Berkshire Driving School - Trailer Socket Project

Project feedback to : john@berkshire-driving-school.co.uk

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On this page: Foreword : Contributors : Standards : Diagrams : Shortcomings : Proposal : Future : Mechanical : Electrical.

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Foreword

An improvement on the multi-pin system currently used to control and power lights and equipment on trailers and semi-trailers and to replace the break-away cable on smaller trailers

Edited by John Silvester
Project leader

First draft 30 November 2003
Latest update 1st January 2007

Project funded by:

THE BERKSHIRE DRIVING SCHOOL
63 Wheatley
Great Hollands
Bracknell
Berkshire RG12 8UG

Version 26.10.06

This project was initiated by John Silvester, owner of The Berkshire Driving School. This business trains clients to draw a trailer and pass a driving test with a trailer. It has been found that the conventional multi-pin trailer plug connecting the electrics has frequently been unreliable and an improvement was needed. John Silvester has had success in the past with electro-mechanical projects. He aims to assemble a team capable of developing an improved trailer socket which will become the new standard for powering and controlling trailer lighting and equipment. The present standards do not allow significant information to be passed back to the towing vehicle and this project is designed to address this shortcoming.

Contributors to this project

John Silvester – project leader
Neil Manuel – logic designer

The present standards

The present standard for connecting a trailer to a vehicle in order to power electrical devices on the trailer is via a multiple pin connector. This connector typically has seven conductors arranged within a round shell. This shell protects the pins from the ingress of dust and water and prevents the conductors from shorting out should they inadvertently become in contact with the vehicle or trailer chassis. One conducting pin is used for one or a few vehicle lights which illuminate together when voltage is applied to that conductor. The many trailer lamps require many conductors. One conductor is used as an earth and carries the conventional current back to the vehicle. This conductor is typically no larger than the other conductors and this restricts the current carrying capability of the connector. The British system needs a similar connector for charging trailer batteries or running a refrigerator.

Current Wiring Diagrams

British system

Connections	Looking at Rear of Socket, Front of Plug
Pin Colour Function 1 Yellow Left Indicator 2 Blue Fog Light 3 White Earth 4 Green Right Indicator 5 Brown Right Side Light 6 Red Brake Lights 7 Black Left Side Light

Continental system

Connections	Looking at Rear of Socket, Front of Plug
Pin Colour Function 1 Yellow Left Indicator 2 Blue Fog Light 3 White Earth (35 amp) 4 Green Right Indicator 5 Brown Right Side Light 6 Red Brake Lights 7 Black Left Side Light 8 Yellow Reversing Light 9 Green Permanent Live 10 Blue Ignition Live 11 Red Earth (35 amp) 12 Black Coupled Trailers 13 White Earth (35 amp)

Shortcomings of the present system

1. Large, often exceeding 800mm diameter
2. The multiple pins and sockets are difficult to clean
3. Limited current carrying ability for all the pins
4. The small pins deform when used regularly and contact is lost
5. The plug is difficult to connect in the dark since it needs to be correctly orientated. Being round, it is often difficult for the operator to find the correct orientation
6. The pins are located mainly by friction and may be easily pulled out
7. The socket is usually located beside the tow-ball and the cable must be slack enough to accommodate a bend between trailer and drawing vehicle
8. The slack cable may touch the ground when the trailer bends the opposite way
9. Many contacts have to be connected and these are in the plug and socket which is subject to road dirt and spray thrown up by turbulence at the back of the towing vehicle
10. There is no provision for monitoring of equipment on the trailer
11. A separate break-away cable has to be attached
12. The current standard has a shutter which has to be lifted. This means that two hands are needed to effect a connection.

