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AQ: AM & FM radio

For AM & FM radio & some data communications adding the QP filter make sense.
Now that broadband, wifi, data communications of all sizes & flavours exist – any peak noise is very likely to cause interuptions & loss of integrity of data – all systems are being ‘cost reduced’ ensuring that they will be more susceptible to noise.
I can understand the reasons for the tightening of the regulations.
BUT, it links in to the other big topic of the moment – the non-linearity of managers.
William is obviously his own manager – I bet if his customer was to ask him to spend an indefinite amount of time fixing all the root causes to meet the spec perfectly without any additional cost it would be a different matter.

Unfortunately for most of us the realities of supervisors wanting projects closed & engineering costs minimized we have to be careful in the choice of phrasing.
Any suggestion that one prototype is ‘passing’ suddenly can be translated to job finished, & even in our case where the lab manager mostly understands, his boss rarely does & the accountant above him – not at all.

It gets worse than that – at the beginning of a project (RFQ) – the question is “how long will EMC take to fix?” with the expectation if a deterministic answer; the usual response of a snort of derision & how long is a piece of string generally translates to 2 weeks & once set in stone becomes a millstone (sorry mile-stone).

We already have a number of designs that while not intentionally using dithering, do use boundary mode PFC circuits which automatically force the switch frequency to vary over the mains cycle. These may become problematic at some future variation of the wording of the EMC specs.

While I have a great deal of sympathy for the design it right first time approach, the bottom line for any company is – it meets the requirement (today) – sell it!!

AQ: Electronic industry standards

You know standards for the electronic industry have been around for decades, so each of the interfaces we have discussed does have a standard. Those standards may be revised but will still be used by all segments of our respective engineering disciplines.

Note for example back in the early 1990s many big companies HP, Boeing, Honeywell … formed a standards board and developed the Software standards( basic recommendations) for software practices for programming of flight systems. It was not the government it was the industry that took on the effort. The recommendations are still used. So an effort is first needed by a meeting of the minds in the industry.

Now we have plenty of standards on the books for the industry, RS-422, RS-232, 802.1 … and the list goes on and on. The point is most of the companies are conforming to standards that may have been the preferred method when that product was developed.

In the discussion I have not seen what the top preferred interfaces are. I know in many of the developments I have been involved in we ended up using protocol converters, Rs-232 to 802.3, 422 to 485 … that’s the way it’s been in control systems, monitoring systems, Launch systems and factory automation. And in a few projects no technology existed for the interface layer, had to build from scratch. Note the evolution of ARPA net to Ethernet to the many variations that are available today.

So for the short hall if I wanted to be more comparative I would use multiple interfaces on my hardware say usb, wireless, and 422. Note for new developments. With the advancement in PSOCS and other forms of program logic interface solutions are available to the engineer.

Start the interface standards with the system engineers and a little research on the characteristic of the many automation components and select the ones that comply with the goals and the ones that don’t will eventually become obsolete. If anything, work on some system standards. If the customer is defining the system loan him a systems engineer, and make the case for the devises your system or box can support, if you find your product falls short build a new version. Team with other automation companies on projects and learn from each other. It’s easy to find issues as to why you can’t succeed because of product differences, so break down the issues into manageable objectives and solve one issue at a time. As they say divide and concur.

AQ: UPS systems commissioning test and inspection procedures

The UPS systems commissioning test and inspection procedures are to conform to;

• BS EN 50091-1:1993 – Specification for Uninterruptible Power Supplies (UPS). General and Safety Requirements, AND

• IEC 62040-3 (Draft Edition – 2) in particular the Efficiency test procedures outlined in its “Annexure-J”.

These procedures to include:

1. Visual Inspection:
a. Visually inspect all equipment for signs of damage or foreign materials.
b. Observe the type of ventilation, the cleanliness of the room, the use of proper signs, and any other safety related factors.

2. Mechanical Inspection:
a. Check all the power connections for tightness.
b. Check all the control wiring terminations and plugs for tightness or proper seating.

3. Electrical Pre-check:
a. Check the DC bus for a possible short circuit.
b. Check input and Bypass power for proper voltages and phase rotation.
c. Check all lamp test functions.

