leisure_ksj,

What level of certification do you have? The CLD-R is a recertification exam for those who already have a CLD certification which is expiring or has recently expired. I would have thought that someone with a CLD would be familiar with multiple loops within a program as this is a rather basic concept.

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OK. I started to suggest that perhaps there was a language issue. Crossrulz answer is correct - queues are overkill to pass a setpoint. A local variable would be OK, but that is not one of the selections. The question really is about getting you to think about the data requirements.