Safer vehicles from new ring encoder technology

New test bed steering robots, attached to a vehicle’s steering column, are using new, high temperature, high power ring encoders that allow engineers to record a complete series of steering inputs from the driver. This can be replayed as many times as needed under the robot’s control in order to take measurements. And according to users of the new technology, it’s simple to use and test data is both accurate and reliable. The steering robot can also be coupled with a ‘motion pack’ – a gyro, three accelerometers and a global positioning system (GPS) – to provide path-following capability so that the robot can be made to follow a path or manoeuvre on the road to within a few centimetres. As the speed is gradually increased, very high lateral accelerations are generated allowing engineers to find the ‘true’ limits of the car. The test bed steering robots, manufactured by Bradford-on-Avon based company AB Dynamics, are currently being used by major automotive and truck OEMs, tyre manufacturers and test houses around the world. The robots all incorporate the ring encoder technology, developed by Renishaw. The steering robot contains a powerful servomotor which is attached to the vehicle’s steering column, together with a steering wheel allowing the car to be steered manually when not under computer control. The robot’s electronics include the necessary motor drivers and amplifiers as well as the means of recording the inputs and processing the test data. AS Tony Best, MD of AB Dynamics explains, “At first we used a rotary encoder from another supplier to control the inputs because it was one of the few encoders on the market with a large enough central bore to allow it to be used with truck steering systems. However, we discovered that they were discontinuing the manufacture of the encoder we were using and so we had to look for a new supplier. We started to employ [Renishaw’s] RGR ring encoder system, which is an encoder tape scale fixed to a stainless steel ring.” He continues: “In the area of transient testing, people in the automotive industry want to have better control of dynamic inputs. There are certain inputs that are difficult to replicate by hand, for example those that are gradually increasing in frequency but which maintain the same amplitude. It’s also very difficult to produce good sinusoidal shapes when you are going very slowly, the tendency is to produce triangular shapes instead. When you’re trying to get good results in order to compare one car with another, or a slightly modified car, you need to have accurate, repeatable, well-defined inputs.” There were some initial problems with the encoder though. During in-house testing at elevated temperatures, the adhesive used to bond the encoder tape scale to the stainless steel ring allowed the tape to slip where the ends joined, causing a ‘blip’ in the signal. Renishaw solved this by developing an RESR ring encoder where graduations are marked directly onto a low profile stainless steel ring, thus eliminating the need for conventional tape scale. Also, the correct alignment of the ring and the readhead is vital. On the steering robot there are clearance holes in the bracket that hold the readhead in position. These must be aligned in the X, Y and Z rotationally. The encoder therefore has a built-in LED that tells the user when correct position is achieved, simplifying the set up of the system. The robots are capable of a wide range of inputs: sinusoidal, steps and ramps at many different frequencies. The firm’s latest version is even more powerful than the original design, with high speed and torque, offering peak velocities of 1800 degrees per second. This technology is important for the automotive industry. Although vehicles of today handle far better than their predecessors, dynamicists and manufacturers of vehicle systems spend a great deal of time and effort testing equipment to ensure it is safe. Lateral acceleration, roll angle, yaw rate and other factors are measured and analysed to allow improvements in road handling for better and safer vehicles. Pointers Ring encoder technology used on vehicle steering robots is providing engineers with reliable test data for improving vehicle handling and road safety 17 of the steering robots are already being used successfully by automotive and truck OEMs, tyre manufacturers and test houses. The robots are capable of a wide range of inputs: sinusoidal, steps and ramps at many different frequencies, offering peak velocities of 1800 degrees per second.