4. Initial UPS Startup:
a. Verify that all the alarms are in a “go” condition.
b. Energize the UPS module and verify the proper DC, walkup, and AC phase on.
c. Check the DC link holding voltage, AC output voltages, and output waveforms.
d. Check the final DC link voltage and Inverter AC output. Adjust if required.
e. Check for the proper synchronization.
f. Check for the voltage difference between the Inverter output and the Bypass source.
g. Perform full-load, step-load, and battery discharge tests using supplier furnished load bank.

AQ: Techniques contribute in control system

1. Any successful methodology is not a simple thing to come by and typically requires a huge commitment in time and money and resources to develop. It will take several generations to hone the methods and supporting tools.

2. Once you get the methods and tools in place, you then face a whole separate challenge of indoctrinating the engineers in the methods.

3. Unique HMI text involves a lot of design effort, implementation, and testing.

Many of the techniques contributed by others in the discussion address faults, but how do you address the “normal” things that can hold up an action such as waiting for a process condition to occur, such as waiting for a level/pressure/temperature to rise above/fall below a threshold or waiting for a part to reach a limit switch?

Some methods allow for a text message that describes each step. When developing these text messages, I focus on what the step’s transition is waiting for, not the actions that take place during the specific step. This helps both the operator to learn the process as well as diagnose what is preventing the machine from advancing to its next step.

I have seen sequencing engines that incorporate a “normal” step time that can be configured for each step and if the timer expires before the normal transition occurs, then you have “hold” condition. While effective, this involves a lot of up-front development time to understand the process and this does not come cheaply (with another nod to John’s big check!).

(Side note on sequential operations: I have used Sequential Function Charts (SFCs/GRAFCET) for over 20 years and find them to be exceptionally well-suited for step-wise operations, both from a development perspective as well as a troubleshooting perspective.)

I have seen these techniques pushed by end users (typically larger companies who have a vested interest in standardization across many sites) as well as OEMs and System Integrators who see these as business advantages in shortening development, startup, and support cycles. Again, these are long-term business investments that require a major commitment to achieve.

AQ: Three Phase Input DC Drive

Controlled bridge rectifiers are not limited to single-phase designs. In most commercial & industrial control systems, AC power is available in three-phase form for maxi mum horsepower & efficiency. Typically six SCRs are connected together, to make a three-phase fully controlled rectifier. This three-phase bridge rectifier circuit has three legs, each phase connected to one of the three phase voltages. It can be seen that the bridge circuit has two halves, the positive half consisting of the SCRs S1, S3, & S5 & the negative half consisting of the SCRs S2, S4, & S6. At any time when there is current flow, one SCR from each half conducts.

The variable DC output voltage from the rectifier sup plies voltage to the motor armature in order to run it at the desired speed. The gate firing angle of the SCRs in the bridge rectifier, along with the maximum positive & negative values of the AC sine wave, determine the value of the motor armature voltage. The motor draws current from the three-phase AC power source in proportion to the amount of mechanical load applied to the motor shaft. Unlike AC drives, bypassing the drive to run the motor is not possible.

Larger-horsepower three-phase drive panels often consist of a power module mounted on a chassis with line fuses & disconnect. This design simplifies mounting & makes connecting power cables easier as well. A three phase input DC drive with the following drive power specifications:

  • Nominal line voltage for three-phase-230/460 V AC
  • Voltage variation-+15%, -10% of nominal
  • Nominal line frequency-50 or 60 cycles per second
  • DC voltage rating 230 V AC line: Armature voltage 240 V DC; field voltage 150 V DC
  • DC voltage rating 460 V AC line: Armature voltage 500 V DC; field voltage 300 V DC

AQ: Why your project failed?

I have contracted with lots of different groups and moving within the same company to save failed projects or project in trouble or impossible to implement and helped these groups to achieve company goal. What I have noticed is that less the managers or groups know less they realize more knowledge or experience can help them. Less they know, less they understand they need help because they don’t know what they need. They think they are just fine until it is too late and a group or company goes under because of it.

