For example, a controller with an open loop system is given in Figure 7.20, which is a DC motor speed control circuit.
Why must an electronic circuit be used to control the speed of this motor? Why not just use a potentiometer to control the speed of the motor by changing the voltage entering the motor?
Figure 7.20: DC Motor Speed Controller
Of course, there are very basic and important arguments to know why not just use a potentiometer to control the speed of a DC motor, namely:
- Because if you use a potentiometer, when the rotation is slow (by reducing the voltage) the motor will lose its power, so if it is given a load it will stop.
- Also, a lot of power is lost in the potentiometer even when it rotates slowly.
- For this reason, electronic circuits are needed, besides that, currently most industrial controls use electronic circuits because they make all work in the industry easier.
The way the above circuit works is:
- The motor speed control circuit above uses a PWM (Pulse Width Modulation) electronic switch circuit, which in principle the switch is in series with the motor. If the motor rotation speed is desired to be low, the switch only turns on for a moment then turns off repeatedly (longer off time than on time) so that the motor rotation speed becomes slow but the voltage is not reduced at all so that the motor power remains. And if the motor speed is desired to be high, this electronic switch will turn on longer than it turns off so that the motor rotates even faster.
- The UJT circuit is obtained as an oscillator circuit that produces positive pulses with a frequency of 400 Hz and this is the input to the next circuit.
- The next circuit is a monostable circuit using a digital IC 74121, which can be said to be stable enough to produce output pulses on pin 1 (output ? of the IC). The negative pulse width of this monostable can be adjusted by potentiometer Rv1. The pulse width can be adjusted from 0.1 ms to approximately 2 ms, if Rv1 is rotated clockwise. This monostable operation can be prevented by making pins 3 and 4 of the IC given logic 1, by turning SW1 off so that the monostable output remains in a high condition (the motor stops rotating). SW1 is the ONOFF control of the motor.
- The pulses from the monostable are fed to the driver circuit, which consists of three transistors, Tr1, Tr2 and Tr3. The purpose of this driver circuit is to ensure that Tr4 is switched rapidly between two possible states, either fully on (saturation) or fully off (cut off). This is necessary so that the power dissipation during switching is kept low. When the output of the monostable is high, Tr1 is conducting and its collector is low. In this condition, Tr3 remains off because its base current is avoided through D1 to the collector of Tr1. At the same time, Tr2 is conducting, ensuring that Tr4 remains off with its base connected to its emitter through Tr2. When the output of the monostable is low, Tr1 becomes off and turns off Tr2 as well. Tr3 now conducts with its base current supplied by R12 and this makes Tr4 fully switched to conduct with its base current supplied by R13.
- The final part is the power switch, which is a transistor Tr4 as explained above. When Tr4 is fully conducting the motor almost receives a voltage of +12 V while the current passing through depends on the size of the pulse period. On the rising / falling side of the pulse, Tr4 is off but D3 is conducting, this is to limit the transient current changes to the motor.
From the explanation of how it works above, of course, there are several things that must be considered, so that when there is a case of damage, it can be handled immediately because it is already known which part is damaged. That is what every technician wants when facing a case of damage to immediately find out the cause of the damage and immediately determine which area is not right and find which components are damaged to be repaired. For that, of course, the technician must first know the circuit that will be repaired so that the cause can be easily found. Several things that need to be considered from the circuit above are:
- When does the monostable make the motor rotate or stop? The motor rotates if SW1 is on, the potentiometer is rotated clockwise (it will get faster), and the output of the monostable is low (logic 0). Conversely, the motor stops rotating when SW1 is off, the potentiometer is at its minimum (counterclockwise), and the output of the monostable is high (logic 1).
- Which transistors work (conduct) when the motor rotates and vice versa? When the motor rotates, the transistors that conduct are Tr3 and Tr4, while the conditions of Tr1 and Tr2 are off (cut off).
- Remember that Tr1 and Tr3 are NPN type transistors while Tr2 and Tr4 are PNP type transistors, which means that their conduction requires input at different bases (see Chapter 4).
- The UJT output is a 400Hz pulse which is an oscillator circuit.
Because in this circuit the output is in the form of motor speed rotation, of course the cases found in the field are only those related to the motor and there are only three of them, namely:
- a) The motor rotates at maximum speed and cannot be controlled.
- b) The motor does not rotate at all in all conditions.
- c) The motor rotates slowly and cannot be controlled.
For case a it can be explained as follows:
The quickest test is the monostable part first. Position SW1 in the off condition, the motor rotation should stop but it turns out to still rotate at maximum. Then measure the output voltage of the monostable, if the monostable is not damaged then the output voltage will be high (above 2 Volts), if below 2 Volts it means the monostable circuit is damaged. If low the possible damage: Potentiometer is open, or R5 is open, or Dz1 is short circuit, or R7 is open.
The next step is to check the driver section, namely the collector of Tr3, the voltage at this point should be 12 Volts when SW1 is off. If it is not 12 Volts, usually if it is damaged the voltage here will be very small, but if it is measured large (12 Volts) it means Tr3 is not problematic.
