All About Servos

All About Servos
By Tony Arnold - StampedeProject.com
RC Car - January 2008

All About Servos
By Tony Arnold - StampedeProject.com

Always abused and often forgotten, servos are core to everything RC. Unfortunately servos are still a bit of a mystery for most of us and either work when plugged in or not. Stealing some notes from Hitec’s RC Brand Manager, Mike Mayberry, we are able to provide some explanations, servo basics, tips, and selection suggestions.

HOW DOES A SERVO WORK
A servo is a fairly simple but precise mechanism composed of an ESC, motor, gear reduction system, and output shaft all conveniently packaged in a little box… basically just a very compact transmission, ESC and motor. A power signal goes to the ESC which spins the motor and in turn the reduction gears and finally the servo output shaft to turn the servo arm left or right.

TORQUE vs. SPEED vs. CURRENT
According to Mike, many of their servos have the same motors, cases, and controller boards, but differ only in the gear reduction. Like everything else in RC, given the same power capabilities it comes down to gearing. Geared with less reduction, you have a servo that reacts fast to input but has less torque, gear that same powerplant with more gear reduction and the torque can greatly increase by sacrificing speed. More torque is almost always a good thing, but sacrificing speed comes down to tolerable limits and a 400Oz/in. servo that takes 4 seconds to move 20 degrees is unusable. Of course by increasing the power of the controller board and motor output with higher performance coreless and brushless motors, much higher torque and faster response is simultaneously possible, but that also means the servo will now draw higher current …nothing comes for free. Hitec’s Ultimate top of the line Coreless Digital Programmable 7955TG titanium gear servo for example provides 333oz/in torque and quick .15 second response time, but pulls more current during operation.

SERVO RATINGS EXPLAINED
What does oz/inch mean? The measurement indicates how much torque a servo will deliver before stalling with a 1” servo horn installed. With a longer servo horn, less torque will be provided and a shorter horn will nte more torque. A 80oz servo will deliver roughly 160oz if you use only a ½” long servo horn, but the speed decreases and overall movement becomes the limiting factors. Servo speed is measured in degrees traveled. Lengthen and shortening the servo horn also affects the server speed output on your linkage. As the arc of the servo arm increases with a longer servo horn mounting point, a faster movement and servo speed is realized, however the longer the mounting point the less the torque provided.

SIZES
Although there are some custom sizes, most servos come in micro, standard, giant, and ¼ scale. For most 1/10^th to 1/6^th scale applications we use standard sized servos, however ¼ scale servos are becoming more popular now that we are all demanding more from our servos in very large scale RCs. Amazingly even micro sized servos are pushing some serious power - Hitec’s micro 225MG metal gear servo delivers over 66oz/in of torque.

PLASTIC VS METAL GEAR
For most of us in the land of cars and trucks, metal gear servos rule the world from a durability perspective. RTRs on the other hand, benefit from the lower overall price of good quality servos with plastic gears. Mike’s generally recommends are metal gears for most ground RCs, but recommends plastic geared servos in airplanes and boats to save weight. Metal gears are now available in everything from brass to titanium to other exotic metals. The strength and lightweight of Hitecs titanium servos provide the best of all worlds.

ANALOG VS. DIGITAL
Technically analog and digital servos share all the same components however the digital servo feature a miniature microprocessor that sends signals at a exponentially higher rate to the motor. This gives the digital servos a more precise feel, response, and even more power due to the higher rate of cycles per second sent to the motor. For example if you only pushed back on a swing once per second to control the position of a swing, it will have time to swing back to you, however if you pushed on the swing with a higher rate, 50 times per second, the swing would remain stationary and therefore provide more control of the position – this is the same situation with Digital vs. Analog. A digital servo’s controller operates at a higher frequency and therefore controls the arm more precisely and delivers more power.

