Electro-Hydraulics Primer

This may be your first venture into electronic control of hydraulic systems. If so, or you just need a refresher course, then please read through our Electro-Hydraulics Primer.

To give you an idea of all the standard modules available from HCT for the Digital Valve Control product, you can view a DVC system architecture and modular configuration layout example.

Electronics is not nearly as scary as a it seems. With basic understanding and simple tools, electronics can be applied and troubleshot successfully. If you would like a quick education on what this electronics stuff is all about and how to tell if it is working, please read on.

The basic medium of exchange is an electron. When many electrons move to a single destination it is called voltage. Voltage is the pressure that causes the electrons to flow. When the electrons move (i.e. flow), it is called Current. Resistance is the restriction to currents flow (like pressure drop in a hydraulic system). More voltage pressure will result in more current flow for a given resistance restriction. Flowing current causes magnetic fields. Magnetic force is proportional to the current flow, ignoring non-ideal effects. Magnetic force causes valve spool and electric motor movement. Just as forcing fluid through a restriction generates heat, forcing current through resistance also generates heat. The heat is measured in wattage. Watts is also the measurement of electric power. Inductance is the property of a coil resisting changes in current flow. More inductance will cause more "inertia". Capacitance is the property of a capacitor resisting changes in voltage. More capacitance will cause more "shock absorbing" action to remove voltage "pressure spikes", or can store energy for later use like an accumulator in a hydraulic system.

Now that the basic concepts have been introduced, let’s take a look at the basic formulas involved. Feel free to fast forward past the next section, but remember where to find it when you need it.

Watts	Volts	Current	Resistance
W = V * I	V = I * R	I = V / R	R = V / I
W = I ² * R	V = W / I	I = W / V	R = V ² / W
W = V ² / R	V = √ (W * R)	I = √ (W / R)	R = W / I ²

Download Electronic Formulas to get these formulas in an EXCEL spread sheet for those times that your calculator batteries or memory has failed.

Current insists on being able to flow back to where it started from, thus the name circuit. Current has to "return to tank" or you "run out" and no power is transferred. You must have two wires hooked up to work. The more positive voltage coming out of the power supply is often referred to as Power, Positive, High side or Plus. While the more negative is Ground, Return, Common, Negative, Low or Minus. The second wire of a circuit can often be the chassis of a mobile system, but care must be exercised that there are no cracks or joints that cause unnoticed breaks or restrictions in the currents' path as they add resistance to the circuit. If more than one circuit uses the same wire, they interact, just as would happen if an undersized hose were used to return the fluid from many valves to tank. The pressure drop across the restriction in the wire is the resistance multiplied by the sum of all the currents using the wire, and all circuits using that wire see this voltage drop. The expected currents should be added up and an appropriately sized wire selected, or individual wires to each load can be used. The gauge and length of the wire affect its resistance. Even a small machine can have long wires if they are routed poorly.

Practice safe electronics by using a fuse. Fuses seldom are fast enough to protect electronic components from damage, but they will usually limit the fire damage and avoid burning up the wiring. Note that fuses, circuit breakers, connectors, relays and switches all have resistance. Fuses and circuit breakers are typically the worst offenders and should be accounted for even at moderate current values (1 to 10 amps). While the other devices are typically quite small and are only a problem at high currents. Corroded contacts can have very high resistance, so use waterproof parts if there is any chance of water, and use "inductive load" rated switches if driving coils. The coil inductance tries to keep the current constant when you try to turn off the switch. Just as damage is likely if you instantly stop a massive, spinning device, trying to instantly turn off current to a coil will result in an arc across the switch if no means of arc suppression is employed. That is basically how spark plugs work

Valve coils have a resistance. This resistance demands a certain amount of voltage to drive enough current to shift the spool fully. The valve manufacturers typically build their coils to have enough resistance that they will not draw too much current and burn up, but will be able to run off of typical 12 or 24 volt power sources. The amount of voltage you have to work with is the battery, alternator or power supply voltage. Extra resistance spread around the circuit may drop enough voltage to prevent proper operation. Valve coils are made of copper wire and have resistance, so forcing current through them causes them to heat up. Copper wire increases its resistance when it gets hot. This heating phenomenon is most troublesome for currents over one amp. Coils can also become hot when hot fluid is run through the valve, or if located next to a running engine. It is hard to predict coil temperature and even harder to get the resistance vs. temperature curve from the manufacturer, not to mention getting the resistance or voltage drop specs on all valve drivers, fuses, switches and etc. Often the "cut and try" approach is used with good success, but the person commissioning the machine must know what to look for if the electronics are not fully shifting the valve. Some coils increase their resistance by over 50% when very hot and no longer draw enough current to fully shift at normal alternator voltages.

Download Wire coil Calculator to get an Excel spreadsheet for calculating the wire size you need and the temperature effects on coil resistance and current. Note that you may also want to check appropriate safety agency tables that apply to your business.

The prototype machine just came out of 6 months of assembly. The electrical schematic is covered in changes and grease. The machine did not work on first power up. Throw off all the electronic devices and return them to the vendor as defective. Do not include symptoms of failure, just write BAD on them.

While that approach may make you Feel better for a while, it does not get your machine out the door and is unlikely to actually fix anything. Better to remain calm, send the boss back to his office, and begin troubleshooting.