The last step is to check the power switch transistor (Tr4), which is automatically the problem, and usually the damage is a short circuit between the emitter and collector on Tr4. Indeed, the final transistor is the transistor that is most vulnerable to damage, because the work of this transistor is almost always at maximum so that it is always hot. So you really have to use a cooler on the Tr4.
Meanwhile, we don't need to look at the UJT circuit, because once Tr4 is replaced and the rotation can be adjusted, it means the oscillator is good.
For case b it can be explained as follows:
Here, even though SW1 has been turned on, it still does not rotate, so there are quite a lot of things to check if you don't know the fastest way, which is to check one by one from the existing circuit parts. This is where the experience of a technician is needed. If after checking the fuse it turns out not to be broken, then the fastest way is used, namely with the middle splitting system (half splitting) as explained in Chapter 2, although the parts are not many, this method is very suitable for this one condition.
The way to isolate half of this system, namely short circuit briefly between the base and emitter of Tr1. Then automatically Tr2 does not work and causes Tr3 and Tr4 to conduct which causes the motor to rotate to the maximum. If this happens then the driver circuit and power switch are not a problem. So just check the oscillator and monostable circuits.
Monostable check is the same as in case a, but it is better to check the oscillator circuit first. Using an oscilloscope, you can check the output of the oscillator, if it produces a 400 Hz pulse, it means the circuit is working, but if not, usually the UJT is damaged.
Of course, if the oscillator works, there must be something wrong with the monostable part. And usually the problem is the monostable IC itself (74121).
For case c it can be explained as follows:
Here the motor rotates slowly even though the potentiometer is at its maximum but there is still a slight change, but if SW1 is turned off then the motor rotation stops. That means the driver circuit and power switch have no problem, because they can still forward the pulses coming out of the monostable. It can be seen when SW1 is turned off the rotation stops.
So the suspected damage is the monostable or oscillator part. But because the potentiometer is still functioning, it means the monostable circuit is working normally.
Of course there is only one more, namely the oscillator circuit. But here the damage does not mean it does not work at all. The oscillator circuit changes its frequency to low. This means that the active components are not a problem, the problem is the passive components that can change the frequency. The determinants of the frequency are R2 and C2. The biggest possibility is that R2 changes to enlarge and the next possibility is that the capacitor changes to enlarge, but for capacitors changing prices is very rare.
Principles of Damage & Failure Tracking
The maintenance section is essential for:
- Keeping equipment in good working condition
- Maintaining the continuity of a company
- Participate in achieving the profits expected by the company.
- Maintenance must be well planned, but must also be supported by human resource skills and complete equipment.
Knowledge of specifications is very important in making or maintaining a tool, so that a tool is produced that is reliable enough and we can maintain it properly and correctly.
Recalibration of equipment needs to be carried out on measuring equipment, to prevent damage or measurement errors in the equipment, recalibration is intended to increase reliability as well.
Reliability is the ability of an item to perform a required function (without failure) under specified conditions for a specified period of time.
Failure is the end of an item's ability to perform its required function.
MTTF (Mean Time To Failure) shows the length of time a component is used until failure occurs. MTTF is for items that cannot be repaired.
MTTF = 1 / FR (hours)
MTBF (Mean Time Between Failure) indicates the duration of use of a system and is usually for repairable items.
MTBF = 1 / total FR (hours)
If a constant failure rate applies, that is, the failures are random, then the formula applies:
Reliability Product Law. For units in series, failure of one part means failure of the entire system.
Rs = RX . Ry .......Rn
Redundancy: used to improve system reliability by placing them in parallel.
Rp = 1-Qp
Where Qp is the unreliability of the parallel system.
Qp = QA . QB ......... Qn
For special cases, two parallel units:
Rp = RX + Ry - Rx . Ry
Factors that influence reliability:
- Design and development: component selection, derating, mechanical layout, prototype testing.
- Production: skills, cooperation and training of workers, production equipment, comfortable working environment (ventilation and lighting), automatic test equipment.
- Storage and shipping: packing, storage and shipping methods.
- Operation: suitable environmental conditions, correct operating methods.
Environments that affect reliability: temperature, pressure, humidity, oxidation, vibration/shock, radiation, fungi and insects.
Avaliability (existence) = MTBF / (MTBF + MTTR)
There are many methods for tracking damage from the simplest to a complex system, and these must be chosen appropriately so that the speed and success rate are high.
We must be able to perform simple active component testing, as well as open or short circuit testing and checking of a circuit.
Computer Based Maintenance & Repair Management System
The Maintenance and Repair Management System explained in the previous chapter is implemented manually. The system can be implemented using a computer.
Computer-assisted Maintenance Management Systems, commonly abbreviated as CMMS (Computerized Maintenance Management Systems) is a software that contains all aspects of an organization's life. Many vendors offer this software for free. The software generally still needs to be modified to suit the conditions or needs of the organization as a user.