Proposed standard

Units are mounted on the towing vehicle and trailer and produces a coded radio signal. This is used to control the lights on the trailer. Signals also pass in the opposite direction and can be used for confirming the code and passing other information from the trailer

• A pair of robust connectors carry current to the trailer.
• The robust connector should be stout enough to act as a break-away cable
• The power cable must be reinforced to act as a break-away cable.
• The connector should be placed directly underneath the tow-ball
• The connector should locate positively and be unlikely to be disconnected should the tow-ball coupling fail. The connector will then act as the break-away cable attachment.
• The standard should allow for future safety devices to be included in the specification.

The standard should be compatible with:

• Bolt on tow balls
• Swan neck tow balls
• Pin and jaw couplings

Future proofing

The design should allow for the introduction of:

• Light emitting diodes
• Antilock trailer brakes
• Acoustic parking sensors
• Antilock brakes
• Electronic stability control
• Overheating bearings
• Rear view cameras
• Tyre pressure monitors
• Charging of trailer batteries
• Monitoring of refrigeration compartments
• Unauthorised access alarm
• Alarm warning of faulty coupling
• Traction motors on the trailer
• Any conceivable electronic additions

Tow-ball mechanical design standards

The tow ball is 50mm diameter

• Tolerances are specified
• The neck dimensions are specified
• The metal used is restricted
• The origin of the metal must be traceable
• Corrosion resistant finishes are restricted
• Clearance round the ball is specified
• Markings and labels are mandatory
• The device must be tested independently and gain approval Generally this is a million pushes and pulls at its rated load
• The break-away cable has to exceed standards

Design notes (mechanical)

Trailers towed by cars and leisure 4WD leisure vehicles in Europe use 50 mm diameter tow-balls.
The absolute maximum towing weight for such vehicles is 1.75 tonnes.
Non-European countries may have higher permitted loads.
Car manufacturers designed towing equipment are often equipped with swan-neck attachments.
After market and leisure 4X4 vehicles usually use tow-balls mounted on a tow-bar or chassis plate.

The standard dimensions are:

• Mounting hole centres 190mm apart
• 15mm diameter
• Bolt head socket diameter 24mm
• End radius 250mm centred around bolt holes
• Allow 16mm diameter for socket clearance. This clearance may be 38mm deep before it would interfere with the torque wrench.
• A mild steel mounting plate for electrical connections is usually sandwiched between the bar and ball.

Ideally the proposed connector would be cast as part of the tow-ball. Due to the complexities of gaining certification for such a device it is proposed that the initial design should fit between bar and ball. It will be born in mind that the design should eventually be incorporated into the tow-ball casting.

• Mounting the connector below the ball will reduce the amount of slack needed to accommodate the vehicle turning.
• This position will also offer some protection should the driver not completely reverse accurately when re-coupling.
• The downside of this arrangement is that the operator will not be able to see the position of the connector below the tow-ball.
• This need not be a problem if the connectors lock together easily and a guiding device is part of the design.
• Exposed contacts should not become completely live until hidden during coupling.
• Only two pairs of contacts are needed, live and earth.
• They will be at a low voltage and low current capability until the trailer transmits a signal back to the towing vehicle.

When the contacts touch, current flowing to the trailer starts the hand-shaking process and the towing vehicle allows the full battery voltage and current capability to be provided to the trailer.

Disconnecting the socket shuts of the main power to the connector.

The rear of a motor vehicle is an inhospitable environment. The two contacts needed for this project will be less susceptible than the current multi-pin arrangement but some environmental protection should still be incorporated into the design.

• The socket should be able to resist the maximum trailer weight, say 1.75 tonnes.
• Connecting and removal should be a one-handed process.
• Type 631 stainless steel has high strength
• Super dolce steel by the Sumitomo (SEI) Steel Wire corporation is as strong and more corrosion resistant
• 1x19 stainless steel wire 4mm diameter will have a breaking strength of about 1.5 tonnes
• Sta-Lok structural cable systems can be used to connect to the wire.