Electrons are usually invisible and silent, which makes it hard to find leaks or restrictions. The well-equipped troubleshooter must have a digital volt / ohm / amp meter, mating connectors or break out boxes, and may need an oscilloscope to see quickly changing voltages or pulses. Our products incorporate LEDs to help trouble shoot your system. NOTE: If our product has flashing LEDs, that usually indicates that it has FOUND a problem, not that it IS a problem!

HCT products typically have a power LED to indicate that there is enough voltage to run our product. The power LED should be steady on! An off or flashing LED indicates a power problem and you must find and fix it before worrying about any other symptoms. Note that on our microprocessor based products, the power LED will flash steadily to indicate that the power supply voltage is higher than the maximum specified. When the power supply is too high it can cause the units to shut down, which also turns off the power LED or makes it flash erratically. Check both the power and ground wires, switches, connectors and fuses to verify they are installed. If no obvious fault is found, use the voltmeter to measure the voltage between power and ground AT the unit (with the unit plugged in and drawing power if possible), verifying against the units power specification. You did keep the spec / users guide that came with the product, right? If it is a standard product, you can get a new one from our web site, www.HighCountryTek.com.

Start at the unit and work your way back toward the power supply until the problem is found. If you do not have a voltmeter, jump the product's power input to the battery to verify that the power LED comes on. Now that you can trust the unit, move the jumper wires further down stream, past switches, wires and connectors until the light goes out.

Most of our products that have variable current outputs with red/green PWM% LED's for each output. these give a relative indication of the current output. If the PWM% LED is fully red, the unit is not trying to drive much current or there is a short circuit. Disconnect the coil at the card and measure its resistance and compare it to the specifications. If the PWM% LED is fully green, the unit can not drive as much current as it wants to, probably due to an open circuit or insufficient voltage for the resistance to be driven. This can also be caused by adjusting the unit to drive more current than is actually required to do the job, so re-adjust the card if the system if functioning correctly but the PWM% LED is fully green. If the system is not operating at the desired speed, disconnect the coil at the card and measure its resistance and compare it to the specifications. If the coil is OK, measure the power supply voltage at the cards' power input and coil output while the card is driving the coil. If the coil voltage is within a volt of the power supply (typical, see cards' specifications), the card is functioning correctly. You may reduce the voltage drop from the power supply to the card by shortening the wiring, using bigger wires or fewer switches and connectors. Choosing a coil rated for less voltage can also solve this problem. If the PWM% LED is off, the unit is not trying to drive that output, then trouble shoot the inputs. Our microprocessor based products flash the PWM% LED to indicate a short (flashing red/off) or and open (flashing green/off).

Most of our products have single color LEDs to indicate the state of simple on/off outputs and some times the state of inputs. Shorted on / off outputs are indicated by rapid flashing and open outputs by slow flashing. Error LEDs come on or flash to indicate a wide variety of problems, see the unit specifications.

A good quality meter that can read DC voltage up to 50 volts, current up to 10 amps and resistance down to 1 ohm should be part of your tool kit. An extra battery is also a good idea. Time is money and mistakes are expensive, so buy a digital meter to avoid having to figure out which analog scale you need to look while the system is causing major noise and confusion. There are many quality meters available, but Radio Shack is often the easiest place to find and has a nice selection to chose from. Very important safety tip: The current range of the meter will measure the maximum current the power supply can put out if you accidentally hook it across the power source! You must also avoid hooking up the current meter incorrectly, by selecting a range that is too low. This action typically blows the fuse in the meter and usually a new fuse cannot be found easily. Current measurements should be made by splicing the meter in series with the circuit. Voltage measurements are made by hooking across the circuit. Resistance measurements should be made (with the circuit turned off and disconnected from the rest of the machine) by hooking across the circuit. If blown current fuses proves to be a problem, HCT recommends that a 1 ohm, 10 watt resistor (Radio Shack or etc.) be temporarily spliced in series with the circuit is to be measured. A voltmeter hooked across the resistor will read the voltage drop due to the current. The current is equal to the voltage for the case of a one-ohm resistor. Note that these resistors are typically 5%, so the accuracy will be less than that of a current meter. Use a 0.1-ohm resistor (typically a special order) if the one-ohm resistor causes too much resistance in the circuit, but remember to multiply the answer by 10 to get the current value. This resistor is also the easiest way to measure dither amplitude and frequency with an oscilloscope. Be sure that the scope is floating with respect to ground (battery power or use two wire AC cord adapter).

All of our microprocessor-based products have an available computer interface to your PC. This is by far the most effective way to setup and trouble shoot our products. We provide all of the information represented by the LED's plus a lot more. Meters and running graphs give quick looks at what your system is doing, while enunciators tell you what the unit is trying to do. Data logging and remote operation can be used to consult with our staff to help debug difficult problems.

Once the unit is correctly wired and known to be functioning as intended, the machine may still not be functioning as you intended. The first step after initial power up and check out is to adjust the unit to your machine. Fluid power systems tend to have quite a bit of variation, so the factory presets on even custom electronics tend to need adjustment for the individual machine. If you have purchased a standard product, the odds of it being setup perfectly for your particular machine are very slim. Adjust the unit before deciding that it is defective. See our users guides for adjustment instructions. Typical adjustments will be setting the minimum and maximum coil currents to eliminate dead bands in the valves operation, adjusting ramps and / or PID constants to set the speed of response to inputs and selecting ranges and timer values on our microprocessor based products. Be aware of the effects of stiction and hysteresis, which may require dither adjustment too.