Cam timer
Encyclopedia
A cam timer is an electromechanical system for controlling a sequence of events automatically.
drives, through a reduction gearbox, a shaft on which is arranged a series of cam
s. Associated with each cam is one or more switch
es. The motor rotates at a fixed speed, and the camshaft follows, usually extremely slowly. Indentations or protrusions on the cams operate the switches at different times. By arrangement of the cams and switches, complex sequences of opening and closing switches can be made. The switches then operate different elements of the controlled system - for example motors, valves, etc.
Most cam timers use a miniature mains synchronous motor
to rotate the mechanism at an accurate constant speed. Occasionally more complex timers with 2 motors are seen.
s, where they are used to drive the washing sequence according to a pre-programmed pattern. They are gradually being superseded by microprocessor
-controlled systems, which have greater versatility, and thus can more easily respond to various feedback.
Historically, cam timers were employed on industrial machines to control repetitive sequencing operations. In this case the cam followers often operated hydraulic valves. Cam timers in industry were superseded with the introduction of PLCs
, which offer improved reliability and allow the operation to be optimised.
With washing machine
cam timers, it is necessary to wait a variable amount of time (for example waiting for a tank of water
to heat up to a preset temperature). to achieve this, the cam motor
itself subject to control by one of its switch
es. The timer
sequence switches the cam motor off, and the motor is started again by the signal from the thermostat
when the required temperature is reached.
Usually washing machines thermostats have less fixed temperature detection points than the number of wash temperatures used. For intermediate temperatures, the cam mechanism uses the stop and wait method to heat to the nearest temp below the one desired, then uses only fixed timing of the heating element to increase the water to the desired temperature.
Some cam timers also have a fast forward mode, where applying power to a point on the controller causes rapid advance of the mechanism. This is often seen on washing machine controllers. Rapid advance can be achieved by moving of gearing, which may be triggered by various means.
Using feedback
and external time delay and other sensory circuits, it is possible to build an electromechanical state machine using a cam timer, which is what such a system within e.g. a washing machine amounts to.
Electronic controllers have largely replaced cam timers for most applications, primarily to reduce business costs, and also to maximise product features.
Cam timers don't have the greater degree of flexibility that cpu-based controllers provide. As well as offering more wash program variations, a cpu based washing machine controller can automatically initiate test cycles, reducing manufacturing costs, and provide fault codes in the field, again reducing repair costs, and providing feedback on real world failure rates and causes. All of these reduce manufacture and business costs.
Description
An electric motorElectric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...
drives, through a reduction gearbox, a shaft on which is arranged a series of cam
Cam
A cam is a rotating or sliding piece in a mechanical linkage used especially in transforming rotary motion into linear motion or vice-versa. It is often a part of a rotating wheel or shaft that strikes a lever at one or more points on its circular path...
s. Associated with each cam is one or more switch
Switch
In electronics, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another....
es. The motor rotates at a fixed speed, and the camshaft follows, usually extremely slowly. Indentations or protrusions on the cams operate the switches at different times. By arrangement of the cams and switches, complex sequences of opening and closing switches can be made. The switches then operate different elements of the controlled system - for example motors, valves, etc.
Most cam timers use a miniature mains synchronous motor
Synchronous motor
A synchronous electric motor is an AC motor distinguished by a rotor spinning with coils passing magnets at the same rate as the power supply frequency and resulting rotating magnetic field which drives it....
to rotate the mechanism at an accurate constant speed. Occasionally more complex timers with 2 motors are seen.
Uses
The most common use for cam timers is in automatic washing machineWashing machine
A washing machine is a machine designed to wash laundry, such as clothing, towels and sheets...
s, where they are used to drive the washing sequence according to a pre-programmed pattern. They are gradually being superseded by microprocessor
Microprocessor
A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...
-controlled systems, which have greater versatility, and thus can more easily respond to various feedback.
Historically, cam timers were employed on industrial machines to control repetitive sequencing operations. In this case the cam followers often operated hydraulic valves. Cam timers in industry were superseded with the introduction of PLCs
Programmable logic controller
A programmable logic controller or programmable controller is a digital computer used for automation of electromechanical processes, such as control of machinery on factory assembly lines, amusement rides, or light fixtures. PLCs are used in many industries and machines...
, which offer improved reliability and allow the operation to be optimised.
Methods used to increase control
The most basic type of cam timer continues to rotate regardless, which is not suitable when needing to wait for events that occur at variable times.With washing machine
Washing machine
A washing machine is a machine designed to wash laundry, such as clothing, towels and sheets...
cam timers, it is necessary to wait a variable amount of time (for example waiting for a tank of water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
to heat up to a preset temperature). to achieve this, the cam motor
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...
itself subject to control by one of its switch
Switch
In electronics, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another....
es. The timer
Timer
A timer is a specialized type of clock. A timer can be used to control the sequence of an event or process. Whereas a stopwatch counts upwards from zero for measuring elapsed time, a timer counts down from a specified time interval, like an hourglass.Timers can be mechanical, electromechanical,...
sequence switches the cam motor off, and the motor is started again by the signal from the thermostat
Thermostat
A thermostat is the component of a control system which regulates the temperature of a system so that the system's temperature is maintained near a desired setpoint temperature. The thermostat does this by switching heating or cooling devices on or off, or regulating the flow of a heat transfer...
when the required temperature is reached.
Usually washing machines thermostats have less fixed temperature detection points than the number of wash temperatures used. For intermediate temperatures, the cam mechanism uses the stop and wait method to heat to the nearest temp below the one desired, then uses only fixed timing of the heating element to increase the water to the desired temperature.
Some cam timers also have a fast forward mode, where applying power to a point on the controller causes rapid advance of the mechanism. This is often seen on washing machine controllers. Rapid advance can be achieved by moving of gearing, which may be triggered by various means.
Using feedback
Feedback
Feedback describes the situation when output from an event or phenomenon in the past will influence an occurrence or occurrences of the same Feedback describes the situation when output from (or information about the result of) an event or phenomenon in the past will influence an occurrence or...
and external time delay and other sensory circuits, it is possible to build an electromechanical state machine using a cam timer, which is what such a system within e.g. a washing machine amounts to.
Replacement with electronic controllers
While still fairly popular, cam timers are mechanical and hence subject to wear and reliability problems. Their reliability record remains good, but there is always some failure rate with mechanical switch contacts.Electronic controllers have largely replaced cam timers for most applications, primarily to reduce business costs, and also to maximise product features.
Cam timers don't have the greater degree of flexibility that cpu-based controllers provide. As well as offering more wash program variations, a cpu based washing machine controller can automatically initiate test cycles, reducing manufacturing costs, and provide fault codes in the field, again reducing repair costs, and providing feedback on real world failure rates and causes. All of these reduce manufacture and business costs.
See also
- ClockClockA clock is an instrument used to indicate, keep, and co-ordinate time. The word clock is derived ultimately from the Celtic words clagan and clocca meaning "bell". A silent instrument missing such a mechanism has traditionally been known as a timepiece...
- TimerTimerA timer is a specialized type of clock. A timer can be used to control the sequence of an event or process. Whereas a stopwatch counts upwards from zero for measuring elapsed time, a timer counts down from a specified time interval, like an hourglass.Timers can be mechanical, electromechanical,...
- Programmable logic controllerProgrammable logic controllerA programmable logic controller or programmable controller is a digital computer used for automation of electromechanical processes, such as control of machinery on factory assembly lines, amusement rides, or light fixtures. PLCs are used in many industries and machines...
(PLC)