Fail Safe Design!!!
Fail Safe Design, In a control system circuit can be design in many different ways but there are not any rules for a perfect circuit design. But engineers are always trying to make a better design. Here I will share a concept is called “fail Safe design” and it is widely used in electronic circuit and equipment.
Have you ever tried to analyze your circuit behavior when any component goes fail, and one step ahead, have you ever tried to design your circuit behavior when any component goes fail? That design is called fail safe design.
In fail safe design we consider some general failure and try to design circuit in such way so that after failure of that particular component or element our circuit didn’t defeat in its purpose. For example here I will talk about a simple circuit for alarm. This alarm can be accessible from four locations. As shown in figure.
Now let’s talk about its reliability, this circuit will work perfectly until it’s all connections are healthy. But let assume if switch 2 gets open (wire cut). What will happen? In emergency we will not able to access alarm from location 2. So as safety point of view this circuit is not reliable because this circuit has not capability to identify own fault. Perhaps you will never know its fault until next checking of Alarm.
So finally question is, “Can we redesign a fail safe circuit for this fault?”
In above circuit we have used “Normally Open” terminal of switch, it means when any one press switch circuit terminal will be connected or it will be in “Normally Close” position. In redesigning we will use “Normally Close” terminal of switch and reconnect all switches as per given circuit.
Here you will see that all switches are connected in series and when any one press any switch “Relay” CR1 goes de-energizes and it cause to energize Siren because for relay we have used “Normally close” contact.
Here purpose of circuit is to generate an alarm when any one access any switches from any location in emergency. Don’t forget that these switches are situated in different location. Now if we consider a fault (wire broken) in switch wires so it will de-energize “Relay” CR1 and contact of “Relay” (which is used “Normally close”) energizes Siren. No doubt it will be a false alarm but it indicate problem in circuit. So it is much better than previous one on safety point of view.
How a fail safe circuit use in Robotics?
As I told earlier there are no perfect rules for designing a circuit. In fact it varies on circuit complexity. Key concept of fail safe condition is, think about failure and try to find an alternate route. For an autonomous bot, its circuit can categories in four parts. These are
- Power Supply circuit of bot
- Feedback and sensor circuit
- Processing Unit of bot
- Its actuating part
Power Supply Circuit of bot: In robotic hardware design the most common mistake is, using same power source for micro-controller and same for its actuator (motors). Now what happens, motors drive heavy current but micro-controller board inbuilt power supply is design only for it board components. Hence result is a not working model.
So as a fail safe designer, we have to separate it in two parts, one for Motor driving circuit and other for micro-controller circuit. If possible use some suitable glass fusses in power distribution path.
Feedback and sensor circuit: Sensors are an essential part of a robot. So our feedback and sensor circuit must have a fail safe design. If you are using a potential free contact in sensor use its NC terminal, it will also indicate failure of your sensor. Using a light sensor is very common in robotic so if you are using a pair of light detectors don’t mix its common terminal. Use separate wiring from sensor to IO board.
Processing Unit of Bot: Now a days processing Unit are available in compact and pre-soldered PCBs so no need to re-engineering in its circuit. Only use some optical signal isolator so that you can ensure safety of you micro-controller board during any wiring mistake.
Bot Actuating Parts (Motors): Small DC & stepper motor is a good choice for a bot, hydraulic actuators are used in industrial robot. I would like to recommend you always use a Motor driving circuit for DC and stepper motor. For motor use a separate power source. If you are using a high speed motor use a speed control H bridge circuit.
The aim of any fail safe design is to create a robust and reliable control system which gives at least a working design in spite of common problems like wiring or component failure.
Hope that you have understand fail safe concept of circuit design and it also can be implement in robotics. Further if you want to know about feedback control go to my blog “Why Learning Feedback Control System is important for a Robotic Project?”