Taking a Step Up in Access Control Design
When it comes to designing access control systems, the integrator has a choice of many programming features in today’s system control panels. System design veterans, however, have also found when it comes to trying to do what the customer wants, the need still arises for customizing some electro-mechanical door security configurations.
This month, we are going to look at a circuit design technique often taught in engineering classes but not to the low-voltage technical trade — “Relay Logic Ladder” (RLL) design. I have observed this technique being included in various trade association low-voltage certification classes, such as one I recently attended by the Door and Hardware Institute (www.dhi.org).
This design format is something that many technicians have looked at on paper, but seldom given much thought. It is not a difficult technique to learn and will help them better organize their customer’s customized access application requests.
The ladder diagrams used in RLL are commonly found in industrial control systems and lend themselves well to any relay, contact, sensor, switch, door strike and low-voltage configuration — something that challenges all contractors as they interface security systems to real world applications such as access control.
Ladder diagrams were originally designed in the 1960s to make the transition simple from relay action to logical programming. Let’s look at some ladder logic design fundamentals and set the foundation for Programmable Logic Controller (PLC) programming opportunities.
Climbing a Ladder of Access
A ladder design looks exactly like a ladder. The circuits we will be looking at will typically have two vertical rails (see diagram). Each rail (L1, L2) is of a different common voltage potential. This will apply to AC or DC low-voltage circuits. It is good practice to have the electrical ground side of the circuit rail on the right.
The horizontal ladder “rungs” are the branches of each segment of the circuit. Each rung represents a unique parallel branch circuit between the power supply rails. Rung wiring can be numbered the same as the wiring labels in the actual installed circuit wiring.
Each rung has three sections: inputs and sensors, contacts or switching, and coils (inductors) or loads. The loads should be kept on the right or closest to the ground rail for short-circuit protection. You can have as many rung circuits as needed or electrically supported by the power supply. Also notice that some input sensor symbols can look very similar to a relay contact but will have a unique label — such as C1 for card reader, which is the relay output from the card reader, or D1 for the door status sensor, which would probably be a magnetic door contact.
In the diagram in the magazine, we have set up a basic access control circuit using a ladder diagram. In this example, a customer has requested to have access control set up on a door so that the following takes place:
- A card swipe reader (C1) is used to remotely notify a guard (DA)
- A timer (T1) will only allow guard notification at certain times
- The guard will be remotely notified of a valid read and an access request light (DA) and camera (VS1) for that door will pop up on the guard shack console
- Upon visual verification, the guard presses a door open button (G1), releasing the maglocks (M1) and the door opens for access
- Indicators at the door, which will be red (R3) on locked and switch to green (G3) when access is granted
- Must provide immediate door access for a roving guard with an access master key (RG1)
- Door to release when fire alarm system is activated (F1)
- A door status (DS) indicator if door is left open
Logical Means to Installation
The circuit branches utilize two of the most common types of Boolean logic. The first is an “AND” logic circuit in which more than one condition must be simultaneously met for a true condition and output device activation. In this case, the card reader (C1) NO (normally open) contact must be activated and the timer (T1) NC (normally closed) contact must not be activated for the control relay one (CR1) relay coil to activate. In turn, that triggers the camera relay (CR2), activating the guard shack door alert light (DA) and switching the video switcher alarm input (VS1) to the door camera position.
The other Boolean logic configuration is the “OR” circuit in which any one of several devices needs to activate in order to get an output device to activate. In the diagram on page 34, you will notice that either G1 momentary NC switch activation from the guard shack or the roving guard’s momentary NC key switch (RG1) will cause power to drop to the maglocks. Can you identify the other conditions that the customer had asked for?
Be Sure to Follow Ladder’s Steps
Remember: When laying out a circuit using a ladder design method, make sure the loads are on the right and the sensors on the left. You can have as many rungs as you need. If you have the chance for many loads being active at the same time, don’t forget to check the overall load to the power supply.
Additionally, don’t forget to check cable voltage drops if the guard shack is a considerable distance away. Relays should all be at the door location as they draw very little current, but can switch larger current. Maybe we should have used one for the maglock?
We just got a small taste of what we can do with RLL designs. As your circuits get more complex, you might want to look at using advanced logic simplification methods such as Karnaugh mapping. As you get more involved with these design techniques, it might be worth considering a PC-based ladder circuit design and simulation program such as the Constructor 2003 from CMH Software.
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