Control Systems and Cabling
Telemetry is the automatic measurement and transmission of data from a distant source to a receiving station.
In the previous chapter, the various ways in which cameras may be moved so that a different field of view may be obtained were discussed. Some means of controlling these positioning devices must be used where movable cameras are present in a system. These control systems are generally referred to as telemetry systems. This name comes from the Greek word meter, to measure, and therefore to control, and tele meaning at a distance, in the same way that television means viewing an object at a distance.
There are many types of control systems available on the market and, as always, each method of controlling a movable camera has its benefits and drawbacks. The purpose of this chapter is to explain the principles of the various types of control systems available and to discuss their advantages and disadvantages.
There are two main ways of configuring the cabling from a controller to remote locations. One is known as daisy chain in which the cable is looped from one unit onto the next and so on. The other is a star configuration in which a separate cable is run from the controller to each location. These types of connection only apply to the control cable. The video cable must always be run from each camera location back to the main control. In other words, the video cable is always in a star configuration.
The daisy chain configuration does not need the last unit to be looped back to the controller. The control system being considered should be checked to ascertain which method of cabling is required. In a large industrial CCTV system, the layout of the site will dictate which type of cabling will be the most economical.
Hard Wired Control Systems
Hard-wired control systems are the simplest way of controlling movable cameras. As the name would suggest the connection between the control panel and the scanner/pan-tilt and motorised lens is direct connection by a length of multicore cable. The cost benefits of such an approach are that no form of telemetry receiver is required at the camera location, neither is a local power supply point necessary at the camera site as all the power for the camera, lens and pan-tilt may be sent over the same cable. The lens functions require a 6 or 12 volt DC supply, which will be provided by the controller. The pan, tilt functions may be 12 volt DC or 24 volt AC.
A typical hard-wired camera installation might be as shown in Diagram 12.2.
The video switching in a system like this would be done with a simple video switcher on to one or two monitors. As there is only one movable camera in the system, it is a simple matter to select the picture from the movable camera on to one of the monitors, and then to control the position and lens of that camera with the hard-wired control panel. Typically, the cable required for connection from the control panel to the movable camera must consist of 12 individual wires, or cores, covered by an overall sheath. This number of cores is needed, as all the functions of the movable camera must be individually sent along the cable.
A typical schedule for such a cable might well be as follows:
Table 12.1 Typical telemetry connections
There are two important factors to be considered in respect of hardwired systems. These are; the safety and cost of installing this multicore cable, the maximum distance at which hardwired pan-tilts may be sited from the controller.
It is obvious to see that the cost per metre of a 12 cored cable will be higher than the single or double pair cable required by other forms of telemetry system. This though is offset by the saving in supplying telemetry receivers and transmitters. In a site where there are several hardwired movable cameras at some distance apart, the cost of the cable may be noticeable in the total price of the system. The second part of this concern is that there are two main types of pan-tilt unit available, 24-volt AC types and 240 volt AC types. The IEE wiring regulations state that 240-volt cables must be run in protective conduit or trunking, for safety reasons. These regulations further state that low voltage cables, such as those conductors used for lens control must not be run in the same conduit. If 240-volt AC pan-tilts are used then all the expense of providing this protection must be considered.
The other limitation of hard-wired controllers is imposed by the voltage drop caused by the resistance of the cable. The current drawn by the pan-tilt unit causes the cable to heat and resist that current. The symptom of this resistance is a drop in the voltage available at the pan tilt. The greater the current drawn by the pan-tilt the greater the voltage drop, therefore the smaller the distance that the pan/tilt can be from the controller before the remaining voltage to the pan/tilt is too small for the pan-tilt motors to work! The limiting voltage drop is about 10% of the total, I.e. 2.4 volts for a 24-volt pan-tilt and 24 volts for a 240-Volt pan-tilt.
Ohms law enables the effect of the resistance of the cable to be calculated. This is given by the following simple formula :
Voltage drop = Current x Resistance (IR drop)
From cable datasheets, the amount of resistance per metre can be obtained. Once that has been found then the resistance of the cable can be calculated. The overall resistance will be for twice the length of the run. This is because there is the resistance of the core feeding the motor and the resistance of the return core to be considered. The current drawn by the pan-tilt can be found in the datasheet of the pan-tilt. The current and resistance obtained can then be put into the formula above to find the voltage drop. If the voltage drop is greater than 10% of the total then there will be problems and a larger core of cable will have to be used. This will have a consequent effect on the cost of the installation.
As an example, a 20-AWG cable might have a resistance of 0.053 Ohms per metre. A pan-tilt with a current consumption of 0.9 Amps is planned for siting 25 metres from the controller.
The total length of the conductor will be twice 25 metres, because of the effect of the supply and return cores. The total resistance would be 50 times 0.053 Ohms = 2.66 Ohms.
The voltage drop will therefore be 0.9 x 2.66 = 2.4 volts. This is the maximum that may be tolerated.
Therefore, the maximum cable run for hardwired control is quite small for 24-volt AC pan-tilts. One option is, of course, to use 240-volt AC pan-tilts. The benefits of such a choice are two fold. First, the 240-volt pan tilt uses much less current that a 24-volt pan tilt and so the voltage drop will be smaller. Furthermore, the 10% maximum voltage drop is 24 volts rather than 2.4 volts and so the effect of any voltage drop is less. However, due to the wiring regulations mentioned earlier, the additional cost of installing conduit or trunking for the 240-volt cables must be incurred. Some equipment manufacturers have approached this problem by developing relay boxes that are installed at the pan-tilt location. A relay box consists of several low voltage relays, one for each function. The low voltage is provided from the controller that operates a relay that switches the mains voltage to the appropriate function.
Such a system would be as shown in Diagram 12.3.
The relay boxes give several advantages:
- The relays use much less current than pan-tilts and so the voltage drop is much less. The payoff is in operating range, up to 4000 metres! Alternatively a much smaller gauge, and consequently cheaper, cable may be used.
- Either type of pan-tilt, whether 24 volt or 240 volt, may be used at any one camera location.
However, there are also two disadvantages:
- 240-volt mains supply points are needed at each movable camera location to power the relay boxes. It is important to remember, though, that these supply points would also be needed with any other form of telemetry.
- There is a cost involved in buying the relay boxes, but these are noticeably less expensive than a telemetry receiver of any other type of system.
The limiting factor for hard wire systems is the number of movable cameras. It can be confusing for the person using the system if there are, say, more than three joysticks for camera control. There are controllers available where several movable cameras, typically six, can be controlled from a single controller. In such a controller, the operator pushes a button to select which camera is to be controlled and the control voltages are switched to the corresponding multicore cable. The operation of such systems is slightly awkward, as the operator must remember to select the same camera on the video switcher as has been selected on the hard wire controller. The effective limit, then, for hard-wired systems is really one or two movable cameras.
This article is an extract from chapter 12 of 'The Principles & Practice of CCTV' which is recognised as the benchmark for CCTV installation in the UK.