I give you an example of one of the project I worked on at Nortel. I was assigned to write project specification for a product working with a director group with 100 designers and testers. During my research to get information to write the product specification I discover deficiency in the hardware they wanted to use that would cause the system reliability and availability unacceptable to the customer and did not meet customer requirement. I proposed design change in one of the interface card and firmware used in the system. The management did not agree with me on this item so I refused to write the specification the way they want it to not expose this deficiency. We had a large meeting with the president of the company with 20 people in that meeting looking at two different presentation to see if they need to change direction or stay on course and move me out of the way to another project activity. I am not the greatest in politics and making things look good when they are not.

The result was that I was moved to different project for 1 year implementing and releasing one more product that made the company lots of money. After a year development, they complete the project and released it to the customer. The customer starts validating the product and had lots of the test cases failing in the area that I proposed to change.
This was a large project and lots of money involve. The customer rejected the product and they went back on the drawing board after getting lots of this equipment on order for this project. The management came back to me and one of my team members to come back to the team and help.

Me and my team member both having experience over 15 years at that point came back and have a solution designing a new complex interface card with microcode firmware and some software to save the project. I and he had to work for 4 months for day and night having design review between two of us at 3 am in the cafeteria to get it done (defining specification to validated working product).

This project was completed and customer accepted this solution. The director group was dismantled and all people in the group were laid off and absolved in other groups in the company and some in the same group. The management groups were smart people with good intention, they were with software background and good intention for the project. They just did not have the knowledge and background to manage the system and hardware level because of lake of knowledge and experience in that area.

You see this in lots of companies when a software designer or manager is successful in their area, they get promoted and manage groups that are out of their area and lots of time they destroy groups or project because of not having the background to identify good or bad direction to go. You will always have engineers to not agree with each other and managements have to make decision to go one way or another. The wrong decision in these cases can destroy a project or company. Not all engineers can present a case in 1 hour to sell you their point of view, Remember they are not lawyer or sales man, they are engineers. So what do you do, Follow the sales man or lawyer to save your project or the reason to drive your decision and if you don’t have the knowledge or experience to lesson to the reason then you will make the wrong decision.

AQ: Single Phase Input DC Drive

Armature voltage-controlled DC drives are constant torque drives, capable of rated motor torque at any speed up to rated motor base speed. Fully controlled rectifier circuits are built with SCRs. The SCRs rectify the supply voltage (changing the voltage from AC to DC) as well as controlling the output DC voltage level. In this circuit, silicon controlled rectifiers S1 & S3 are triggered into conduction on the positive half of the input waveform & S2 & S4 on the negative half. Freewheeling diode D (also called a suppressor diode) is connected across the armature to provide a path for release of energy stored in the armature when the applied voltage drops to zero. A separate diode bridge rectifier is used to convert the alternating current to a constant direct current required for the field circuit.

Single-phase controlled bridge rectifiers are commonly used in the smaller-horsepower DC drives. The terminal diagram shows the input & output power & control terminations available for use with the drive. Features include:

  • Speed or torque control
  • Tachometer input
  • Fused input
  • Speed or current monitoring (0-10 V DC or 4-20 mA)

AQ: What if

Many years ago we used to call this the “what ifs?”. Part of the design phase is when we model what we think the system is meant to do. Just as important is how the system is meant to react when things are not going well, the abnormal situations or what ifs?

Your client will tell you how their machine or process works, well he will describe how he thinks it works. This is OK as a starting point but we need to consider the scenarios of “what If” something goes wrong? Scenarios is also a good word as scenarios paint a situation that can be described to the customer for his comment.

For example on a compressor control project what if the lube oil pump fails on the compressor, how do we alarm this to the Operator, should we trip the compressor or do we start the back-up oil pump (if there is one). As you look at the system you can pick out various components and generate likely scenarios that you can discuss with the client. Using this approach gives more of a real world feel to your client meetings that are likely to generate a deeper insight into how the system is meant to work.

All scenarios do not have to be centered around abnormal situations but can also relate to things that need to be considered as part of normal operation. For example we might look at how a duty/standby pump system works? One scenario might relate to duty pump failure but another scenario might consider rotating the duty and standby pumps to even out wear and tear? You might also have manual mode and auto mode scenarios to consider?