Wire and fitting standards manufactured to the forma B.S. 1290: 1983
now to the new standards: bs. 302.pt2: en 12385. bs en13414-1
B.S. 302: Part 1 1987
B.S. 302: Part 2 1987
All corresponding Test certificates supplied

manufactured to the forma B.S. 1290: 1983
now to the new standards: bs. 302.pt2: en 12385. bs en13414-1
B.S. 302: Part 1 1987
B.S. 302: Part 2 1987
All corresponding Test certificates supplied

Trailer Number Plates

The ability for a numberplate to be read is covered by Statutory. Instrument 1971 No 450 - The Road Vehicles (Registration & Licensing) Regulations 1971. Part Ill - Exhibition of Licences and Registration Marks states the following:

Requirements:

1. The provisions of this regulation shall apply to vehicles, other than works trucks and agricultural machines, first registered on or after 1 October 1938.
2. The registration mark of the vehicle shall be fixed and displayed on both the front and the back of the vehicle, so that in normal daylight the letters and figures are easily legible from every part of the 'relevant area', the diagonal of the square governing the 'relevant area' being 75 feet.
3. For the purposes of this regulation, the expression 'relevant area' in relation to a registration mark on a vehicle means the area contained in a square described on the ground, either in front of or behind the vehicle, where one corner of the square is below the middle of the registration mark and the diagonal of the square from that corner is parallel to the longitudinal axis of the vehicle, but excluding any part of the square within 10 feet of the vehicle - see diagram.

It is an offence to allow the numberplate to be obscured. This is now a fixed penalty offence, and therefore more likely to be policed. Obviously, this is of some concern to towbar fitters and to drivers of vehicles with towbar attachments.

Electrical design standards

(Rough draft)

• The lead content of solder is reduced
• The device must comply with standards concerning the transmission of electrical signals
• Factors include transmission frequency
• Baud rate
• Transmission power

Design notes (Electrical)

• Manchester encoding timed by the rise and fall of the signal
• Max bit rate 2,400 bps
• ASCII, divide bps by 10 to get cps (characters per second) because 1 start and 8 data and 1 stop bits ASCII will be 240 bytes (or characters) per second
• Half duplex more than halved the data rate since each unit must wait before receiving before transmitting so signals don´t collide and Tx must stabilise
• Therefore 50 bytes per second in reality (20ms)
• Full duplex speeds this up

Possible use of XBee 2.4 GHz modules

History

• ZigBee is the name of a wireless communication standard.
• It is a development of the virtually defunct Home RF (replaced by Wireless fidelity or Wi-Fi)
• The standard is supported by major players and they have formed a consortium called the Zigbee Alliance.
• XBee is the trade name of MaxStream. These units are new to the market at the start of this project.
• The units are half duplex ie they alternately transmit and receive.

Advantages

• The modules are Bi-directional (most modules only transmit in one direction) It has 16 separate channels so 16 networks can be addressed.
• Current consumption is very low so heat build up will not be a problem.
• They have transmitting power which can be set within the legal European standards (10mW)
• The units also are low cost.
• Transmission speed is quite low but this is not a problem in this application.
• Each unit has a unique addressing system whereby the units have unique factory configured serial number.
• The modules have in-built data-packet and error-checking to ensure reliable data transmission.
• They can be easily be interfaced with computers and microcontrollers.
• A PICAXE-XBee module is also available

Specification of the XBee module

• Range: in the order of 30 m
• Transmit power: 1mW
• Data rate: 250kbps
• Receiver sensitivity: -92dBm
• Supply voltage: 2.8-3.4V
• Typical transmit current: 45mA
• Typical receive current: 50mA
• Power down current: less than 10 micro amps
• Frequency: 2.4gHz
• Dimensions: 244mm by 276mm
• Operating temperature: -40 to 85C
• Number of pins: 20
• Number of channels: 16 direct sequence channels, software selectable
• Agency approvals: FCC Part 15.247 Industry Canada (IC) Europe

Figure 1—This is a simplified view of the actual XBee hardware shown in Photos 1 and 2. As long as you don´t overrun the buffers, ZigBee things work as designed.

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