Computerization of maintenance and repair management allows all information to be available in all sections related to the maintenance function, such as managers, supervisors, planners, warehouse personnel, and accounting departments.
Advantages of Computerized Maintenance Management
- Increase efficiency
- Reduce Maintenance Costs
- Reduce equipment down-time (repair time) costs
- Increase the service life of the tool
- Produces a record of a tool's maintenance history, to make it easier to plan maintenance and repair costs.
- Generate maintenance results reports in the format required by users and management.
CMMS can be used to monitor all equipment maintenance and repair costs through:
- Monitoring of WO costs through the WO implementation schedule
- Monitoring inventory and purchasing of goods, to avoid stockpiling of goods in the warehouse. For vendors, this information is used to determine the most appropriate time to deliver goods.
- Monitoring the Preventive Maintenance Schedule (JPP), to prevent excessive maintenance (overmaintenance), and can increase up-time and extend the service life of the equipment.
In general, CMMS consists of 4 modules, namely:
- Work Order Planning and Scheduling
- Maintenance Inventory Control
- Module for updating Preventive Maintenance data
- Maintenance Report
How to Localize Damage?
Localizing Faults in Simple Circuits
Component damage can be identified through the symptoms of damage. The technician's job is to interpret the symptoms of damage. The knowledge needed here is the characteristics of each component.
Each fault shows unique symptoms, for example changes in circuit operation, changes in output signals, dc bias levels, etc.
Localizing Faults in Complex Circuits
Basically, a complex system consists of several circuit blocks (sub-systems) which have different functions.
To determine component damage in a circuit consisting of hundreds or thousands of components is certainly not easy. Therefore, divide the system into several blocks according to the function of each block, such as the example in Figure 1-8: RF Signal Generator Circuit below. Test the performance of each block. Start testing from its power source, then continue to the next blocks. In this way, if there is a block that is not functioning properly, it will be easy to identify.
Task
Find a manual of an electronic system, such as a TV, Video player or others. Look at the circuit diagram. From the circuit, make a block diagram, then show it to your teacher, ask, whether you have drawn it correctly. To increase your insight, you can exchange pictures with your friends who have different pictures.
Definition of Corrective Maintenance
Corrective maintenance will be related to damage detection, determining the location of damage, and repairing or replacing damaged parts. The stages of corrective maintenance can be seen in Figure 1.3.
Figure 1.3. Corrective Maintenance Stages
Work Aids
Work aids are all tools that can be used by technicians or experts to determine the type and location of damage to the system being checked. This can be a maintenance manual, test equipment (multi-meter, oscilloscope, logic probe, and so on), and or special equipment (for example for measuring instrument calibration). You can learn about test equipment specifically in other chapters in this book. When we buy electronic equipment (and also other equipment), for example a radio tape, it is usually given a Manual for operating instructions and maintenance instructions or how to overcome problems with the equipment. The form and format of the maintenance manual vary greatly, depending on the manufacturer of the equipment. An example of the maintenance manual format can be seen in Table 1.2.
Figure 1.4: Diagnostic Aids
Maintenance manuals are also available in the form of flowcharts, as shown in Figure 1.6. The system to be analyzed in this example is a regulator. Figure 1.5 is a block diagram of the regulator to be examined.
A good manual contains:
- Description of the system and how to operate it
- System performance specifications
- Theory of Operations (system, block diagram and or circuit)
- Maintenance methods (preventive & emergency response)
- Spare parts list
- Mechanical layout
Understanding Preventive Maintenance
In a broad sense, preventive maintenance includes engineering and management aspects. In engineering, preventive maintenance includes: detecting and/or correcting the use of existing equipment, through statistical analysis of existing failures or errors or based on existing repair records. This work must be carried out properly by people who are truly experts in their fields and with the right frequency (for example twice a year).
If it is too frequent, it will not only increase maintenance costs, but will also reduce the productivity and efficiency of the company's work. The data in Figure 1.2. shows that damage often occurs at the beginning of the use of the tool. This can be caused by worker negligence and/or internal damage to components from the tool manufacturer (this is called product failure). Level of damage
The tool's service life will decrease after workers get used to using the tool. After passing the critical period, the tool will experience more frequent problems, so repairs will be carried out more often, until the tool's service life runs out. At this time, it means that the tool can no longer be repaired.
In the field of management, maintenance activities include: making a list of jobs, determining the number and qualifications (expertise) of technicians needed, estimating how long the job will take, planning a schedule for implementing the job, and predicting maintenance and repair costs. All of these activities are usually listed in a control sheet.
The most important thing in preventive maintenance is to determine the Job List. The main purpose of making a job list is to remind workers about: what tools need to be serviced, what the technician or worker should do (eg measuring or testing current or voltage at a certain point, cleaning the tool, replacing components, and so on), This list will also include the maintenance implementation procedures that must be carried out. listed:
The job list should be compiled by various stakeholders (manufacturers, mechanics, experts, contractors, insurance companies, government, related associations, distributors, consultants and various product users).