What you have to remember is that most clients are not control systems experts. They might and probably will struggle with flow charts or any other pseudo code expression type formats that describe how you think the client’s system is meant to work? You have to tailor your approach to match your audience and that is also very important when you produce your documentation, you do not want to lose valuable information just because the client doesn’t fully understand what you are trying to tell him? Also make sure that you spend time with your client. Walk the client through your design, do this face to face as much as possible and do it more than once! Getting feedback on a regular basis helps to eliminate the dreaded word “REWORK”! Also taking this partnership approach builds a good relationship with your client.

For the machine builders your client might be in your own company? Remember same company or not they are your client and your success in no small way depends on your relationship with them.

Add the modes of operation and abnormal situations to your system model and develop the methods of how you will flag these situations to the Operator and Maintenance Engineer. Alarm Management is dealt with by EEMUA 191. If you do nothing else then read this document it will help you to set up alarm workshops, alarm reviews, alarm prioritization and rationalization and how to develop an effective alarm management structure for your system.

AQ: DC Drives Basic Operation Principles

DC drives vary the speed of DC motors with greater efficiency & speed regulation than resistor control circuits. Since the speed of a DC motor is directly proportional to armature voltage & inversely proportional to field current, either armature voltage or field current can be used to control speed. To change the direction of rotation of a DC motor, either the armature polarity can be reversed, or the field polarity can be reversed.

DC drive diagram

The block diagram of a DC drive system made up of a DC motor & an electronic drive controller. The shunt motor is constructed with armature & field windings. A common classification of DC motors is by the type of field excitation winding. Shunt wound DC motors are the most commonly used type for adjustable-speed control. In most instances the shunt field winding is excited, as shown, with a constant-level voltage from the controller. The SCR (silicon controller rectifier), also known as thyristor, of the power conversion section converts the fixed-voltage alternating current (AC) of the power source to an adjustable-voltage, controlled direct current (DC) output which is applied to the armature of a DC motor. Speed control is achieved by regulating the armature voltage to the motor. Motor speed is directly proportional to the voltage applied to the armature.

The main function of a DC drive is to convert the fixed applied AC voltage into a variable rectified DC voltage.

SCR switching semiconductors provide a convenient method of accomplishing this. They provide a controllable power output by phase angle control. The firing angle, or point in time where the SCR is triggered into conduction, is synchronized with the phase rotation of the AC power source. The amount of rectified DC voltage is controlled by timing the input pulse current to the gate. Applying gate current near the beginning of the sine-wave cycle results in a higher aver age voltage applied to the motor armature. Gate current applied later in the cycle results in a lower average DC output voltage. The effect is similar to a very high speed switch, capable of being turned on & off at an infinite number of points within each half-cycle. This occurs at a rate of 60 times a second on a 60-Hz line, to deliver a precise amount of power to the motor.

AQ: System operation

Our PSA unit (meaning Pressure Swing Adsorption) uses 5 adsorption vessels. The process itself is a batch process, but in order to run in a continuous process plant, each of the 5 vessels can complete all the adsorption process but at the same time, each of them is in a different status of the sequence (i.e. gas in, gas out, adsorption, pressurizing, depressurizing, cleaning, etc). The sequence is mainly controlled by time and pressure condition in each step of the sequence, by managing several valves (I think 5 by vessel, but I’m not sure right now).

Panel operator experienced some problems with valve 1 (gas entry) in vessel 2 because it should open but immediately it received the close command. Instruments technician check that orders coming from the DCS were OK, and also check the valves by injecting the open order, so, they and operation staff concluded that “the program has some kind of problem”.

Some time ago, I spent a lot of time studying the operation manual of this unit and the code written to control it and I wrote a document merging both knowledge. In page 9, I described a condition (an exclusive pressure difference between vessel and gas coming in the vessel) avoiding valve 1 opening during adsorption stage. I explained this condition to operation staff and they confirm that the values were right and that the excessive delta P really exist so, the decided to check back the valve 1 (already checked), discovering a problem (the stem moved, but the disk not).

Conclusion:
– If operation staff know properly the process, they know about this condition, but this could be solved with a properly designed HMI (i.e. including and alarm indicating “valve 1 closed by excessive deltaP”).
– The initial inspection of the valve didn’t show anything wrong, but stem and disk were disconnected.
– If we didn’t dig into the code, this problem, solved in less of an hour could take several hours.