CONTROLLING BREAKAGE
According to Mike, most breakage occurs due to poor EPA (End Point Adjustment) and gear strength. Adjusting EPA is critical in maintaining the life of servos as continual over extension at stall or hard stop points causes enormous internal stress on all servo components such as a current spike in the controller, gears binding and the motor stalling. EPA should be set to stop arm movement just before binding of linkages or to limit movement for other purposes such as specifying end-points on a crawler dig setup. If you don’t have a fancy TX that allows easy EPA adjustment digitally such as Hitec’s PCX transmitter, a simple $29.99 gadget called the Servo Extender from Servo City can be plugged in between your RX and servo to adjust EPA and even centering. The best situation regardless of your transmitter’s capabilities is to use programmable servos in conjunction with a Servo Programmer to digitally program EPAs and other servo functions. Many Hitec servos can be easily programmed using Hitec’s own HPF10, HPF-20 or PC/Field Servo Programmers. The Servo Programmer is a handy device that performs a number of functions including servo centering and general testing of all servo brands, and on Hitec programmable servos reset to factory default, EPA and center points, direction of rotation, dead band width, failsafe point, speed (reduction only). Overall a cooler running servo that doesn't get it's parts mechanically slammed around will last far longer than on that constantly strains against the limits or binding linkages.

SERVO PROGRAMMING
The work of servo tuning has changed radically from relying on a high end radio for EPA, Centering, and servo speed. Because we are now demanding more and more from our servos, Hitec has been at the forefront of delivering programmable servo options. With Hitec's proprietary programmable digital servos and Hitec's PC based HPP-21 and stand-alone HFP-20 servo programmers we can do all sorts of neat stuff... set EPA, center, test, set speed, rest factory defaults, direction of rotation changes, dead band width, fail-safe point, increase/decrease overload, set thermal shut down parameters, and servo resolution. I never thought I would need these features, however the more I use the programing capabilities of my Hitec 7955TG for crawling the more I understand the need for servo tuning. These capabilities add up to friendlier initial setup, improved performance, and longer lasting servos.

HOW TO CHOOSE A SERVO
Usually we buy servos with money left after we buy a kit, but servos are the life blood of control from your RC and without a reliable, fast, or powerful enough servo, that $800 kit will be careening off the barriers in no time. According to Mike, "Many people try to get by with a less expensive and less powerful servo and end up running the servo at it’s maximum power output. The obvious result are failures due to gear stress and heat, but moving to a metal gear servo only masks the real heat problem is most cases." Running a servo near it’s max output also puts un-needed stress on the servo's motor and controller which generates heat. Mike calls this the "circle of death" where as heat increases the electrics become less efficient and less powerful, the servo strains harder which generates more heat… on and on to the point where thermal overload and servo damage can occur. Basically a less powerful servo will fail and have a far shorter life than a higher power model. The best solution is to replace a hot running servo with a higher power model or start with a high power servo to begin with. Mike mentioned a good rule it multiplying your vehicle’s weight in ounces by 1.5 to calculate a minimum recommended servo torque (oz/in) rating at 5V. For example the new 9.5lb (152oz.) E-Revo should have a minimum 228oz/in servo handling steering. A better rule is to double the weight of the car or load, that way the servo is never working at it's maximum. Braking servos are notorious for failure and Mike attributes this to a mis-perception that a braking servo doesn't need to have a lot of power. The reality though is that braking servos can have larger load spikes than steering servos each and every time the brakes are applied. Mike recommends at least a 120oz/in metal geared servo for braking duties.

POWERING YOUR SERVO
Mike also warned that most manufacturers rate their servos at 5V and 6V, however most ESCs typically pass only 5V, so that 250oz/in servo may be delivering significantly less at 5V. There are a couple ways around this issue. The first is easy, just use a higher output ESC. Another option is to utilize a BEC (Battery Eliminator Circuit) such as the Castle BEC to step down higher currents to a servo and/or RX unit's 5V-6V range. Some people even feed the + & - wires on the servo directly from the BEC or battery pack and only connect the center signal wire to the receiver.