Relays, solenoids, and servos
According to the statistics I have seen, the average age of the American farmer is just under 60 years old. That means they grew up during a time that was a confluence of the Cold War and the Space Age, watching The Jetsons on TV, if they had one with reception.
The wonder of sitting behind the steering wheel of a tractor and moving a lever or flipping a switch to control something that only a few years prior required a person to do dramatically increased productivity and reduced exposure to accidents.
As farms prospered, the hand-crank tractor was replaced by a shiny new one with electric start, and that introduced to many a relay into an electrical circuit. In like fashion, solenoids were found to lock shift levers on combines or to evoke an action in a hay baler.
As more automated controls were integrated into machinery and other equipment, servos became common. With the introduction of drones and other technology, the use of relays, solenoids, and servos is increasing. Though relatively simplistic devices, if misunderstood, they can be hard to diagnose when an issue occurs. The days of just replacing a part instead of understanding how it works and why it fails are long over, due to cost, a lack of access to the component as well as the need for timely repairs.
Relay remotely controls devices
Relay is a very common device not only found on equipment, cars, and trucks but throughout the farm and your home. Regardless of where they are, their operation and function remain the same.
The appreciation for the need of a relay is found in the understanding of electrical load in a circuit. A basic fact of electricity is that the higher the load (the more work that is required to be performed), the more amperage that is necessary. To carry the higher current flow, a thicker (heavier) wire is required.
A relay allows a high electrical load to be controlled remotely. Since a common use of a relay is for a starter, lets look at what would be required in that circuit if a relay is not employed. In this example, a wire the diameter of the battery cables would need to be run to the ignition switch and then back to the starter.
Also, the ignition switch would have to be robust and, thus, large enough to carry the load of starter. Think of a set of jumper cables running into and out of the dashboard of any machine. Not too practical!
A relay consists of an electromagnet that consumes very little amperage and is remotely controlled by a switch, which doesn’t need to be robust.
The electromagnet controls a heavy-gauge set of contacts (usually a disk) that has high amperage capacity and connects two terminals. The one terminal is from the battery (positive side of the battery) and the other goes to the starter. When you turn the ignition key, low-current battery voltage energizes the electromagnet, which, in turn, has contacts connecting the incoming battery cable to the outgoing one to the cranking motor.
When you release the ignition, the voltage to the electromagnet shuts off, and it is spring-loaded to disconnect the circuit.
The potential problem areas for a relay include:
• The energize voltage is either too low or suffers from a poor ground.
• The energize solenoid winding has either high resistance (too weak to pull in), is internally grounded (current bypassing windings, no movement), or is mechanically stuck due to rust or corrosion.
• The high-amperage contacts have burned and are not passing current from the feed to the load.
• The high-amperage connections are corroded or loose.
To properly diagnose a relay requires the use of a volt-ohm meter so that a value can be assigned to the pull-in voltage, resistance of the windings, and the integrity of the feed and load side. These diagnostic steps apply to any relay. The only difference would be the voltage and resistance of the windings.
What is a solenoid
The difference between a relay and a solenoid is that the latter is used to evoke action and not transfer current. A solenoid has two wires: power and ground. It is an electromagnet (as in the pull-in part of a relay), but it usually connects to a mechanical device such as a rod or lever.
When the solenoid is energized, it moves the part it is connected to. Once the power is shut off, it is spring-loaded to return to a rest position. For example, a round baler may use a solenoid that is controlled by a micro switch in the unit to first lock the end gate. When the bale is ready to be ejected, the voltage supply or ground is shut off and the solenoid is at rest and the gate opens.
A solenoid can be controlled by switching either the ground or voltage supply.
It can also be made to default (no power) in any position, depending on where the internal spring resides. Thus, it can be normally open or normally closed.
With electrical circuits, the term normally identifies its position without the circuit being evoked.
The potential problem areas with a solenoid are the voltage and ground, the resistance of the internal windings, and the condition of the part it is controlling.
A volt-ohm meter is required to check it.
A servo moves incrementally
Servo is employed when something needs to be moved incrementally and accurately. A common use of a servo would be in some autosteer systems, but they are found in many other applications such as drones, planters, combines, etc.
A servo consists of a stepper motor that is pulsed on and off and can move a predetermined range for every pulse. In most applications, the polarity to the electric motor is reversed to change direction. A DC motor’s direction is determined by the polarity of the circuit.
To be considered a servo and not just a stepper motor, an integral potentiometer is required so that the position of the motor can be monitored and changed.
Most servo applications are directly mounted to the device they are operating, so there is no stack-up of tolerance from a worn intermediate component.
Traditionally, a servo will be fed with system voltage, and the ground circuit will be pulsed on and off for it to achieve desired position. The servo needs to be powerful enough to operate what it is tasked to do and responsive to every control pulse.
As with the relay and solenoid, a servo needs to be checked with a volt-ohm meter against the proper specifications. Issues that can occur with a servo include:
• High resistance of the stepper motor windings (operating slowly).
• A short in the stepper motor windings (no movement).
• Voltage and ground supply.
• Control circuit failure.
• Potentiometer failure or skewing (false location data).
• Mechanical binding or any other issue with the servo or the device it is controlling.
As we task more chores of machinery and add complexity, then relays, solenoids, and servos will be commonplace in everything from your tractor to your smartphone-controlled grain dryer system.